June 2, 2025 • 47 mins
Welcome back to the RadOnc Smart Review Physics Series! In P8, we stressed the importance of Quality Assurance – making sure our LINACs work correctly. But QA relies on having an accurate baseline measurement of the machine's output. How do we establish that baseline? That's where Calibration comes in. Today, in Episode P9: Calibration Essentials, we'll demystify the process of calibrating a LINAC, focusing on the current standard protocol in the US: the AAPM's Task Group 51 report, or TG-51.
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(00:00):
OK, so let's talk about radiation therapy, getting the
dose delivered to the patient exactly right.
Well, it isn't just important, is it?
It's really absolutely fundamental.
It really is. Everything relies on it.
Yeah, every single dose calculation, every plan, every
time we use our Linux, it all hinges on having a completely
accurate baseline measurement. That's right, because if that
(00:21):
initial measurement is off, evenjust a tiny bit.
Then everything that follows is off too systematically wrong for
every patient treated on that machine.
Precisely. It's the absolute bedrock, you
could say, of clinical dissymmetry, and establishing
that precise baseline involves this critical process called
calibration. Calibration.
Right, it's how we figure out the absolute dose output of the
(00:43):
Linux, basically linking the machine's own unit, the monitor
unit or MU to a known absolute absorbed dose.
Exactly. So today let's let's unpack
that. Let's really get into how Linux
are calibrated. We're going to dive into this
standard protocol used here in the United States.
And that standard, the one that's really become the gold
standard, you know, is the American Association of
(01:04):
Physicists in Medicine's Task Group 51 report.
TG51 everyone just called it TG 51.
TG51. That's the one.
OK, PG 51. Let's settle in and explore what
this actually involves. And we've talked before about
how Linux make these high energyphoton and electron beams and
that we measure the radiation dose using detectors, usually
(01:25):
ionization chamber. Right, those precision
instruments. So calibration, specifically
following TG 51 is that crucial step.
It connects the number of Mus wedial up on the machine to a
precise, absolute absorbed dose,but it has to be under very
specific, repeatable conditions.Think of it like meticulously,
(01:45):
probably braiding a really sensitive scientific instrument.
You absolutely need to establishits response against a known
absolute standard and in a controlled environment.
So with the linac, we need to becompletely certain that when we
tell it, say, deliver 100 Mus, the machine gives a predictable
and really precise amount of absorbed dose, maybe that's 100
(02:09):
centigrade right at a very specific point inside a water
phantom. And that verified relationship,
the MUS to dose, that's the foundation.
It's the essential starting point for every single
calculation you then do for every patient treatment.
That analogy of calibrating a critical instrument that really
fits TG 51 then is like the instruction manual, the recipe
(02:30):
for getting that MU to dose relationship defined precisely.
Precisely. And the TG51 protocol which came
out from the AAP PM back in 1999. 1999 OK.
It really represented a significant step forward, a kind
of paradigm shift actually in how we approach Linux
calibration compared to the older ways.
A shift. How is it different?
Well, the most crucial change and honestly the defining
(02:52):
feature of TG 51 is that it's based directly on measurements
of absorbed dose to water. Absorbed dose to water.
OK. This was a big departure from
earlier protocols like the AAP Ms. own TG21 protocol.
That one was from 1983. TG21 right?
I remember that one. In TG21 it was based on
measuring exposure in air and then you needed conversion
(03:15):
factors, these things called F factors, to translate that air
measurement into dose absorbed in a medium like water or
tissue. Ah.
OK. So TG 21 measured in air, then
calculated to get dose in tissue.
Yeah, but TG51 measures dose directly in something that acts
like tissue. Exactly, you got it.
(03:35):
TG51's direct absorbed dose to water basis is, well, it's a
major point, something you absolutely have to remember if
you're studying this stuff. Absorbed dose to water got.
It it simplifies the whole thingconceptually and it aligns much
more directly with what we actually care about delivering
to the patient's tissue absorbeddose.
Because the human body is mostlywater, essentially.
Right. In terms of how it interacts
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with high energy radiation, the body is largely water
equivalent. So measuring directly in water
just makes more sense. It's inherently more relevant.
OK. Absorbed dose to water as the
foundation, that's a huge take away.
What are some other like fundamental characteristics or
requirements of TG51? OK, let's walk through the key
features. We've hit the first one, the
measurement medium. For that primary absolute
(04:18):
calibration, the protocol says measurements must be done in a
water phantom. It must be water, not air, not
those solid plastics. And the specific step, the
absolute calibration, No, you must use water.
And the reason, again, for insisting on water?
Well there are several really good reasons.
Like we said, water is the most relevant material to human
(04:39):
tissue for these mega voltage beams.
It's radiation interaction properties.
You know, things like it's density, stopping power, how it
scatters radiation. They're extremely well known and
stable, reliable, very reliable.Plus, water is every where.
It's relatively cheap, and it lets you position the ionization
chamber very, very precisely. You can put it exactly where you
(05:00):
need it, at specific depths, specific locations,
reproducibly. So it provides a standard
consistent environment for the measurement.
Exactly. So just a hammer at home,
absorbed dose to water is the basis and water is the required
medium for the actual measurement.
That level of detail and reproducibility, it sounds
(05:20):
absolutely crucial for making ita standard.
It is, which brings us to the second key, feature,
traceability. Traceability meaning.
TG51 relies on a system where the ionization chamber you use
in your clinic for calibration, well, it has to be calibrated
itself against a higher standard.
OK, so my tumor needs calibrating too.
Right. And this calibration is done by
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an accredited DOS Symmetry Calibration Laboratory or an
AECO, OK. And these labs are accredited
specifically because their calibrations are traceable.
They link back to national primary standards of absorbed
dose, like the ones maintained by NIST, the National Institute
of Standards and Technology herein the US.
Ah. So there's a chain from the
national lab like NIST down to the ADCL and then down to my
(06:04):
specific chamber that I use in the clinic.
Exactly a direct chain of traceability.
The ADCL gives you a calibrationcertificate specifically for
your chamber and the key piece of information on there is the
absorb dose to water calibrationfactor, but specifically for a
cobalt 60 gamma ray beam. Cobalt.
And this factor has a very specific notation in TG51.
(06:27):
It's N sub D comma W for cobalt 60.
N sub D comma W cobalt 60. OK, let's break that down again
just to be sure. N is the calibration factor,
right? D means it's for absorbed dose.
The subscript WT means absorb dose to water.
Yep, and the Cobalt 60 subscripttells you the type of being used
at the ADCL for that calibration.
(06:48):
Perfect. You nailed the notation.
Yeah. And what that number actually
represents that N sub D comma W for cobalt 60 value, it's the
absorbed dose to water, usually in Gray or centigrade that
corresponds to 1 unit of corrected reading from your
specific ionization chamber whenit's placed under reference
conditions in the 80 CLS cobalt 60 beam.
So it connects my chambers reading to the real dose based
(07:11):
on that cobalt 60 standard. Yeah, exactly.
It's how the absolute accuracy of the national standard gets
transferred to your working clinical chamber.
It basically turns your chamber into a traceable transfer
standard. Got it.
That factor is like the bridge. It links my chamber signal back
to the fundamental definition ofdose.
All referenced back to NIST via that Cobalt 60 beam at the ADCL.
(07:34):
OK, what else does TG51 cover? Key feature #3 What it applies
to The protocol gives you detailed procedures and all the
factors needed for calibrating both high energy photon beams
and electron beams that come from our clinical Linux.
Photons and electrons good. Right.
These are the 2 main types of beams we use in external beam
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therapy. So having one unified protocol
for both is, you know, very practical.
So it's the go to standard for the beams we use every day?
Yeah. What about the final
foundational feature? Frequency of recalibration.
This is important. Keep that traceability chain
intact and make sure you're measurements stay accurate
overtime. TG51 requires that your
ionization chamber and the electrometer you use with it
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must be recalibrated by an ADCL at least every two years.
Every two years. OK, so a regular checkup to make
sure my transfer standard, my chamber system hasn't drifted or
changed. Exactly.
Keeps everything tied back to the primary standard accurately.
So just to quickly recap the foundations, TG 51 based on
absorbed dose to water measurements must be in water
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accuracy traceable via an ADCL'sN sub D comma.
W factor for cobalt 60 covers both photons and electrons and
recalibration every two years. Those are the big rules, the
nonnegotiables. You got it.
Perfect setup. OK, let's shift gears now into
the actual how to How do we actually perform the photon beam
calibration using TG51? What does the setup look like?
(09:00):
OK, so you start by carefully setting up your calibrated ion
chamber and electrometer system,and this goes into a large water
phantom 1 where you can precisely control the
conditions. Like those big tanks we see?
Exactly. Usually the chamber is mounted
on a scanning system. This lets you position it really
accurately in three dimensions, Up, down, left, right, in, out.
And I assume there are very specific reference conditions
(09:22):
needed for this measurement. Can't just stick it anywhere.
Absolutely not. The protocol defines these very
rigidly. That's crucial for
reproducibility. The standard depth of
measurement in the water is fixed at 10 centimeters deep. 10
centimeters, OK. The field size is also standard
10cm by 10 centimeters. Now this field size is defined
(09:42):
either at that measurement depth, the 10 centimeters.
If you're using what's called a source to surface distance, or
SSD setup, that's where the water surface is 100 centimeters
from the source. SSD setup 100 centimeters to the
surface. Or the 10 by 10 field size can
be defined at the ISO center if you're using a source to axis
distance or SAD setup. In the SAD case, the chamber
(10:05):
itself is placed at 100 centimeters from the source,
which is the ISIS center. The 10 by 10 field is defined
there at ISIS center. So either way, it's basically a
10 by 10 field at 100 centimeterdistance related point and the
measurement depth is 10 centimeters deep in water.
Standard field, standard depth, standard distance.
Precisely these specific conditions were chosen
(10:27):
carefully. 10 centimeters deep is generally deep enough in the
water to get what's called charged particle equilibrium for
typical linac photon energies. Right where the electrons are
steady. Exactly.
But it's also still practical for doing routine measurements.
So once your setup is perfect, chamber at 10 centimeter depth,
correct distance, 10 by 10 field, you deliver a known
(10:47):
number of Mus from the linac, typically something like 100 or
200 MUS, and you record the raw reading, the charge or current
from your electrometer, let's call that raw reading M raw, M
raw. And as we've definitely talked
about before, that raw reading isn't the final answer.
It needs correcting, right? We can't just use that raw
signal straight up. That's absolutely right.
(11:08):
You're ahead of the game. To get the true signal, the one
that really corresponds to the ionization produced just in the
chamber sensitive air cavity, you have to apply several
correction factors to that M raw.
OK. What kind of Corrections?
Well, there's the correction fortemperature and pressure,
usually written as P sub TP. That normalizes your reading to
(11:30):
standard at atmospheric conditions because air density
affects the reading. Right, PTP.
Got it. Then there's the correction for
Ion Recombination, P sub ion. This accounts for the fact that
not all the ions created by the radiation actually get collected
by the chamber's voltage. Some recombine before they're
measured. There's also a correction for
polarity effects P subpole. Sometimes the chamber reads
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slightly differently depending on whether the collecting
voltage is positive or negative.This corrects for that people,
and if your electrometer itself has its own calibration factor
separate from the chamber, theremight be AP sub elect factor
too. Applying all of those
corrections to MROG gives you the fully corrected chamber
reading, and in the TT 51 world we just call that clean
(12:15):
corrected reading. MMM.
Simple enough. So M is the clean signal after
we've accounted for the environment and how the chamber
isn't perfectly ideal. Now the $1,000,000 question, how
do we turn that corrected reading M into the actual
absolute absorb dose to water atthat reference point?
OK, here's where the core TG51 photon dose formula comes in.
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It's actually pretty elegant theabsorb dose to water which we
write as D sub WD. Sub W dose to water.
For your specific clinical photon beam which has a certain
quality Q measured at that reference depth of 10
centimeters in water is calculated like this D sub W = M
* K sub Q * N D comma W for cobalt 60.
(12:59):
OK, say that again. D sub W = M * K sub Q times NDW
for cobalt 60. That's the one.
All right. Let's break that down.
M is our fully corrected reading.
We just talked about N sub D comma W for cobalt 60.
Is that ADCL calibration factor we got earlier that tells us how
our chamber reads in a cobalt 60beam for a known dose to water?
(13:19):
Exactly. It's your traceable link back to
the standard, but remember it's specific to cobalt 60 energy.
Right, which means K sub Q must be the missing piece, the beam
quality conversion factor. Bingo, that K sub Q factor is
the absolutely crucial piece here.
Its whole job is to take that N sub D comma W factor, which
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remember is only strictly valid in a cobalt 60 beam, and adjust
it or convert it so that it becomes applicable to your
specific clinical photon beam which has a different energy
spectrum or quality queue. OK, why is that adjustment
needed? Doesn't my chamber just read
dose? Shouldn't like 1 Gray be 1 Gray
no matter the beam energy? That's a really common question,
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and it would be nice if it were that simple.
But no, an ionization chamber doesn't respond exactly the same
way to beams of different energySpectra, even if the absorbed
dose delivered to the water at that point is identical.
Why not? Well, the chambers response
depends on a bunch of complex physics.
Things like how the radiation interacts in the chamber wall
material, the type of gas in thecavity, usually air, how
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secondary electrons are generated and where they deposit
their energy. All these things are somewhat
energy dependent. A cobalt 60 beam has a very
specific gamma ray spectrum, around 1.25 MEVI on average.
A high energy linac photon beam is different.
It's a Bremstrelung spectrum created in the target then
shaped by a flattening filter. It's a much broader range of
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energy. So the different energy mix
effects how the chamber sees thedose.
Exactly. The K sub Q factor accounts for
that difference. It corrects for the change in
your specific ionization chamber's response because of
the spectral differences betweenthe reference cobalt 60 beam and
your clinical beam. Q.
So K sub Q is basically answering the question, OK, my
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chamber reads this much dose in cobalt 60, how much differently
will it read in this particular linac beam for the exact same
actual dose delivered to the water And K sub Q is that
correctional multiplier? That's a perfect way to
conceptualize it conceptually. K sub Q answers.
How differently does my specificionization chamber model respond
to this clinical beam quality cue compared to how it responded
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back to ADCL and the Cobalt 60 reference beam?
And how is it derived? Is it measured or calculated?
It's derived using pretty complex physics calculations.
These involve things like ratiosof restricted stopping, powers
of water to air, corrections forthe chamber wall, material and
thickness, central electrode effects, and other energy
dependent factors. It's all calculated relative to
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the response in cobalt 60. TG 51 provides tables of these
calculated K sub Q values. But how do I know which K sub Q
value to use from the table? How do I determine my clinical
beams quality cue? Good question.
In TG51 for photon beams, the quality cue is specified by a
single measured parameter. It's the percentage depth dose
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at 10 centimeters depth. But, and this is important, it's
specifically for the photon component only.
Photon component only. What does that mean?
Well, especially for higher energy linac beams, when the
beam first enters the water, there's some electron
contamination mixed in with the photons near the surface.
Those surface electrons. Right.
For the beam quality specifier, we want a measure that reflects
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just the photon energy spectrum,not the contaminating electrons.
So we need the PDD at 10 centimeters depth.
That's due only to the photons. This is usually approximated by
measuring the PDD curve and thenusing specific methods to
estimate and subtract the electron contribution at
shallower depths, or by using a thin lead foil to filter out
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electrons during the measurement.
OK. So it's not just the raw PDD
measurement. Not quite.
It's often denoted as PDD of 10 sub X, where that little X
explicitly means photons only. PD of 10 sub X Yeah.
And how do I actually get that value for my Beam?
You have to measure the central axis percentage depth dose curve
for your specific photon beam energy and field size.
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The reference 10 by 10100 SSD. Then you use established
techniques like analyzing the shape of the curve near the
surface or using that lead foil method I mentioned to figure out
the PDD value at 10 centimeters depth after removing the
influence of contaminant electrons.
It's a really crucial measurement to get right.
OK, sounds like a careful measurement is needed.
Once I have my PDD of 10 sub X value, what then?
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Once you have that number for your Beam, you go to the
extensive tables provided in theTG51 protocol document itself.
These tables list K sub Q valuesfor many common models of
ionization chambers across a whole range of PDD of 10 sub X
values. So I find my chamber model in
the table. Exactly.
You find the row or section corresponding to your specific
chamber model. Then you find the PDD of 10 sub
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X value you measured. You might need to interpolate
between list of values and read off the corresponding K sub Q
factor. OK, so measure PDD of 10 sub X
for my beam. Look up my specific chamber
model in the TG 51 tables. Find the K sub Q for my measured
PDD 10X. Then I plug that K sub Q along
with my corrected reading M and my ADCL factor NDW for cobalt 60
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into the formula dose in KQNDWCO60 and boom.
That gives me the absolute absorb dose at 10 centimeters
depth in water for the musi delivered.
That is the standard TG51 photoncalibration process in a
nutshell. You got it.
Phew. OK, hey, that feels logical.
And just one quick but importantdetail about positioning the
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chamber, which he sort of touched on earlier.
It's about the effective point of measurement.
Right where the measurement is actually seen by the chamber.
Exactly, for the standard cylindrical ionization chambers
like the farmer type chambers, as most clinics use for this,
you don't actually place the geometric center of the chamber
right at the 10 centimeter reference depth.
Because of how the air cavity affects the electron paths, the
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chamber effectively measures thedose slightly upstream from its
physical center. TG51 specifies that for the
cylindrical chambers you need toposition the chamber so that
this effective point of measurement is at the 10
centimeter depth. And where's that effective
point? It's calculated as 0.6 times the
radius of the chamber's air cavity, and that distance is
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measured upstream towards the beam source from the chamber's
central axis. 0.6 times the cavity radius upstream.
OK, so I need to know my chamber's radius and shift it
slightly downstream so that point hits 10 centimeters.
Precisely. It's a small shift, but
important for accuracy. Now, if you were using a
parallel plate chamber for photons, which isn't typical for
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routine calibration but possible, then the reference
point is different. For parallel plate chambers, you
place the inside surface of the proximal plate.
That's the upstream plate, the one the beam hits first, right
at the reference step. 10 centimeters for photons.
Great, different rules for different chamber shapes makes
sense. It's about accounting for where
the ionization is actually beingcollected and averaged.
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Those little details, they really matter for getting the
absolute dose right. They absolutely do.
Precision is key here. OK, photon calibration seems
pretty clear now. Reference conditions corrected
reading M and DW from the ADCL and the crucial K sub Q factor
based on PDD of 10 sub X. What about electron beam
calibration using TG51? Is it like basically the same
(20:33):
process? Well, it shares the same
fundamental goal, determine the absolute dose to water under
reference conditions. But the specifics of the setup
and especially the factors needed in the dose formula are
quite different. This reflects the unique physics
of how electrons interact compared to photons.
Different physics, different rules.
Yeah, OK. How does the setup differ?
The setup itself is still done in a water phantom that parts
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the same. You're still measuring absorbed
dose to water and you typically use a standard electron
applicator or cone attached to the neck head.
You know, like a 10 by 10cm or maybe a 15 by 15 centimeter cone
size. Right, the cones that shape the
electron field. Exactly.
And the distance is also usuallystandard, typically a 100
centimeter SSD source to surfacedistance.
(21:17):
OK, so Stillwater standard cone size, standard distance sounds
similar so far. What's the first big difference
compared to photons then? The reference step we call it D
sub ref for electrons, unlike photons where it's always 10
centimeters for electrons the reference depth is not fixed.
It changes depending on the energy of the electron beam.
(21:37):
OK, so the depth we measure at depends on the electron energy.
How do we figure out what depth to use for a specific energy?
It's determined based on the electron beam quality specifier
used in TG51. Just like PDD of 10 sub X
specifies photon quality. For electrons, the quality
specifier is R. 50R50 What does that stand for?
(21:58):
R50 is defined as the depth in water measured along the beam
central axis where the absorbed dose has fallen to 50% of its
maximum value, D Max R50. Yeah, the depth where the dose
is 50% of the maximum dose. OK so to find R50I have to
measure show the electron beams central axis depth dose curve
first. Exactly.
(22:18):
You do a depth scan with your chamber, usually in the water
tank, Find the peak dose depth DMax, find the dose value that's
half of that peak, and then findthe depth where the dose drops
to that 50% level. That depth is your R50 value.
Got it. Measure the PDD curve.
Find D Max, find 50% of D Max. Find the depth for that 50%
dose. That's R50.
Precisely. And R50 is really important
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because it does two things. First, it tells you the quality
of the electron beam. Second, once you have R50 for
your beam, you use it to calculate the reference depth D
sub ref using a specific formulagiven in TG51.
OK, what's the formula for D subref?
The formula is D sub ref equals ( 0.6 times R 50 ) -, 0.1
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centimeters. D sub ref equals OK 0.6 * R
fifty. Then subtract 0.1 centimeters.
So the depth we actually calibrate at is directly
calculated from that R50 value. That's right, the reference
depth is linked directly to how deeply the electrons penetrate,
which is what R50 measures. And why that specific depth?
Why .650 point 1 centimeters? Good question.
(23:24):
This depth D sub ref is chosen very deliberately.
It always lies on the steep falling portion of the electron
depth dose curve, significantly deeper than the depth of maximum
dose D Max. OK, on the downward slope after
the peak, why there? Measuring in this region of
steep dose fall off provides better sensitivity to the
electron beams energy and spectrum.
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Small changes in electron energywill cause more noticeable
shifts in the dose at D sub ref compared to measuring near the
surface or right at the D Max peak where the curve is flatter.
So it's a more sensitive spot tocalibrate, reflecting the beam
energy better. Makes sense calibrating where
the dose is changing predictablybased on energy.
OK, variable depth based on R50.That's a key difference.
What's another critical difference for electron
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calibration compared to photons?The choice of ionization
chamber. This is much more critical for
electrons, and in some cases theprotocol dictates exactly what
you must use. This is probably key difference
#2 for electrons. Mandatory chamber type outs.
For low energy electron beams, and TG51 defines this
specifically as beams with nominal energies of about 6 mega
(24:30):
electron volts or less. Or more precisely, beams with an
R50 value of 2.6 centimeters or less.
R50 less than or equal to 2.6 centimeters.
OK, low energy. For those beams you must use a
parallel plate type ionization chamber, not a cylindrical 1.
Must use a parallel plate. Why is such a strict rule there?
Why can't I use my usual farmer chamber?
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Because for these low energy electrons, the dose changes very
rapidly with depth, especially near the surface and around D
Max, the dose gradients are incredibly steep.
OK. Steep fall off.
Right cylindrical chambers like the Farmer type have a
relatively large air cavity volume compared to these rapid
changes. When you put that cylindrical
chamber into these shallow depths with steep gradients, it
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does 2 bad things. First, it significantly perturbs
the electron field. It displaces water, causes
scattering changes. Second, it averages the dose
over its entire volume. But the dose isn't uniform over
that volume because the gradientis so steep.
Exactly. It averages the dose in a region
where the dose is changing really fast.
This leads to inaccurate measurements.
(25:37):
You're not measuring the dose ata specific point anymore.
OK, so for low energy electrons,the cylindrical chambers
basically mess up the bean they're trying to measure and
give an inaccurate average reading.
Got it. Whereas parallel plate chambers
are designed differently. They have a very thin entrance
window and a shallow disc shapedcavity.
This design minimizes how much they disturb the electron field
(25:59):
and they measured those much closer to a single plane.
So they give a much more accurate measurement in those
tricky low energy, steep gradient situations.
Makes perfect sense. So low energy electrons, R50 2.6
centimeters, almost mandatory parallel plate chamber.
What if I have a higher energy electron beam, say 12 maybe or
18 maybe, where R50 is much larger?
(26:22):
Good question. For higher energy electron beams
where R50 is greater than 2.6 centimeters, you are allowed to
use a cylindrical chamber like your farmer chamber.
OK, so I can use it then. You can, but there's a catch.
Using a cylindrical chamber for any electron beam calibration,
regardless of the energy, means you have to include an
(26:42):
additional correction factor in the dose calculation formula.
Another correction factor just for using the cylindrical
chamber. Yep, which makes the electron
dose formula inherently a bit more complex in the photon
formula, especially if you opt for the cylindrical chamber
more. Factors.
OK, I'm braced. Hit me with the full electron
dose formula from TG51. All right, here it is the
absorbed dose to water D sub W for an electron beam, a quality
(27:05):
Q which we now know is specifiedby R50 measured at the reference
depth D sub ref is calculated asD sub W = M.
Corrected reading M, Got it. Yeah, multiplied by P
subgradient for quality QP. Subgradient.
OK, new one. Multiplied by K prime sub R50.
K Prime sub R50 another new one.Multiplied by K sub.
Equal yet another new one. Multiplied by N sub D comma W
(27:28):
for cobalt 60. OK, whoa, deep breath.
Let's recap that monster D sub W= M * P sub gradient times K
prime sub R 50 * K sub equal times NDW for cobalt 60.
That's the beast. OK.
We know M is the fully correctedreading from the electrometer,
right? And we know N sub D comma W for
(27:50):
Cobalt 60 is our trusty ADCL calibration factor linking back
to the cobalt 60 standard. Correct, same factor used for
photons. So let's tackle new ones.
What is K sub equal? OK, K sub equal.
This is called the photon to electron conversion factor, and
importantly, it's specific to your ionization chamber model.
Photon to electron conversion. Wait, what does it convert?
(28:11):
Remember that your main ADCL calibration factor N sub D comma
W was determined in a cobalt 60 photon beam.
K sub equals job is basically toconvert that photon beam
calibration factor into an effective calibration factor
that's appropriate for use in electron beams.
It accounts for the fundamental differences in how the chamber
responds to high energy photons versus electrons.
(28:34):
OK, so it bridges the gap between the Co 60 photon
calibration and using the chamber in an electron beam.
Precisely. And you find this factor K sub
equal in the TG 51 tables. It's listed for specific chamber
models. Look up K sub equal for my
chamber in the tables. Got it.
Next K prime sub R50 What's that?
K prime sub R50. This is the electron quality
(28:56):
conversion factor. It plays a role similar to K sub
Q for photons. OK, so it corrects for the
specific bean quality. Exactly.
It adjusts the calibration to account for the specific
electron bean quality, which as we established, is defined by
your measured R50 value. It corrects for the fact that
the chambers response will vary slightly depending on the
electron energy spectrum represented by R50.
(29:17):
So just like KQ depends on PDD 10 yeah X for photons, KR50
depends on R50 for electrons. You got it.
And again you look up this K prime sub R50 factor in the TG
51 tables. The value will depend on your
measured R50 and also on the type of chamber you are using,
whether it's a cylindrical or a parallel plate chamber.
Look up K prime R 50 based on myR50 value and my chamber type
(29:41):
makes sense. And finally the last new factor
P sub gradient. P sub gradient.
This is the gradient correction factor.
Now listen closely here. This factor is only needed and
only applied if you are using a cylindrical ionization chamber
for your electron beam calibration.
Only for cylindrical. Chamber only for cylindrical.
(30:02):
If you followed the rule and areusing a parallel plate chamber,
either because you had to for low energy or chose to for high
energy, then P sub gradient is simply equal to 1.0.
No correction needed from this factor.
OK, P gradient equal 1.0 for parallel plate chambers.
Why is it needed only for cylindrical ones?
Remember how we determine the electron reference depth D sub
(30:23):
ref. Yeah, it's calculated from R50
and it lies on the steep fallingpart of the depth dose curve.
Exactly. It's in a region of high dose
gradient. That means the dose is changing
rapidly with just small changes in depth, right.
A cylindrical chamber, as we discussed has a significant
physical size, especially along the beam direction.
It's reading represents an average of the dose deposited
(30:44):
across its entire air cavity volume.
OK, but if the dose is changing rapidly across that volume
because you're in a steep gradient, the average dose the
chamber reads isn't necessarily the same as the dose at the
precise single point defined as D sub breath, which is related
to the chamber center adjusted for effective point of
measurement. It's that averaging problem
(31:06):
again, like trying to measure the exact speed of a car when
it's rapidly accelerating or decelerating using a speedometer
that takes a few seconds to update the reading lags behind
the instantaneous value. That's a fantastic analogy.
That's exactly the issue. P sub gradient corrects for this
averaging effect. It accounts for the fact that
the cylindrical chamber is smearing out the dose
measurement in a non uniform steep dose gradient field.
(31:30):
Its value depends on the steepness of the gradient, which
relates to R50 and the specific geometry, mainly the radius of
your cylindrical chamber model. OK.
And parallel plate chambers don't need it because.
Because they're sensitive volumeis very thin along the beam
direction. They essentially measured dose
much closer to a single plane. The inside surface of the front
(31:50):
window is the reference point, so they don't suffer from that
volume averaging problem in the gradient nearly as much.
Hence P sub gradient is just onepoint O for them.
Got it. So electrons need more factors.
Potentially there's K sub equal to convert the photon
calibration to electrons, there's K prime sub R50 to
adjust for the specific electronenergy based on R50.
(32:11):
And if you use the cylindrical chamber, there's piece of
gradient to correct for dose averaging in the steep fall off
region at D sub ref. You've summarized it perfectly.
It is definitely a bit more involved than the photon side,
but each of those factors is there to address a specific
physical reality about measuringelectron beams accurately with
an ionization chamber calibratedin a cobalt 60 photon beam.
(32:33):
OK, so once you've done all thatset up correctly, taken the raw
reading M, applied all the environmental and chamber
corrections to get M, identifiedall the necessary TG 51 factors,
KQ for photons or key call KR50,and maybe P grading for
electrons. Looked them up in the tables and
plugged everything into the correct formula.
(32:54):
You finally get that D sub W theabsolute absorb dose to water
delivered by your linac for the number of musu set under those
very specific reference conditions.
That's the end point of the TG51calculation itself.
You've determined the absolute output under reference
conditions. For example, if I delivered say
100 Mus for my measurement, and my whole TG51 calculation spit
(33:14):
out a dose of maybe 100.5 centigrade at the reference
point, then my Linux output under those reference conditions
is just that dose divided by theMUS.
So 100.5 centigrade divided by 100 MUS equals 1.005 centigrade
per MU. Exactly, you've nailed it.
That centigrade per MU number isthe crucial output figure
(33:37):
derived from this entire meticulous process.
That number, That specific centigrade per MU value, that's
kind of the Holy Grail of this whole calibration, isn't it?
That's the number that everything else in patient dose
calculation is going to rely. On it truly is the absolute
foundation, this calibrated doseper MU value.
Sometimes it gets adjusted slightly to represent the dose
(33:59):
at D Max using a PDD or TMR measurement, but it's still
based on this absolute value. This is the starting point for
calculating the MUS needed for every single patient treatment.
Every single one. Every single one.
When you're planning a patient treatment, you take this
fundamental baseline output, this dose per MU, and then you
apply all the other factors thataccount for the specifics of
(34:20):
that patient's setup. Oh, things like output factors
for different field sizes or shapes, depth dose factors like
PDD's or TMR's to get the dose correct at the actual tumor
depth, the inverse square factorto account for any non standard
distance. Maybe wedge factors, tray
factors, the list goes on. Right.
All those relative factors we use in planning.
(34:41):
Exactly. But all those factors are
relative. They modify the dose relative to
that absolute dose per MU that you established so carefully
through the TG51 calibration. If that absolute starting point
is wrong, all the relative adjustments won't fix it.
OK, let's really hammer this home then.
Yeah. Why is this calibration process
following TG 51 so rigorously? Why is it not just, you know,
(35:04):
some physics busy work, but absolutely fundamental to safe
and effective radiation therapy?Why is getting this single
number right so mission critical?
Well, the number one reason without a doubt is dose accuracy
for the patient. Dose accuracy.
As we said right at the start, any error you make in this
initial absolute calibration, itdoesn't just stay there, it
propagates directly through every single patient treatment
(35:26):
performed on that linac afterwards.
Every single one. Every single one.
If your calibration is off by let's say, 3% high, then every
single dose delivered to every patient on that machine will be
systematically 3% higher than intended.
And for many treatments, a 3% error or maybe 5% can have real
(35:47):
clinical consequences. It could potentially impact
tumor control rates if you're consistently underdosing, or it
could significantly increase therisk of serious side effects
normal tissue toxicity if you'reconsistently overdosing.
And there have been cases where calibration errors have actually
caused harm. Sadly, yes.
There have been documented incident, serious incidents,
(36:08):
where patient harm resulted directly from systematic errors
in linac calibration. It underscores how critical this
is. So a small percentage error in
this one number, mirrored under ideal conditions in a tank of
water, translates directly into a potential problem, a risk for
every single patient, every single day that machine is used.
It absolutely does. That's the weight of it.
Second big reason, Consistency. Consistency.
(36:30):
Using a standardized, widely accepted protocol like TG51
ensures there's a high degree ofconsistency in dosimetry between
different Linux, even different models from different
manufacturers, and crucially, between different clinics and
hospitals. So everyone is speaking the same
dose language. Essentially, yes.
If a patient is prescribed, say 200 centigrade per fraction,
(36:50):
following a standard like TG51 gives us confidence that 200
centigrade delivered by machine A over at institution X is truly
equivalent to 200 centigrade delivered by machine B across
the country at institution Y. And that's.
It's vital for things like clinical trials, right?
Comparing results. Absolutely crucial for multi
institutional clinical trials. You can't compare patient
(37:12):
outcomes if you're not absolutely sure the dose
delivery was standardized and accurate across all
participating sensors. TG51 provides that essential
common language and baseline fordose.
OK. Dose, accuracy, consistency,
What else? 3rd, I'd say understanding
limitations and uncertainties byactually going through the TG51
(37:33):
process meticulously by understanding each correction
factor, where it comes from, thephysics behind it, and all the
required measurement conditions.Like the specific positioning
rules or needing that parallel plate chamber for low energy
electrons. Exactly.
By understanding all that detail, you as the medical
physicist gain a much deeper understanding of the whole
(37:55):
process and, importantly, the potential sources of error or
uncertainty within it. Knowing where things could
potentially go wrong. Right.
And that knowledge is vital. It helps you evaluate your own
measurements, critically troubleshoot if something looks
weird, and make sure your ongoing quality assurance
program is robust enough to catch any potential problems
before they could ever impact a patient.
(38:17):
It informs best practices. Makes sense and #4.
And 4th, it's a cornerstone of commissioning new equipment.
Commissioning. OK, Setting up a new machine?
Yeah. When a brand new Linnec is
installed in a clinic, performing the TG51 absolute
calibration is one of the very first, most fundamental
measurements you do. You simply cannot confidently
(38:38):
collect all the other beam data needed for the treatment
planning system. You know the output factors,
depth, dose curves, beam profiles, all that stuff.
Or even think about starting to treat patients until you have
accurately established that absolute dose per MU output
under the TG51 reference conditions.
It's the anchor point. Absolute starting point for
(38:58):
verifying the machine and getting it ready for clinical.
Use exactly It's the cornerstoneupon which all the other beam
data measurements and all subsequent clinical calculations
are built. OK, so let's try to wrap that
importance up. Accurate TG51 calibration gives
us that absolutely essential traceable dose per MU number.
It's done under specific ideal reference conditions for both
(39:21):
photons and electrons. It uses a properly calibrated
chamber from an ADCL and it applies specific factors KQ,
kcal, KR50P gradient to account for beam quality and chamber
response based on measured parameters like PDD 10X and R50
and that final output number. It's the absolute linchpin for
(39:41):
calculating every single dose delivered to every single
patient. Couldn't have said it better
myself. It is absolutely not an
exaggeration to say that performing this calibration
correctly is arguably the singlemost critical measurement that a
clinical medical physicist performs.
You know, it really feels like in physics, especially medical
physics, everything gets described as absolutely
essential or critical or foundational or vital doesn't.
(40:02):
It it's true, it does seem that way sometimes.
Maybe it's because honestly, in this field, if just one link in
that chain of measurements and calculations is weak, the whole
thing can potentially fall apart, particularly when patient
safety is writing on it. Yeah, you can almost hear the
the board examiners or the review committees asking now
tell me, was this absolutely foundational, critical, vital
step performed correctly and documented meticulously?
(40:25):
Exactly, makes you sweat just thinking about it.
Speaking of board exams and review committees and just
making sure these truly vital concepts are locked in.
Maybe we should do a quick boardreview blitz.
Just reinforce some of these absolute must remember points
from TG51. Perfect idea.
Let's do some rapid fire questions.
Yeah, good practice. OK.
Question number one, according to the TG51 protocol, what is
(40:48):
the required measurement medium for performing the absolute
absorbed dose calibration of a clinical linear accelerator?
OK, medium, That has to be water.
TG 51 is fundamentally an absorbed dose to water protocol.
That was the big shift from TD 21's air karma basis.
So the absolute calibration measurement itself must be
performed in a water phantom. Correct water.
(41:09):
It is question 2 for photon beamcalibration under TG51.
How is the beam quality conversion factor K sub Q
determined? Specifically, what measured
parameter is K sub Q based on OKphoton KQ?
That depends on the beam qualityspecifier, which is PDD of 10
sub X. That's the percent of depth
those at 10 centimeters depth, but for the photon component
(41:32):
only with electronic contamination removed or
accounted for. You measure that PDD 10X value
for your beam and then use it tolook up the correct K sub Q
value for your specific ion chamber model in the TG51 table.
Absolutely right. PDD of 10 sub X characterizes
the photon energy spectrums penetration and that dictates
the K sub Q. Excellent question three.
(41:52):
According to TG51, what is the formula used to determine the
reference depth D sub ref for electron beam calibration?
The electron depth formula, OK, in AD sub ref equals, let me
think, 0.6 times R 50 ) -, 0.1 centimeters.
Yeah, D sub ref equals 0.6 R 50.1cm and it scales to the
electron beam quality R50, whichitself is the depth of the 50%
(42:15):
dose level on the central axis PD curve.
Nicely recalled that formula is key.
And finally question #4 this is a practical application a
medical physicist is setting up to calibrate A6 mega electron
Volt electron beam. They perform a measurement and
determine that the R50 for this particular beam is 2.3
centimeters. According to the recommendations
(42:38):
in TG51 regarding chamber selection, which type of
ionization chamber should they absolutely use for this
calibration measurement? OK, six MEVI electrons R50 is
2.3 centimeters. That R50 value 2.3 centimeters
is less than the threshold of 2.6 centimeters that TG 51 sets
for low energy electrons. Therefore, the protocol requires
(42:58):
him to use a parallel plate ionization chamber.
A cylindrical chamber is not appropriate here because of
perturbation effects in the steep gradients of this low
energy beam. Perfect reasoning and correct
answer. Parallel plate is mandatory in
that situation. Those are definitely some high
yield points to have absolutely solidified, whether you're
studying for boards or just ensuring robust and accurate
(43:18):
clinical practice day-to-day. Yeah, it's a complex protocol,
definitely have a lot of details, but getting these
foundational elements, the medium, the traceability, the
reference conditions, the quality specifiers, the factors,
the Chamber rules, getting thosereally internalized feels
critical. It truly is.
So we spent this time really digging into how TG51 gives us
that crucial single number, the dose per MU under very specific
(43:42):
idealized reference conditions. We got the Photon formula M
times KQ times NDW and the Utronformula which is a bit longer, M
* P gradient if needed, times K or 50 times Kegel times NDW, all
starting from that traceable NDWcobalt 60 factor from the ADCL
and using measured beam quality specifiers like PDD 10X and R50.
(44:02):
That output number is our absolute starting point, our
anchor. That baseline dose per MU under
reference conditions is precisely what this whole
process characterizes. It's the foundation.
But here's something to really chew on as we wrap up.
Maybe a final thought. TG51 gives us this incredibly
precise number, right? The dose per MU, but it gives it
to us at one specific point in space, 10 centimeters deep for
(44:26):
photons or D sub ref for electrons, inside a perfectly
uniform idealized tank of water for a simple 10 by 10 square
field. Right, under very controlled,
very specific conditions. So how do we as medical
physicists take that one number,determine under such ideal
conditions, and then confidentlyuse it to calculate the actual
monitor units needed to treat a tumor that's, you know, buried
(44:48):
deep inside a patient's unique, complex body?
Have the $1,000,000 question that follows calibration.
Yeah, that tumor isn't always at10 centimeters depth or AD sub
ref. It might need a weirdly shaped
field, not simple square. We might be using multiple beams
coming in from different angles,passing through lungs, hitting
bone. It's way more complicated than
the water tank. And that right there is where
(45:10):
the rest of clinical dissymmetryand the treatment planning
system come into play. You use that absolute
calibration number from TG 51 asyour non negotiable starting
point, your anchor. But then you need a whole
arsenal of additional data and calculations.
Like all the relative dose data.Exactly the off axis ratios to
know the dose away from the center.
The output factors for all the different field sizes and ships.
(45:33):
The machine can make the full depth dose.
Curds or tissue maximum ratios, TMRS to predict dose at any
depth. Scatter factors, corrections for
B modifiers like wedges or blocks, Algorithms to handle how
the beam goes through different tissue densities like lung or
bone. It's a huge amount of extra
information and calculation built on top of that initial
(45:54):
calibration number. It is.
The TG51 calibration provides the absolute foundation, the
100% dose reference point, but the complex structure built on
top of it, accurately calculating the dose
distribution throughout a heterogeneous patient geometry
for a specific treatment plan. That's a whole other incredibly
complex layer of physics and computation.
So the question really is, how does the accuracy we achieve in
(46:18):
that initial idealized calibration actually ripple
through that entire complex chain of subsequent measurements
and calculations? Where are the otential weak
links or pitfalls in that chain?How do we ensure that the dose
accuracy we established so carefully in the quiet water
phantom is truly maintained whenwe're treating a real,
(46:39):
breathing, moving person? It's a fantastic question, and
honestly it's something every clinical physicist needs to keep
at the forefront of their mind. It really highlights why both
the foundational calibration andall the subsequent patient
specific calculations and checksrequire such meticulous
attention to detail, constant vigilance and a really deep
understanding of all the physicsinvolved at every step.
(47:00):
Definitely something to think about.
You can find notes from today and join the discussion over at
radonsmartlearn.com. And please subscribe so you
don't miss our next exploration.Thanks for tuning in.
OK, so let's talk about radiation therapy, getting the
dose delivered to the patient exactly right.
Well, it isn't just important, is it?
It's really absolutely fundamental.
It really is. Everything relies on it.
Yeah, every single dose calculation, every plan, every
time we use our Linux, it all hinges on having a completely
accurate baseline measurement. That's right, because if that
(00:21):
initial measurement is off, evenjust a tiny bit.
Then everything that follows is off too systematically wrong for
every patient treated on that machine.
Precisely. It's the absolute bedrock, you
could say, of clinical dissymmetry, and establishing
that precise baseline involves this critical process called
calibration. Calibration.
Right, it's how we figure out the absolute dose output of the
(00:43):
Linux, basically linking the machine's own unit, the monitor
unit or MU to a known absolute absorbed dose.
Exactly. So today let's let's unpack
that. Let's really get into how Linux
are calibrated. We're going to dive into this
standard protocol used here in the United States.
And that standard, the one that's really become the gold
standard, you know, is the American Association of
(01:04):
Physicists in Medicine's Task Group 51 report.
TG51 everyone just called it TG 51.
TG51. That's the one.
OK, PG 51. Let's settle in and explore what
this actually involves. And we've talked before about
how Linux make these high energyphoton and electron beams and
that we measure the radiation dose using detectors, usually
(01:25):
ionization chamber. Right, those precision
instruments. So calibration, specifically
following TG 51 is that crucial step.
It connects the number of Mus wedial up on the machine to a
precise, absolute absorbed dose,but it has to be under very
specific, repeatable conditions.Think of it like meticulously,
(01:45):
probably braiding a really sensitive scientific instrument.
You absolutely need to establishits response against a known
absolute standard and in a controlled environment.
So with the linac, we need to becompletely certain that when we
tell it, say, deliver 100 Mus, the machine gives a predictable
and really precise amount of absorbed dose, maybe that's 100
(02:09):
centigrade right at a very specific point inside a water
phantom. And that verified relationship,
the MUS to dose, that's the foundation.
It's the essential starting point for every single
calculation you then do for every patient treatment.
That analogy of calibrating a critical instrument that really
fits TG 51 then is like the instruction manual, the recipe
(02:30):
for getting that MU to dose relationship defined precisely.
Precisely. And the TG51 protocol which came
out from the AAP PM back in 1999. 1999 OK.
It really represented a significant step forward, a kind
of paradigm shift actually in how we approach Linux
calibration compared to the older ways.
A shift. How is it different?
Well, the most crucial change and honestly the defining
(02:52):
feature of TG 51 is that it's based directly on measurements
of absorbed dose to water. Absorbed dose to water.
OK. This was a big departure from
earlier protocols like the AAP Ms. own TG21 protocol.
That one was from 1983. TG21 right?
I remember that one. In TG21 it was based on
measuring exposure in air and then you needed conversion
(03:15):
factors, these things called F factors, to translate that air
measurement into dose absorbed in a medium like water or
tissue. Ah.
OK. So TG 21 measured in air, then
calculated to get dose in tissue.
Yeah, but TG51 measures dose directly in something that acts
like tissue. Exactly, you got it.
(03:35):
TG51's direct absorbed dose to water basis is, well, it's a
major point, something you absolutely have to remember if
you're studying this stuff. Absorbed dose to water got.
It it simplifies the whole thingconceptually and it aligns much
more directly with what we actually care about delivering
to the patient's tissue absorbeddose.
Because the human body is mostlywater, essentially.
Right. In terms of how it interacts
(03:56):
with high energy radiation, the body is largely water
equivalent. So measuring directly in water
just makes more sense. It's inherently more relevant.
OK. Absorbed dose to water as the
foundation, that's a huge take away.
What are some other like fundamental characteristics or
requirements of TG51? OK, let's walk through the key
features. We've hit the first one, the
measurement medium. For that primary absolute
(04:18):
calibration, the protocol says measurements must be done in a
water phantom. It must be water, not air, not
those solid plastics. And the specific step, the
absolute calibration, No, you must use water.
And the reason, again, for insisting on water?
Well there are several really good reasons.
Like we said, water is the most relevant material to human
(04:39):
tissue for these mega voltage beams.
It's radiation interaction properties.
You know, things like it's density, stopping power, how it
scatters radiation. They're extremely well known and
stable, reliable, very reliable.Plus, water is every where.
It's relatively cheap, and it lets you position the ionization
chamber very, very precisely. You can put it exactly where you
(05:00):
need it, at specific depths, specific locations,
reproducibly. So it provides a standard
consistent environment for the measurement.
Exactly. So just a hammer at home,
absorbed dose to water is the basis and water is the required
medium for the actual measurement.
That level of detail and reproducibility, it sounds
(05:20):
absolutely crucial for making ita standard.
It is, which brings us to the second key, feature,
traceability. Traceability meaning.
TG51 relies on a system where the ionization chamber you use
in your clinic for calibration, well, it has to be calibrated
itself against a higher standard.
OK, so my tumor needs calibrating too.
Right. And this calibration is done by
(05:41):
an accredited DOS Symmetry Calibration Laboratory or an
AECO, OK. And these labs are accredited
specifically because their calibrations are traceable.
They link back to national primary standards of absorbed
dose, like the ones maintained by NIST, the National Institute
of Standards and Technology herein the US.
Ah. So there's a chain from the
national lab like NIST down to the ADCL and then down to my
(06:04):
specific chamber that I use in the clinic.
Exactly a direct chain of traceability.
The ADCL gives you a calibrationcertificate specifically for
your chamber and the key piece of information on there is the
absorb dose to water calibrationfactor, but specifically for a
cobalt 60 gamma ray beam. Cobalt.
And this factor has a very specific notation in TG51.
(06:27):
It's N sub D comma W for cobalt 60.
N sub D comma W cobalt 60. OK, let's break that down again
just to be sure. N is the calibration factor,
right? D means it's for absorbed dose.
The subscript WT means absorb dose to water.
Yep, and the Cobalt 60 subscripttells you the type of being used
at the ADCL for that calibration.
(06:48):
Perfect. You nailed the notation.
Yeah. And what that number actually
represents that N sub D comma W for cobalt 60 value, it's the
absorbed dose to water, usually in Gray or centigrade that
corresponds to 1 unit of corrected reading from your
specific ionization chamber whenit's placed under reference
conditions in the 80 CLS cobalt 60 beam.
So it connects my chambers reading to the real dose based
(07:11):
on that cobalt 60 standard. Yeah, exactly.
It's how the absolute accuracy of the national standard gets
transferred to your working clinical chamber.
It basically turns your chamber into a traceable transfer
standard. Got it.
That factor is like the bridge. It links my chamber signal back
to the fundamental definition ofdose.
All referenced back to NIST via that Cobalt 60 beam at the ADCL.
(07:34):
OK, what else does TG51 cover? Key feature #3 What it applies
to The protocol gives you detailed procedures and all the
factors needed for calibrating both high energy photon beams
and electron beams that come from our clinical Linux.
Photons and electrons good. Right.
These are the 2 main types of beams we use in external beam
(07:55):
therapy. So having one unified protocol
for both is, you know, very practical.
So it's the go to standard for the beams we use every day?
Yeah. What about the final
foundational feature? Frequency of recalibration.
This is important. Keep that traceability chain
intact and make sure you're measurements stay accurate
overtime. TG51 requires that your
ionization chamber and the electrometer you use with it
(08:16):
must be recalibrated by an ADCL at least every two years.
Every two years. OK, so a regular checkup to make
sure my transfer standard, my chamber system hasn't drifted or
changed. Exactly.
Keeps everything tied back to the primary standard accurately.
So just to quickly recap the foundations, TG 51 based on
absorbed dose to water measurements must be in water
(08:37):
accuracy traceable via an ADCL'sN sub D comma.
W factor for cobalt 60 covers both photons and electrons and
recalibration every two years. Those are the big rules, the
nonnegotiables. You got it.
Perfect setup. OK, let's shift gears now into
the actual how to How do we actually perform the photon beam
calibration using TG51? What does the setup look like?
(09:00):
OK, so you start by carefully setting up your calibrated ion
chamber and electrometer system,and this goes into a large water
phantom 1 where you can precisely control the
conditions. Like those big tanks we see?
Exactly. Usually the chamber is mounted
on a scanning system. This lets you position it really
accurately in three dimensions, Up, down, left, right, in, out.
And I assume there are very specific reference conditions
(09:22):
needed for this measurement. Can't just stick it anywhere.
Absolutely not. The protocol defines these very
rigidly. That's crucial for
reproducibility. The standard depth of
measurement in the water is fixed at 10 centimeters deep. 10
centimeters, OK. The field size is also standard
10cm by 10 centimeters. Now this field size is defined
(09:42):
either at that measurement depth, the 10 centimeters.
If you're using what's called a source to surface distance, or
SSD setup, that's where the water surface is 100 centimeters
from the source. SSD setup 100 centimeters to the
surface. Or the 10 by 10 field size can
be defined at the ISO center if you're using a source to axis
distance or SAD setup. In the SAD case, the chamber
(10:05):
itself is placed at 100 centimeters from the source,
which is the ISIS center. The 10 by 10 field is defined
there at ISIS center. So either way, it's basically a
10 by 10 field at 100 centimeterdistance related point and the
measurement depth is 10 centimeters deep in water.
Standard field, standard depth, standard distance.
Precisely these specific conditions were chosen
(10:27):
carefully. 10 centimeters deep is generally deep enough in the
water to get what's called charged particle equilibrium for
typical linac photon energies. Right where the electrons are
steady. Exactly.
But it's also still practical for doing routine measurements.
So once your setup is perfect, chamber at 10 centimeter depth,
correct distance, 10 by 10 field, you deliver a known
(10:47):
number of Mus from the linac, typically something like 100 or
200 MUS, and you record the raw reading, the charge or current
from your electrometer, let's call that raw reading M raw, M
raw. And as we've definitely talked
about before, that raw reading isn't the final answer.
It needs correcting, right? We can't just use that raw
signal straight up. That's absolutely right.
(11:08):
You're ahead of the game. To get the true signal, the one
that really corresponds to the ionization produced just in the
chamber sensitive air cavity, you have to apply several
correction factors to that M raw.
OK. What kind of Corrections?
Well, there's the correction fortemperature and pressure,
usually written as P sub TP. That normalizes your reading to
(11:30):
standard at atmospheric conditions because air density
affects the reading. Right, PTP.
Got it. Then there's the correction for
Ion Recombination, P sub ion. This accounts for the fact that
not all the ions created by the radiation actually get collected
by the chamber's voltage. Some recombine before they're
measured. There's also a correction for
polarity effects P subpole. Sometimes the chamber reads
(11:54):
slightly differently depending on whether the collecting
voltage is positive or negative.This corrects for that people,
and if your electrometer itself has its own calibration factor
separate from the chamber, theremight be AP sub elect factor
too. Applying all of those
corrections to MROG gives you the fully corrected chamber
reading, and in the TT 51 world we just call that clean
(12:15):
corrected reading. MMM.
Simple enough. So M is the clean signal after
we've accounted for the environment and how the chamber
isn't perfectly ideal. Now the $1,000,000 question, how
do we turn that corrected reading M into the actual
absolute absorb dose to water atthat reference point?
OK, here's where the core TG51 photon dose formula comes in.
(12:37):
It's actually pretty elegant theabsorb dose to water which we
write as D sub WD. Sub W dose to water.
For your specific clinical photon beam which has a certain
quality Q measured at that reference depth of 10
centimeters in water is calculated like this D sub W = M
* K sub Q * N D comma W for cobalt 60.
(12:59):
OK, say that again. D sub W = M * K sub Q times NDW
for cobalt 60. That's the one.
All right. Let's break that down.
M is our fully corrected reading.
We just talked about N sub D comma W for cobalt 60.
Is that ADCL calibration factor we got earlier that tells us how
our chamber reads in a cobalt 60beam for a known dose to water?
(13:19):
Exactly. It's your traceable link back to
the standard, but remember it's specific to cobalt 60 energy.
Right, which means K sub Q must be the missing piece, the beam
quality conversion factor. Bingo, that K sub Q factor is
the absolutely crucial piece here.
Its whole job is to take that N sub D comma W factor, which
(13:40):
remember is only strictly valid in a cobalt 60 beam, and adjust
it or convert it so that it becomes applicable to your
specific clinical photon beam which has a different energy
spectrum or quality queue. OK, why is that adjustment
needed? Doesn't my chamber just read
dose? Shouldn't like 1 Gray be 1 Gray
no matter the beam energy? That's a really common question,
(14:01):
and it would be nice if it were that simple.
But no, an ionization chamber doesn't respond exactly the same
way to beams of different energySpectra, even if the absorbed
dose delivered to the water at that point is identical.
Why not? Well, the chambers response
depends on a bunch of complex physics.
Things like how the radiation interacts in the chamber wall
material, the type of gas in thecavity, usually air, how
(14:22):
secondary electrons are generated and where they deposit
their energy. All these things are somewhat
energy dependent. A cobalt 60 beam has a very
specific gamma ray spectrum, around 1.25 MEVI on average.
A high energy linac photon beam is different.
It's a Bremstrelung spectrum created in the target then
shaped by a flattening filter. It's a much broader range of
(14:45):
energy. So the different energy mix
effects how the chamber sees thedose.
Exactly. The K sub Q factor accounts for
that difference. It corrects for the change in
your specific ionization chamber's response because of
the spectral differences betweenthe reference cobalt 60 beam and
your clinical beam. Q.
So K sub Q is basically answering the question, OK, my
(15:05):
chamber reads this much dose in cobalt 60, how much differently
will it read in this particular linac beam for the exact same
actual dose delivered to the water And K sub Q is that
correctional multiplier? That's a perfect way to
conceptualize it conceptually. K sub Q answers.
How differently does my specificionization chamber model respond
to this clinical beam quality cue compared to how it responded
(15:29):
back to ADCL and the Cobalt 60 reference beam?
And how is it derived? Is it measured or calculated?
It's derived using pretty complex physics calculations.
These involve things like ratiosof restricted stopping, powers
of water to air, corrections forthe chamber wall, material and
thickness, central electrode effects, and other energy
dependent factors. It's all calculated relative to
(15:50):
the response in cobalt 60. TG 51 provides tables of these
calculated K sub Q values. But how do I know which K sub Q
value to use from the table? How do I determine my clinical
beams quality cue? Good question.
In TG51 for photon beams, the quality cue is specified by a
single measured parameter. It's the percentage depth dose
(16:12):
at 10 centimeters depth. But, and this is important, it's
specifically for the photon component only.
Photon component only. What does that mean?
Well, especially for higher energy linac beams, when the
beam first enters the water, there's some electron
contamination mixed in with the photons near the surface.
Those surface electrons. Right.
For the beam quality specifier, we want a measure that reflects
(16:32):
just the photon energy spectrum,not the contaminating electrons.
So we need the PDD at 10 centimeters depth.
That's due only to the photons. This is usually approximated by
measuring the PDD curve and thenusing specific methods to
estimate and subtract the electron contribution at
shallower depths, or by using a thin lead foil to filter out
(16:53):
electrons during the measurement.
OK. So it's not just the raw PDD
measurement. Not quite.
It's often denoted as PDD of 10 sub X, where that little X
explicitly means photons only. PD of 10 sub X Yeah.
And how do I actually get that value for my Beam?
You have to measure the central axis percentage depth dose curve
for your specific photon beam energy and field size.
(17:15):
The reference 10 by 10100 SSD. Then you use established
techniques like analyzing the shape of the curve near the
surface or using that lead foil method I mentioned to figure out
the PDD value at 10 centimeters depth after removing the
influence of contaminant electrons.
It's a really crucial measurement to get right.
OK, sounds like a careful measurement is needed.
Once I have my PDD of 10 sub X value, what then?
(17:39):
Once you have that number for your Beam, you go to the
extensive tables provided in theTG51 protocol document itself.
These tables list K sub Q valuesfor many common models of
ionization chambers across a whole range of PDD of 10 sub X
values. So I find my chamber model in
the table. Exactly.
You find the row or section corresponding to your specific
chamber model. Then you find the PDD of 10 sub
(18:01):
X value you measured. You might need to interpolate
between list of values and read off the corresponding K sub Q
factor. OK, so measure PDD of 10 sub X
for my beam. Look up my specific chamber
model in the TG 51 tables. Find the K sub Q for my measured
PDD 10X. Then I plug that K sub Q along
with my corrected reading M and my ADCL factor NDW for cobalt 60
(18:25):
into the formula dose in KQNDWCO60 and boom.
That gives me the absolute absorb dose at 10 centimeters
depth in water for the musi delivered.
That is the standard TG51 photoncalibration process in a
nutshell. You got it.
Phew. OK, hey, that feels logical.
And just one quick but importantdetail about positioning the
(18:46):
chamber, which he sort of touched on earlier.
It's about the effective point of measurement.
Right where the measurement is actually seen by the chamber.
Exactly, for the standard cylindrical ionization chambers
like the farmer type chambers, as most clinics use for this,
you don't actually place the geometric center of the chamber
right at the 10 centimeter reference depth.
Because of how the air cavity affects the electron paths, the
(19:07):
chamber effectively measures thedose slightly upstream from its
physical center. TG51 specifies that for the
cylindrical chambers you need toposition the chamber so that
this effective point of measurement is at the 10
centimeter depth. And where's that effective
point? It's calculated as 0.6 times the
radius of the chamber's air cavity, and that distance is
(19:27):
measured upstream towards the beam source from the chamber's
central axis. 0.6 times the cavity radius upstream.
OK, so I need to know my chamber's radius and shift it
slightly downstream so that point hits 10 centimeters.
Precisely. It's a small shift, but
important for accuracy. Now, if you were using a
parallel plate chamber for photons, which isn't typical for
(19:49):
routine calibration but possible, then the reference
point is different. For parallel plate chambers, you
place the inside surface of the proximal plate.
That's the upstream plate, the one the beam hits first, right
at the reference step. 10 centimeters for photons.
Great, different rules for different chamber shapes makes
sense. It's about accounting for where
the ionization is actually beingcollected and averaged.
(20:10):
Those little details, they really matter for getting the
absolute dose right. They absolutely do.
Precision is key here. OK, photon calibration seems
pretty clear now. Reference conditions corrected
reading M and DW from the ADCL and the crucial K sub Q factor
based on PDD of 10 sub X. What about electron beam
calibration using TG51? Is it like basically the same
(20:33):
process? Well, it shares the same
fundamental goal, determine the absolute dose to water under
reference conditions. But the specifics of the setup
and especially the factors needed in the dose formula are
quite different. This reflects the unique physics
of how electrons interact compared to photons.
Different physics, different rules.
Yeah, OK. How does the setup differ?
The setup itself is still done in a water phantom that parts
(20:55):
the same. You're still measuring absorbed
dose to water and you typically use a standard electron
applicator or cone attached to the neck head.
You know, like a 10 by 10cm or maybe a 15 by 15 centimeter cone
size. Right, the cones that shape the
electron field. Exactly.
And the distance is also usuallystandard, typically a 100
centimeter SSD source to surfacedistance.
(21:17):
OK, so Stillwater standard cone size, standard distance sounds
similar so far. What's the first big difference
compared to photons then? The reference step we call it D
sub ref for electrons, unlike photons where it's always 10
centimeters for electrons the reference depth is not fixed.
It changes depending on the energy of the electron beam.
(21:37):
OK, so the depth we measure at depends on the electron energy.
How do we figure out what depth to use for a specific energy?
It's determined based on the electron beam quality specifier
used in TG51. Just like PDD of 10 sub X
specifies photon quality. For electrons, the quality
specifier is R. 50R50 What does that stand for?
(21:58):
R50 is defined as the depth in water measured along the beam
central axis where the absorbed dose has fallen to 50% of its
maximum value, D Max R50. Yeah, the depth where the dose
is 50% of the maximum dose. OK so to find R50I have to
measure show the electron beams central axis depth dose curve
first. Exactly.
(22:18):
You do a depth scan with your chamber, usually in the water
tank, Find the peak dose depth DMax, find the dose value that's
half of that peak, and then findthe depth where the dose drops
to that 50% level. That depth is your R50 value.
Got it. Measure the PDD curve.
Find D Max, find 50% of D Max. Find the depth for that 50%
dose. That's R50.
Precisely. And R50 is really important
(22:40):
because it does two things. First, it tells you the quality
of the electron beam. Second, once you have R50 for
your beam, you use it to calculate the reference depth D
sub ref using a specific formulagiven in TG51.
OK, what's the formula for D subref?
The formula is D sub ref equals ( 0.6 times R 50 ) -, 0.1
(23:00):
centimeters. D sub ref equals OK 0.6 * R
fifty. Then subtract 0.1 centimeters.
So the depth we actually calibrate at is directly
calculated from that R50 value. That's right, the reference
depth is linked directly to how deeply the electrons penetrate,
which is what R50 measures. And why that specific depth?
Why .650 point 1 centimeters? Good question.
(23:24):
This depth D sub ref is chosen very deliberately.
It always lies on the steep falling portion of the electron
depth dose curve, significantly deeper than the depth of maximum
dose D Max. OK, on the downward slope after
the peak, why there? Measuring in this region of
steep dose fall off provides better sensitivity to the
electron beams energy and spectrum.
(23:46):
Small changes in electron energywill cause more noticeable
shifts in the dose at D sub ref compared to measuring near the
surface or right at the D Max peak where the curve is flatter.
So it's a more sensitive spot tocalibrate, reflecting the beam
energy better. Makes sense calibrating where
the dose is changing predictablybased on energy.
OK, variable depth based on R50.That's a key difference.
What's another critical difference for electron
(24:07):
calibration compared to photons?The choice of ionization
chamber. This is much more critical for
electrons, and in some cases theprotocol dictates exactly what
you must use. This is probably key difference
#2 for electrons. Mandatory chamber type outs.
For low energy electron beams, and TG51 defines this
specifically as beams with nominal energies of about 6 mega
(24:30):
electron volts or less. Or more precisely, beams with an
R50 value of 2.6 centimeters or less.
R50 less than or equal to 2.6 centimeters.
OK, low energy. For those beams you must use a
parallel plate type ionization chamber, not a cylindrical 1.
Must use a parallel plate. Why is such a strict rule there?
Why can't I use my usual farmer chamber?
(24:52):
Because for these low energy electrons, the dose changes very
rapidly with depth, especially near the surface and around D
Max, the dose gradients are incredibly steep.
OK. Steep fall off.
Right cylindrical chambers like the Farmer type have a
relatively large air cavity volume compared to these rapid
changes. When you put that cylindrical
chamber into these shallow depths with steep gradients, it
(25:16):
does 2 bad things. First, it significantly perturbs
the electron field. It displaces water, causes
scattering changes. Second, it averages the dose
over its entire volume. But the dose isn't uniform over
that volume because the gradientis so steep.
Exactly. It averages the dose in a region
where the dose is changing really fast.
This leads to inaccurate measurements.
(25:37):
You're not measuring the dose ata specific point anymore.
OK, so for low energy electrons,the cylindrical chambers
basically mess up the bean they're trying to measure and
give an inaccurate average reading.
Got it. Whereas parallel plate chambers
are designed differently. They have a very thin entrance
window and a shallow disc shapedcavity.
This design minimizes how much they disturb the electron field
(25:59):
and they measured those much closer to a single plane.
So they give a much more accurate measurement in those
tricky low energy, steep gradient situations.
Makes perfect sense. So low energy electrons, R50 2.6
centimeters, almost mandatory parallel plate chamber.
What if I have a higher energy electron beam, say 12 maybe or
18 maybe, where R50 is much larger?
(26:22):
Good question. For higher energy electron beams
where R50 is greater than 2.6 centimeters, you are allowed to
use a cylindrical chamber like your farmer chamber.
OK, so I can use it then. You can, but there's a catch.
Using a cylindrical chamber for any electron beam calibration,
regardless of the energy, means you have to include an
(26:42):
additional correction factor in the dose calculation formula.
Another correction factor just for using the cylindrical
chamber. Yep, which makes the electron
dose formula inherently a bit more complex in the photon
formula, especially if you opt for the cylindrical chamber
more. Factors.
OK, I'm braced. Hit me with the full electron
dose formula from TG51. All right, here it is the
absorbed dose to water D sub W for an electron beam, a quality
(27:05):
Q which we now know is specifiedby R50 measured at the reference
depth D sub ref is calculated asD sub W = M.
Corrected reading M, Got it. Yeah, multiplied by P
subgradient for quality QP. Subgradient.
OK, new one. Multiplied by K prime sub R50.
K Prime sub R50 another new one.Multiplied by K sub.
Equal yet another new one. Multiplied by N sub D comma W
(27:28):
for cobalt 60. OK, whoa, deep breath.
Let's recap that monster D sub W= M * P sub gradient times K
prime sub R 50 * K sub equal times NDW for cobalt 60.
That's the beast. OK.
We know M is the fully correctedreading from the electrometer,
right? And we know N sub D comma W for
(27:50):
Cobalt 60 is our trusty ADCL calibration factor linking back
to the cobalt 60 standard. Correct, same factor used for
photons. So let's tackle new ones.
What is K sub equal? OK, K sub equal.
This is called the photon to electron conversion factor, and
importantly, it's specific to your ionization chamber model.
Photon to electron conversion. Wait, what does it convert?
(28:11):
Remember that your main ADCL calibration factor N sub D comma
W was determined in a cobalt 60 photon beam.
K sub equals job is basically toconvert that photon beam
calibration factor into an effective calibration factor
that's appropriate for use in electron beams.
It accounts for the fundamental differences in how the chamber
responds to high energy photons versus electrons.
(28:34):
OK, so it bridges the gap between the Co 60 photon
calibration and using the chamber in an electron beam.
Precisely. And you find this factor K sub
equal in the TG 51 tables. It's listed for specific chamber
models. Look up K sub equal for my
chamber in the tables. Got it.
Next K prime sub R50 What's that?
K prime sub R50. This is the electron quality
(28:56):
conversion factor. It plays a role similar to K sub
Q for photons. OK, so it corrects for the
specific bean quality. Exactly.
It adjusts the calibration to account for the specific
electron bean quality, which as we established, is defined by
your measured R50 value. It corrects for the fact that
the chambers response will vary slightly depending on the
electron energy spectrum represented by R50.
(29:17):
So just like KQ depends on PDD 10 yeah X for photons, KR50
depends on R50 for electrons. You got it.
And again you look up this K prime sub R50 factor in the TG
51 tables. The value will depend on your
measured R50 and also on the type of chamber you are using,
whether it's a cylindrical or a parallel plate chamber.
Look up K prime R 50 based on myR50 value and my chamber type
(29:41):
makes sense. And finally the last new factor
P sub gradient. P sub gradient.
This is the gradient correction factor.
Now listen closely here. This factor is only needed and
only applied if you are using a cylindrical ionization chamber
for your electron beam calibration.
Only for cylindrical. Chamber only for cylindrical.
(30:02):
If you followed the rule and areusing a parallel plate chamber,
either because you had to for low energy or chose to for high
energy, then P sub gradient is simply equal to 1.0.
No correction needed from this factor.
OK, P gradient equal 1.0 for parallel plate chambers.
Why is it needed only for cylindrical ones?
Remember how we determine the electron reference depth D sub
(30:23):
ref. Yeah, it's calculated from R50
and it lies on the steep fallingpart of the depth dose curve.
Exactly. It's in a region of high dose
gradient. That means the dose is changing
rapidly with just small changes in depth, right.
A cylindrical chamber, as we discussed has a significant
physical size, especially along the beam direction.
It's reading represents an average of the dose deposited
(30:44):
across its entire air cavity volume.
OK, but if the dose is changing rapidly across that volume
because you're in a steep gradient, the average dose the
chamber reads isn't necessarily the same as the dose at the
precise single point defined as D sub breath, which is related
to the chamber center adjusted for effective point of
measurement. It's that averaging problem
(31:06):
again, like trying to measure the exact speed of a car when
it's rapidly accelerating or decelerating using a speedometer
that takes a few seconds to update the reading lags behind
the instantaneous value. That's a fantastic analogy.
That's exactly the issue. P sub gradient corrects for this
averaging effect. It accounts for the fact that
the cylindrical chamber is smearing out the dose
measurement in a non uniform steep dose gradient field.
(31:30):
Its value depends on the steepness of the gradient, which
relates to R50 and the specific geometry, mainly the radius of
your cylindrical chamber model. OK.
And parallel plate chambers don't need it because.
Because they're sensitive volumeis very thin along the beam
direction. They essentially measured dose
much closer to a single plane. The inside surface of the front
(31:50):
window is the reference point, so they don't suffer from that
volume averaging problem in the gradient nearly as much.
Hence P sub gradient is just onepoint O for them.
Got it. So electrons need more factors.
Potentially there's K sub equal to convert the photon
calibration to electrons, there's K prime sub R50 to
adjust for the specific electronenergy based on R50.
(32:11):
And if you use the cylindrical chamber, there's piece of
gradient to correct for dose averaging in the steep fall off
region at D sub ref. You've summarized it perfectly.
It is definitely a bit more involved than the photon side,
but each of those factors is there to address a specific
physical reality about measuringelectron beams accurately with
an ionization chamber calibratedin a cobalt 60 photon beam.
(32:33):
OK, so once you've done all thatset up correctly, taken the raw
reading M, applied all the environmental and chamber
corrections to get M, identifiedall the necessary TG 51 factors,
KQ for photons or key call KR50,and maybe P grading for
electrons. Looked them up in the tables and
plugged everything into the correct formula.
(32:54):
You finally get that D sub W theabsolute absorb dose to water
delivered by your linac for the number of musu set under those
very specific reference conditions.
That's the end point of the TG51calculation itself.
You've determined the absolute output under reference
conditions. For example, if I delivered say
100 Mus for my measurement, and my whole TG51 calculation spit
(33:14):
out a dose of maybe 100.5 centigrade at the reference
point, then my Linux output under those reference conditions
is just that dose divided by theMUS.
So 100.5 centigrade divided by 100 MUS equals 1.005 centigrade
per MU. Exactly, you've nailed it.
That centigrade per MU number isthe crucial output figure
(33:37):
derived from this entire meticulous process.
That number, That specific centigrade per MU value, that's
kind of the Holy Grail of this whole calibration, isn't it?
That's the number that everything else in patient dose
calculation is going to rely. On it truly is the absolute
foundation, this calibrated doseper MU value.
Sometimes it gets adjusted slightly to represent the dose
(33:59):
at D Max using a PDD or TMR measurement, but it's still
based on this absolute value. This is the starting point for
calculating the MUS needed for every single patient treatment.
Every single one. Every single one.
When you're planning a patient treatment, you take this
fundamental baseline output, this dose per MU, and then you
apply all the other factors thataccount for the specifics of
(34:20):
that patient's setup. Oh, things like output factors
for different field sizes or shapes, depth dose factors like
PDD's or TMR's to get the dose correct at the actual tumor
depth, the inverse square factorto account for any non standard
distance. Maybe wedge factors, tray
factors, the list goes on. Right.
All those relative factors we use in planning.
(34:41):
Exactly. But all those factors are
relative. They modify the dose relative to
that absolute dose per MU that you established so carefully
through the TG51 calibration. If that absolute starting point
is wrong, all the relative adjustments won't fix it.
OK, let's really hammer this home then.
Yeah. Why is this calibration process
following TG 51 so rigorously? Why is it not just, you know,
(35:04):
some physics busy work, but absolutely fundamental to safe
and effective radiation therapy?Why is getting this single
number right so mission critical?
Well, the number one reason without a doubt is dose accuracy
for the patient. Dose accuracy.
As we said right at the start, any error you make in this
initial absolute calibration, itdoesn't just stay there, it
propagates directly through every single patient treatment
(35:26):
performed on that linac afterwards.
Every single one. Every single one.
If your calibration is off by let's say, 3% high, then every
single dose delivered to every patient on that machine will be
systematically 3% higher than intended.
And for many treatments, a 3% error or maybe 5% can have real
(35:47):
clinical consequences. It could potentially impact
tumor control rates if you're consistently underdosing, or it
could significantly increase therisk of serious side effects
normal tissue toxicity if you'reconsistently overdosing.
And there have been cases where calibration errors have actually
caused harm. Sadly, yes.
There have been documented incident, serious incidents,
(36:08):
where patient harm resulted directly from systematic errors
in linac calibration. It underscores how critical this
is. So a small percentage error in
this one number, mirrored under ideal conditions in a tank of
water, translates directly into a potential problem, a risk for
every single patient, every single day that machine is used.
It absolutely does. That's the weight of it.
Second big reason, Consistency. Consistency.
(36:30):
Using a standardized, widely accepted protocol like TG51
ensures there's a high degree ofconsistency in dosimetry between
different Linux, even different models from different
manufacturers, and crucially, between different clinics and
hospitals. So everyone is speaking the same
dose language. Essentially, yes.
If a patient is prescribed, say 200 centigrade per fraction,
(36:50):
following a standard like TG51 gives us confidence that 200
centigrade delivered by machine A over at institution X is truly
equivalent to 200 centigrade delivered by machine B across
the country at institution Y. And that's.
It's vital for things like clinical trials, right?
Comparing results. Absolutely crucial for multi
institutional clinical trials. You can't compare patient
(37:12):
outcomes if you're not absolutely sure the dose
delivery was standardized and accurate across all
participating sensors. TG51 provides that essential
common language and baseline fordose.
OK. Dose, accuracy, consistency,
What else? 3rd, I'd say understanding
limitations and uncertainties byactually going through the TG51
(37:33):
process meticulously by understanding each correction
factor, where it comes from, thephysics behind it, and all the
required measurement conditions.Like the specific positioning
rules or needing that parallel plate chamber for low energy
electrons. Exactly.
By understanding all that detail, you as the medical
physicist gain a much deeper understanding of the whole
(37:55):
process and, importantly, the potential sources of error or
uncertainty within it. Knowing where things could
potentially go wrong. Right.
And that knowledge is vital. It helps you evaluate your own
measurements, critically troubleshoot if something looks
weird, and make sure your ongoing quality assurance
program is robust enough to catch any potential problems
before they could ever impact a patient.
(38:17):
It informs best practices. Makes sense and #4.
And 4th, it's a cornerstone of commissioning new equipment.
Commissioning. OK, Setting up a new machine?
Yeah. When a brand new Linnec is
installed in a clinic, performing the TG51 absolute
calibration is one of the very first, most fundamental
measurements you do. You simply cannot confidently
(38:38):
collect all the other beam data needed for the treatment
planning system. You know the output factors,
depth, dose curves, beam profiles, all that stuff.
Or even think about starting to treat patients until you have
accurately established that absolute dose per MU output
under the TG51 reference conditions.
It's the anchor point. Absolute starting point for
(38:58):
verifying the machine and getting it ready for clinical.
Use exactly It's the cornerstoneupon which all the other beam
data measurements and all subsequent clinical calculations
are built. OK, so let's try to wrap that
importance up. Accurate TG51 calibration gives
us that absolutely essential traceable dose per MU number.
It's done under specific ideal reference conditions for both
(39:21):
photons and electrons. It uses a properly calibrated
chamber from an ADCL and it applies specific factors KQ,
kcal, KR50P gradient to account for beam quality and chamber
response based on measured parameters like PDD 10X and R50
and that final output number. It's the absolute linchpin for
(39:41):
calculating every single dose delivered to every single
patient. Couldn't have said it better
myself. It is absolutely not an
exaggeration to say that performing this calibration
correctly is arguably the singlemost critical measurement that a
clinical medical physicist performs.
You know, it really feels like in physics, especially medical
physics, everything gets described as absolutely
essential or critical or foundational or vital doesn't.
(40:02):
It it's true, it does seem that way sometimes.
Maybe it's because honestly, in this field, if just one link in
that chain of measurements and calculations is weak, the whole
thing can potentially fall apart, particularly when patient
safety is writing on it. Yeah, you can almost hear the
the board examiners or the review committees asking now
tell me, was this absolutely foundational, critical, vital
step performed correctly and documented meticulously?
(40:25):
Exactly, makes you sweat just thinking about it.
Speaking of board exams and review committees and just
making sure these truly vital concepts are locked in.
Maybe we should do a quick boardreview blitz.
Just reinforce some of these absolute must remember points
from TG51. Perfect idea.
Let's do some rapid fire questions.
Yeah, good practice. OK.
Question number one, according to the TG51 protocol, what is
(40:48):
the required measurement medium for performing the absolute
absorbed dose calibration of a clinical linear accelerator?
OK, medium, That has to be water.
TG 51 is fundamentally an absorbed dose to water protocol.
That was the big shift from TD 21's air karma basis.
So the absolute calibration measurement itself must be
performed in a water phantom. Correct water.
(41:09):
It is question 2 for photon beamcalibration under TG51.
How is the beam quality conversion factor K sub Q
determined? Specifically, what measured
parameter is K sub Q based on OKphoton KQ?
That depends on the beam qualityspecifier, which is PDD of 10
sub X. That's the percent of depth
those at 10 centimeters depth, but for the photon component
(41:32):
only with electronic contamination removed or
accounted for. You measure that PDD 10X value
for your beam and then use it tolook up the correct K sub Q
value for your specific ion chamber model in the TG51 table.
Absolutely right. PDD of 10 sub X characterizes
the photon energy spectrums penetration and that dictates
the K sub Q. Excellent question three.
(41:52):
According to TG51, what is the formula used to determine the
reference depth D sub ref for electron beam calibration?
The electron depth formula, OK, in AD sub ref equals, let me
think, 0.6 times R 50 ) -, 0.1 centimeters.
Yeah, D sub ref equals 0.6 R 50.1cm and it scales to the
electron beam quality R50, whichitself is the depth of the 50%
(42:15):
dose level on the central axis PD curve.
Nicely recalled that formula is key.
And finally question #4 this is a practical application a
medical physicist is setting up to calibrate A6 mega electron
Volt electron beam. They perform a measurement and
determine that the R50 for this particular beam is 2.3
centimeters. According to the recommendations
(42:38):
in TG51 regarding chamber selection, which type of
ionization chamber should they absolutely use for this
calibration measurement? OK, six MEVI electrons R50 is
2.3 centimeters. That R50 value 2.3 centimeters
is less than the threshold of 2.6 centimeters that TG 51 sets
for low energy electrons. Therefore, the protocol requires
(42:58):
him to use a parallel plate ionization chamber.
A cylindrical chamber is not appropriate here because of
perturbation effects in the steep gradients of this low
energy beam. Perfect reasoning and correct
answer. Parallel plate is mandatory in
that situation. Those are definitely some high
yield points to have absolutely solidified, whether you're
studying for boards or just ensuring robust and accurate
(43:18):
clinical practice day-to-day. Yeah, it's a complex protocol,
definitely have a lot of details, but getting these
foundational elements, the medium, the traceability, the
reference conditions, the quality specifiers, the factors,
the Chamber rules, getting thosereally internalized feels
critical. It truly is.
So we spent this time really digging into how TG51 gives us
that crucial single number, the dose per MU under very specific
(43:42):
idealized reference conditions. We got the Photon formula M
times KQ times NDW and the Utronformula which is a bit longer, M
* P gradient if needed, times K or 50 times Kegel times NDW, all
starting from that traceable NDWcobalt 60 factor from the ADCL
and using measured beam quality specifiers like PDD 10X and R50.
(44:02):
That output number is our absolute starting point, our
anchor. That baseline dose per MU under
reference conditions is precisely what this whole
process characterizes. It's the foundation.
But here's something to really chew on as we wrap up.
Maybe a final thought. TG51 gives us this incredibly
precise number, right? The dose per MU, but it gives it
to us at one specific point in space, 10 centimeters deep for
(44:26):
photons or D sub ref for electrons, inside a perfectly
uniform idealized tank of water for a simple 10 by 10 square
field. Right, under very controlled,
very specific conditions. So how do we as medical
physicists take that one number,determine under such ideal
conditions, and then confidentlyuse it to calculate the actual
monitor units needed to treat a tumor that's, you know, buried
(44:48):
deep inside a patient's unique, complex body?
Have the $1,000,000 question that follows calibration.
Yeah, that tumor isn't always at10 centimeters depth or AD sub
ref. It might need a weirdly shaped
field, not simple square. We might be using multiple beams
coming in from different angles,passing through lungs, hitting
bone. It's way more complicated than
the water tank. And that right there is where
(45:10):
the rest of clinical dissymmetryand the treatment planning
system come into play. You use that absolute
calibration number from TG 51 asyour non negotiable starting
point, your anchor. But then you need a whole
arsenal of additional data and calculations.
Like all the relative dose data.Exactly the off axis ratios to
know the dose away from the center.
The output factors for all the different field sizes and ships.
(45:33):
The machine can make the full depth dose.
Curds or tissue maximum ratios, TMRS to predict dose at any
depth. Scatter factors, corrections for
B modifiers like wedges or blocks, Algorithms to handle how
the beam goes through different tissue densities like lung or
bone. It's a huge amount of extra
information and calculation built on top of that initial
(45:54):
calibration number. It is.
The TG51 calibration provides the absolute foundation, the
100% dose reference point, but the complex structure built on
top of it, accurately calculating the dose
distribution throughout a heterogeneous patient geometry
for a specific treatment plan. That's a whole other incredibly
complex layer of physics and computation.
So the question really is, how does the accuracy we achieve in
(46:18):
that initial idealized calibration actually ripple
through that entire complex chain of subsequent measurements
and calculations? Where are the otential weak
links or pitfalls in that chain?How do we ensure that the dose
accuracy we established so carefully in the quiet water
phantom is truly maintained whenwe're treating a real,
(46:39):
breathing, moving person? It's a fantastic question, and
honestly it's something every clinical physicist needs to keep
at the forefront of their mind. It really highlights why both
the foundational calibration andall the subsequent patient
specific calculations and checksrequire such meticulous
attention to detail, constant vigilance and a really deep
understanding of all the physicsinvolved at every step.
(47:00):
Definitely something to think about.
You can find notes from today and join the discussion over at
radonsmartlearn.com. And please subscribe so you
don't miss our next exploration.Thanks for tuning in.
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