June 2, 2025 • 25 mins
Welcome back to the RadOnc Smart Review Physics Series! In P5, we explored the powerful machines, especially LINACs, that generate high-energy photon (X-ray) beams. Now that we have these beams, what happens when they actually hit the patient? In Episode P6: Photon Interactions & Basic Dose Concepts, we'll uncover how these energetic photons interact with matter, transfer their energy, and lead to the concepts of Kerma and Absorbed Dose.
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(00:00):
Welcome to Radonk Smart Review Physics Edition.
Today we're diving into something really fundamental in
radiation oncology physics. What actually happens when those
high energy photon beams we generate, you know, hit the
patient? That's right.
We've talked about making the beams, but now we need to get
into how they interact, how theyactually transfer their energy,
(00:21):
and how that leads us to concepts like karma and absorb
dose. This stuff is crucial really for
everything from dose calculationalgorithms to understanding
imaging contrast and even designing shielding.
OK, let's unpack this. First off, a really key
distinction. We've mentioned charged
particles before, like electronsand protons, and how they cause
(00:41):
direct ionization. They just barge through, messing
with atoms directly. But photons are X-rays and gamma
rays. They're different.
Exactly. Photons have no charge, right?
So they don't 'cause that continuous trail of ionization
as they move, they or what we call indirectly ionizing.
What's fascinating here is they can travel quite a way, then
suddenly have a significant interaction.
And in that interaction. They transfer energy to a
(01:04):
charged particle, usually an electron, and that electron then
races off and causes the ionization and excitation in the
surrounding tissue. So the photon sort of starts the
process, but it's the secondary particle that does the bulk of
the local work. And as these interactions
happen, photons get taken out ofthe original beam.
Some get absorbed completely, some just get scattered in a
(01:26):
different direction. This whole removal process is
called attenuation, right? And for a nice simple beam where
all the photons have the same energy, a mono energetic beam.
This attenuation follows a really predictable pattern.
It's exponential. Think of it like radioactive
decay, but instead of time, we're looking at thickness of
material. Ah, OK, so there's an equation
(01:47):
for that. Yeah, the number of photons you
have left, let's call it N at thickness X or NX, is related to
the number you started with. N 0 by NX equals N 0 times
raised to the power of minus MU times X.
OK, N 0 is the initial number, Xis the thickness of the material
and that Greek letter MU. What's that?
That's the linear attenuation coefficient MU.
(02:08):
It basically represents a fraction of photons that get
removed from the beam per unit thickness.
So it's units are often like percentimeter or millimeter
inverse. And it's value depends on.
Oh, it depends heavily on the photon energy, and also on the
material itself, specifically its density, which we often
write as rho and it's atomic number Z.
We've got some pretty useful rules of thumb for water or
(02:30):
tissue, which is what we mostly care about clinically.
Like for six MV photons, attenuation is about 3% per
centimeter. Roughly, yeah.
And for 15 MV photons it drops to about 2% per centimeter.
And in something much less dense, like lung tissue, it
attenuates way less, maybe only 1/3 or 1/4 of what water does,
(02:50):
because MU depends directly on density.
OK, so since that linear coefficient MU is density
dependent, we often use something else.
Exactly. We often use the mass
attenuation coefficient that's simply MU divided by the density
rho. By dividing out the density, you
get a value that's more characteristic of the material
itself, you know, regardless of whether it's solid, liquid, or
gas. That makes sense.
So it's interactions per gram maybe?
(03:12):
Pretty much the units are typically centimeters squared
per gram. It tells you about the
probability of interaction per unit mass, which is often more
fundamental when comparing different materials or tissues.
Right, so attenuation tells us photons are disappearing from
the beam. But in radiation therapy, we
don't just want them gone, we need them to deposit energy to
(03:34):
kill cancer cells. How do we track that energy
deposition? That's the crucial next step.
This raises an important question.
We need to know how much energy is actually being handed over.
For this we use the energy transfer coefficient written as
MO's subtr. This accounts for the average
energy that gets transferred from the photons to the kinetic
(03:55):
energy of those charged particles, those secondary
electrons right at the interaction site.
But hang on those fast electrons, they don't
necessarily dump all their energy right there at that exact
spot, do they? They travel a bit.
They certainly do, and sometimes, especially if they're
high energy electrons moving through high Z materials, they
can lose some of that kinetic energy by radiating it away as
(04:15):
brim strolling photons. Brim photons.
The breaking radiation. Exactly.
And those brim photons can travel quite far, potentially
escaping the local area we're interested in.
So to account for just the energy that gets deposited
locally through ionization and excitation by those electrons,
we use the energy absorption coefficient MU sub EN.
OK, so MU sub EN focuses just onthe locally deposited energy.
(04:40):
That means MU sub EN must be less than or maybe equal to MU
sub TR. Always the relationship is MU
sub EN equals MU sub TR times the quantity 1 -, g That G
factor is simply the fraction ofthe initial kinetic energy given
to the secondary electrons that gets re radiated away as
brimstrola. And G would increase with.
(05:01):
G increases with the electrons energy and with the atomic
number Z of the material they'retraveling through.
More energy, more chance to radiate higher Z, stronger
braking fuel. OK.
And the mass energy absorption coefficient MU sub EN divided by
rho. That's the one most relevant for
calculating the actual dose to positive.
Precisely that quantity tells usthe energy deposited locally per
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unit mass, and this leads us directly to two absolutely vital
concepts, Kerma and absorb dose.Kerma KERMA, that stands for
kinetic energy released per unitmass, right?
Correct. It's formally defined as the sum
of the initial kinetic energies of all the charged particles set
in motion by uncharged particleslike our photons, within a
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little volume element divided bythe mass of that volume.
Units are joules per kilogram, same as Gray.
Same units joules per kilogram, which we call the Gray, and
kerma itself can be it. Total Kerma K is divided into
collision kerma K sub coal and radiative kerma K sub rad.
Collision kerma is the part transferred to electrons that
they then lose through collisions, ionization and
excitation. That's the part we care about
(06:04):
for biological effects. And collision kerma is related
to that G factor again. Yep, collision kerma K sub coal
equals total kerma K * 1 -, g. It's the locally deposited
fraction. Radiative kerma is the part lost
as brim. OK.
So Kerma is fundamentally about the energy transferred to the
electrons at the point of interaction.
(06:25):
Absorb dose D is about the energy that's actually absorbed
locally from those electrons as they slow down and travel
through the tissue. That's the critical distinction.
Think about a mega voltage photon beam entering a patient
right at the surface. The number of socons hitting is
highest, so the photon Fluence is maximum.
Therefore, Kerma is maximum right at the surface.
(06:47):
But the absorbed dose isn't maximum there.
No, it's actually quite low. Those electrons that get knocked
loose are mostly traveling forward deeper into the tissue.
They haven't had the distance yet to deposit much of their
energy right there in that superficial layer.
This difference is the physical basis for the skin sparing
effect we value so much in MV therapy.
OK. So as the beam goes deeper.
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As the beam penetrates, 2 thingshappen.
Kerma starts to decrease becausethe photons are being
attenuated, but the electrons set in motion near the surface
are now reaching deeper depths and depositing their energy.
Plus, new electrons are being generated deeper down, so the
absorb Joe's actually builds up with depth.
Until it reaches a pick D Max. Exactly absorb dose peaks at the
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depth of maximum dose D Max. The depth of D Max depends on
the beam energy. Higher energy beings have longer
range electrons, so D Max is deeper.
And beyond D Max. Beyond D Max, the absorb dose
starts to decrease as well, mainly because the Kerma, the
production of new electrons, is falling off due to photon
attenuation. The dose curve roughly parallels
(07:51):
the Kerma curve beyond D Max, but it shifted deeper because of
the forward range of those electrons.
And it's in this region beyond DMax that we approach something
called transient electronic equilibrium.
That's the term. It means that for any small
volume, the number of electrons carrying energy into that volume
is roughly balanced by the number carrying energy out plus
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the energy that's actually deposited within the volume by
electrons slowing down. South dose and karma are related
there. In this region, under transient
electronic equilibrium, the absorbed dose D becomes
proportional to the collision kerma K sub Col.
If you had perfect electronic equilibrium where electrons
(08:31):
stopped exactly where they were created, dose would equal
collision kerma. But for MV beams, it's usually
transient equilibrium. OK, so the key take away here,
Kerma is highest at the surface,always decreasing with depth.
Absorbed dose starts low, buildsup to D Max, then decreases, and
crucially before D Max Kerma is greater than absorbed dose.
(08:52):
You got it. That relationship is
fundamental. So now we know about energy
transfer and dose, but how do the photons actually interact to
kick off this whole process? You mentioned 5 main ways,
right? Let's get into the mechanisms.
There are 5 main interaction processes photons can undergo.
1st at very, very low energies, there's Rayleigh scattering,
sometimes called coherent scattering.
(09:12):
OK, what's on? It's pretty simple actually.
The photon interacts with the whole atom, makes the electron
cloud kind of wiggle for a moment, and then the atom
reemits A photon with the exact same energy, just going off in a
different direction. So no energy is actually
deposited. Nope, no energy loss, no
ionization, it just changes direction.
It's really only significant at very low energies and doesn't
(09:35):
play much role in therapy or even diagnostic imaging.
Frankly, pretty negligible for us.
OK, negligible. What's next?
And more important, especially at lower like diagnostic
energies. That would be the photoelectric
effect. This one is dominant at the
lower end of the kilovoltage range.
Think typical diagnostic X-ray energies.
And how does this one work? This is a true absorption event.
(09:56):
The incoming photon hits a tightly bound inner shell
electron, often from the K or L shell.
The photon completely disappears, vanishes.
And all its energy goes where? All of its energy is transferred
to that electron minus the energy that was holding the
electron in its shell, its binding energy.
That electron gets ejected from the atom with kinetic energy.
We call it a photoelectron. OK.
(10:17):
And the probability of this happening, remember you said it
depends strongly on Z and energy.
Very, very strongly. This is critical.
The probability shoots up rapidly with the atomic number
of the atom. Roughly is Z ^3 so high Z
materials are much more likely to have photoelectric
interactions Z. Cubed.
Wow. Yeah, huge dependence.
And it drops off just as dramatically as the photon
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energy increases, roughly as 1 /e ^3.
Also, the photon has to have enough energy to kick the
electron out more than its binding energy.
This leads to sharp jumps in attenuation called absorption
edges. What happens in the atom after
the electron is gone? You've got a vacancy, an empty
spot in an inner electron shell.An electron from a higher outer
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shell will quickly drop down to fill that hole.
When it does, it releases energyeither as a characteristic X-ray
photon specific to that element,or by kicking out another
electron, which we call an Augerelectron.
And the energetic photoelectron itself.
That photoelectron, plus any characteristic X-rays or Auger
electrons produced, deposits itsenergy very locally, right near
the site of the original interaction.
(11:22):
Relevance. You mentioned diagnostic X-rays.
Absolutely dominant for diagnostic X-rays, typically in
the 20 to 100 key range. Yeah, and that Z cube dependence
is why we get such great contrast between bone, which has
a high effect of Z and soft tissue, which has a low Z on our
X-ray images. It's all photoelectric effect.
OK. Super important for imaging now
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moving up in energy into the therapy range.
What takes over? Now we get to the king of
therapeutic interactions, Compton scattering.
This is the dominant process from around say 30 key all the
way up to maybe 25 or 30 MEVI. What happens in Compton
scattering? Here the incoming photon
interacts with a loosely bound outer shell electron.
(12:06):
These electrons are bound so loosely compared to the photons
energy that we can basically treat them as free electrons
just sitting there at rest. OK, so it hits a free electron.
Right. The photon transfers some of its
energy to this electron, knocking it out of the atom.
This ejected electron is called a Compton electron, or sometimes
a recoil electron. The photon itself doesn't
disappear, but it loses energy and scatters off in a different
(12:27):
direction. So unlike photoelectric, the
photon survives, just with less energy.
Exactly. Energy and momentum are
conserved in the whole interaction distributed between
the scattered photon and the Compton electron.
Now the probability dependence here is key and very different
from photoelectric, right? Hugely different, and this is a
massive point for therapy. Yes, the probability of Compton
(12:49):
scattering does decrease as photon energy increases, but
much more slowly than photoelectric, roughly as 1 / e.
Okay, 1 / E but the dependence on Z.
This is the kicker. It's almost independent of the
atomic number Z per gram of material.
Bone and soft tissue attenuate photons almost identically via
the Compton effect. Wait really?
(13:10):
So why does bone look different on an MV port film then just
density? Mostly density, yeah.
Compton does depend on the electron density, the number of
electrons available per gram of material.
Most materials like water, soft tissue, even bone have very
similar numbers of electrons pergram.
The exception is hydrogen. Hydrogen.
Why? Hydrogen has no neutrons, just a
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proton and an electron O for itsmass.
It has roughly double the numberof electrons per gram compared
to most other elements found in tissue.
Fat has more hydrogen, so slightly higher electron
density, but overall the Z dependence is minimal compared
to photoelectric. Interesting.
And the energy sharing depends on the angle.
It does. The photon just has a glancing
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collision. It scatters at a small angle and
loses very little energy. If it scatters at 90°, its
energy drops towards .511 mevi. The electron rest mass energy.
Exactly, and if it scatters straight back 180°, the photon
energy gets even lower, approaching .255 MEVI or half
the electron rest mass energy. And the electron, where does it
(14:15):
go? To conserve momentum, the
Compton electron always gets ejected in a generally forward
direction relative to the incoming photon.
OK. Relevance.
Clinically, this is our main dose mechanism.
This is absolutely how our therapeutic mega voltage photon
beams deposit the vast majority of their dose and soft tissue.
Those Compton electrons are the primary agents causing
ionization and excitation withinthe tumor volume.
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It's also, unfortunately, a major source of scattered
radiation that fogs up our images, both diagnostic and MV
ports. Got it.
Compton is king for therapy dose.
What happens if we go even higher in energy like above 10
or 20 mevi? Then we start to see pair
production become significant. This interaction has a definite
energy threshold. The incoming photon must have at
(14:58):
least 1.02 wave of energy. Why that specific?
Number because that's exactly twice the rest mass energy of an
electron .511 mere V Compare reduction.
A high energy photon passes close to the strong electric
field of an atomic nucleus. The nucleus itself.
Yes, the field of the nucleus and the photons energy
spontaneously converts into mass.
Specifically, it creates a pair of particles, an electron, and
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it's antiparticle, a positron. Wow, matter creation.
Pretty much EMC squared in action.
Any energy the photon had above the 1.022 and go D threshold is
shared between the electron and positron as kinetic energy,
making them fly off. And the probability of this
happening. It increases with photon energy
once you're above the threshold,and importantly, it increases
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significantly with the atomic number of the nucleus involved,
roughly as Z ^2. So high Z materials are much
more prone to pair production. Z ^2 this time.
OK, what happens to that positron?
It's antimatter. It is.
It travels a short distance, losing its kinetic energy like
an electron does, but once it slows down enough, it inevitably
encounters an electron. Matter meets antimatter.
(16:02):
Annihilation. Annihilation.
They both disappear and their combined rest mass energy is
converted back into pure energy,specifically to 0.511 ME photons
that fly off in opposite directions 180° part to conserve
momentum. And those 2.511 mevi photons,
that's the basis for PT imaging,right?
Detecting those pairs. Exactly that annihilation
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radiation is what PT scanners detect.
There's also a related process called triplet production, where
the interaction happens near an orbital electron instead of the
nucleus. It has a higher threshold 2.044
MEVI and is less common. So, relevance of pair production
and therapy. It becomes an important
contributor to attenuation and dose deposition for beams with
(16:47):
energies significantly above, say, 10 MEVI.
It starts to compete with and eventually overtake Compton
scattering at very high energies, especially in high Z
materials like shielding. It's why higher energy beans can
sometimes have slightly better penetration, but it also
complicates things. OK, one more interaction to
cover. The last one, generally
requiring the highest energies is photo disintegration.
(17:08):
Photo disintegration sounds dramatic.
It is here a very high energy photon doesn't just interact
with the electrons of the nucleus's field, it gets
absorbed directly by the nucleusitself.
Into the nucleus? Yep.
This dumps a lot of energy into the nucleus, making it highly
unstable. To get back to a more stable
state, the nucleus immediately ejects A particle, most commonly
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A neutron, but sometimes a proton or even an alpha
particle. What kind of energy does this
take? The threshold energy depends on
the specific nucleus, but for the materials typically found in
a linear accelerator treatment head, like tungsten in the
target or lead tungsten in the collimators, the threshold is
usually around 8 to 10 mega electron volts. 8 to 10 MV That
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sounds familiar. It should.
This is the primary reason why we get unwanted neutron
contamination from Linux. Operating above about 10 MV,
those high energy photons from the beam hit the high Z
components in the head. And knock neutrons out.
Photo Neutrons. Exactly.
Photo neutrons. This is a really big deal for
radiation protection. Neutrons are hard to shield,
(18:11):
requires special door to vines of materials like polyethylene
or concrete, and they contributean unwanted whole body dose to
the patient, which has implications for secondary
cancer risk. Wow OK so operating above 10 MV
really changes the game regarding shielding and safety
because of photo disintegration.It really does.
It's a major consideration. So let's pull this all together.
We've got these five interactions, each dominant in
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different energy ranges and depending differently on Z.
How does knowing all this actually influence our clinical
decisions? Well, it dictates our imaging
choices. For starters, we use kilovoltage
beams for diagnostic CTS and X-rays because we want that Z ^3
dependence of the photoelectric effect to give a sharp contrast
between bone and soft tissue. And conversely, MV portal images
(18:57):
look washed out. Because they're formed mainly by
Compton scattering, which barelycares about Z.
The little contrast you see is mostly due to differences in
density and thickness. Makes sense And for planning
therapy? Knowing Compton is dominant for
our typical MV beams and tissue simplifies dose calculation in a
way we don't need super complex corrections for the Z of bone
versus tissue like we would withKV beams.
(19:20):
The main thing we need to account for accurately is the
difference in electron density, which is closely related to
physical density. So density corrections are
paramount for MV planning systems.
Absolutely. And the choice of beam energy
itself involves these trade-offs.
Going to higher MV, say 15 or 18MV, might give slightly deeper
penetration, partly due to the onset of pair production, which
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can be good for deep tumors. But you cross that photo
disintegration threshold around 10 MV, introducing neutrons.
So you need much more shielding in the room, have your doors
maybe a maze, and you have that extra patient dose component to
consider. It's a balance. 6 MB is often a
sweet spot. Good skin sparing, decent
penetration, no neutrons. And shielding design itself must
be tailored to the dominant interaction.
(20:04):
Right, totally. For a diagnostic X-ray room, low
energy KV lead is fantastic. It's high Z maximizes the
photoelectric effect, efficiently absorbing those
photons. But for a high energy linac
vault? Lead isn't enough.
Well, for the primary beam you need sheer mass to handle
Compton scattering, so thick concrete is the main barrier.
Lead might be used strategically, but concrete
(20:25):
provides the bulk. And if you're above 10 MV you
also need neutron shielding. That's where the hydrogen and
concrete helps. Or adding layers of polyethylene
inside the door comes in. You have to stop photons and
neutrons. It all comes back to the physics
of these interactions. OK, let's crystallize some of
this time for some clinical pearls.
The absolute must remember points for boards and practice.
(20:47):
All right, Pearl, one photoelectric effect.
Think kilovoltage energies. Think Z cube dependence.
Think inverse E cube dependence.Think imaging contrast.
Pearl 2 Compton scattering. Think mega voltage therapy
energies. Think largely independent of Z
Think dependent on electron density.
Think roughly 1 / E dependence. Think dose deposition in tissue.
(21:07):
Pearl three pair production. Think threshold energy greater
than 1.022 MV becomes important at high MV, especially above
1015 MV. Probability increases with
energy and with Z ^2. Think high energy, high Z.
Pearl 4 Photo Disintegration. Think threshold energy around 8
to 10 MEVI Think neutron contamination in Linux operating
(21:29):
above 10 MV. Think shielding implications and
patient safety. And don't forget that rough 5050
rule for water, Photoelectric and Compton crossover around 30.
Cave, Compton and pair production crossover somewhere
around 24 to 30. Mebi gives you a quick sanity
check for which process dominates.
Excellent pearls, ready for a quick board blitz?
Let's test these concepts. Question one which photon
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interaction process is most responsible for the difference
in image contrast between bone and soft tissue on a diagnostic
kilovoltage X-ray? OK, diagnostic KV contrast
between bone high Z and tissue low Z.
That's got to be the photoelectric effect because of
its Z cube dependence. Correct explanation.
The probability of the photoelectric effect is roughly
proportional to Z ^3 bones. Higher Z causes much greater
(22:13):
attenuation of KB photons via this effect, creating contrast
Question 2A10 mega electron Voltphoton beam is incident on
alleged shield. Lead has zeal 82.
Which photon interaction processis likely dominant within the
lead at this energy? 10 Mevi is getting up there and
lead is high Z Compton dominatesin water up to maybe 25 Mevi,
(22:34):
but pair production increases with Z ^2.
So in high Z lead pair production probably takes over
much earlier. I'll say pair production.
You got it. Explanation.
Pair production Z ^2 dependence makes it dominant over Compton
at lower energies in high Z materials compared to low Z
materials by 10. Nevian lead pair production is
likely dominant. Question 3.
Compton's scattering probabilityis primarily dependent on a
(22:57):
atomic number. ZB mass number, AC electron
density, electrons per gram, D photon energy only.
Compton, we said it's almost Z independent, but depends on how
many electrons are available to hit so C electron density.
Perfect explanation. Compton probability is largely
independent of Z but directly proportional to the number of
electrons per unit mass. Question 4.
(23:18):
Neutron contamination becomes a significant concern for linear
accelerators operating above what approximate photon energy?
A1 Mevio. B6 mevi.
C10 mevi. D25 Mevi.
That's the photo disintegration threshold we talked about.
Around 8 to 10 Bev is where it starts to matter in linac
material. So C10 maybe?
Excellent explanation. Photo disintegration creating
(23:40):
photo neutrons generally has a threshold around 810 mevi in
relevant linac materials like the target and collimators.
OK, so let's quickly summarize what we covered today.
We established that photons are indirectly ionizing.
They interact via processes thatremove them from the beam
attenuation, which follows that exponential equation.
And we made that key distinctionbetween Kermo, the kinetic
(24:02):
energy released to electrons andabsorb dose, the energy actually
deposited locally. That explained the dose buildup
effect D Max and the concept of electronic equilibrium.
Then we walked through the five key interactions.
The almost negligible Rayleigh scattering.
The photoelectric effect, dominant at KV energies with its
strong Z cube dependence, crucial for imaging.
(24:24):
Compton scattering, dominant at therapy.
MV energies depending on electron density and responsible
for dose and tissue pair production needing over 1.022
MEVI and depending on Z ^2 important at high energies.
And finally, photo disintegration kicking in above
810 mevi and causing neutron contamination.
And understanding how these interactions compete and
dominate based on photon energy and the Z of the material, well,
(24:48):
it really does underpin just about everything, doesn't it?
From why our images have contrast to how TPS algorithms
calculate dose, to how we safelydesign treatment faults.
It really does. Thinking about how these
microscopic events, these individual photon interactions,
add up to create the macroscopicdose distributions we shape and
deliver to patients, it's prettyincredible when you step back
(25:11):
and think about it, a real reminder of the fundamental
physics driving our clinical practice.
Absolutely. And building right on this, next
time we'll get into how we actually go about measuring that
absorbed dose accurately in the clinic.
We'll talk about different typesof detectors and the theories
behind them, like Brag Ray cavity theory.
Sounds great. Remember, you can find
completepracticeoralboardsover@radonsmartlearn.com.Be sure to subscribe to Radon
(25:36):
Smart Review for our next session.
Thanks for tuning in.
Welcome to Radonk Smart Review Physics Edition.
Today we're diving into something really fundamental in
radiation oncology physics. What actually happens when those
high energy photon beams we generate, you know, hit the
patient? That's right.
We've talked about making the beams, but now we need to get
into how they interact, how theyactually transfer their energy,
(00:21):
and how that leads us to concepts like karma and absorb
dose. This stuff is crucial really for
everything from dose calculationalgorithms to understanding
imaging contrast and even designing shielding.
OK, let's unpack this. First off, a really key
distinction. We've mentioned charged
particles before, like electronsand protons, and how they cause
(00:41):
direct ionization. They just barge through, messing
with atoms directly. But photons are X-rays and gamma
rays. They're different.
Exactly. Photons have no charge, right?
So they don't 'cause that continuous trail of ionization
as they move, they or what we call indirectly ionizing.
What's fascinating here is they can travel quite a way, then
suddenly have a significant interaction.
And in that interaction. They transfer energy to a
(01:04):
charged particle, usually an electron, and that electron then
races off and causes the ionization and excitation in the
surrounding tissue. So the photon sort of starts the
process, but it's the secondary particle that does the bulk of
the local work. And as these interactions
happen, photons get taken out ofthe original beam.
Some get absorbed completely, some just get scattered in a
(01:26):
different direction. This whole removal process is
called attenuation, right? And for a nice simple beam where
all the photons have the same energy, a mono energetic beam.
This attenuation follows a really predictable pattern.
It's exponential. Think of it like radioactive
decay, but instead of time, we're looking at thickness of
material. Ah, OK, so there's an equation
(01:47):
for that. Yeah, the number of photons you
have left, let's call it N at thickness X or NX, is related to
the number you started with. N 0 by NX equals N 0 times
raised to the power of minus MU times X.
OK, N 0 is the initial number, Xis the thickness of the material
and that Greek letter MU. What's that?
That's the linear attenuation coefficient MU.
(02:08):
It basically represents a fraction of photons that get
removed from the beam per unit thickness.
So it's units are often like percentimeter or millimeter
inverse. And it's value depends on.
Oh, it depends heavily on the photon energy, and also on the
material itself, specifically its density, which we often
write as rho and it's atomic number Z.
We've got some pretty useful rules of thumb for water or
(02:30):
tissue, which is what we mostly care about clinically.
Like for six MV photons, attenuation is about 3% per
centimeter. Roughly, yeah.
And for 15 MV photons it drops to about 2% per centimeter.
And in something much less dense, like lung tissue, it
attenuates way less, maybe only 1/3 or 1/4 of what water does,
(02:50):
because MU depends directly on density.
OK, so since that linear coefficient MU is density
dependent, we often use something else.
Exactly. We often use the mass
attenuation coefficient that's simply MU divided by the density
rho. By dividing out the density, you
get a value that's more characteristic of the material
itself, you know, regardless of whether it's solid, liquid, or
gas. That makes sense.
So it's interactions per gram maybe?
(03:12):
Pretty much the units are typically centimeters squared
per gram. It tells you about the
probability of interaction per unit mass, which is often more
fundamental when comparing different materials or tissues.
Right, so attenuation tells us photons are disappearing from
the beam. But in radiation therapy, we
don't just want them gone, we need them to deposit energy to
(03:34):
kill cancer cells. How do we track that energy
deposition? That's the crucial next step.
This raises an important question.
We need to know how much energy is actually being handed over.
For this we use the energy transfer coefficient written as
MO's subtr. This accounts for the average
energy that gets transferred from the photons to the kinetic
(03:55):
energy of those charged particles, those secondary
electrons right at the interaction site.
But hang on those fast electrons, they don't
necessarily dump all their energy right there at that exact
spot, do they? They travel a bit.
They certainly do, and sometimes, especially if they're
high energy electrons moving through high Z materials, they
can lose some of that kinetic energy by radiating it away as
(04:15):
brim strolling photons. Brim photons.
The breaking radiation. Exactly.
And those brim photons can travel quite far, potentially
escaping the local area we're interested in.
So to account for just the energy that gets deposited
locally through ionization and excitation by those electrons,
we use the energy absorption coefficient MU sub EN.
OK, so MU sub EN focuses just onthe locally deposited energy.
(04:40):
That means MU sub EN must be less than or maybe equal to MU
sub TR. Always the relationship is MU
sub EN equals MU sub TR times the quantity 1 -, g That G
factor is simply the fraction ofthe initial kinetic energy given
to the secondary electrons that gets re radiated away as
brimstrola. And G would increase with.
(05:01):
G increases with the electrons energy and with the atomic
number Z of the material they'retraveling through.
More energy, more chance to radiate higher Z, stronger
braking fuel. OK.
And the mass energy absorption coefficient MU sub EN divided by
rho. That's the one most relevant for
calculating the actual dose to positive.
Precisely that quantity tells usthe energy deposited locally per
(05:22):
unit mass, and this leads us directly to two absolutely vital
concepts, Kerma and absorb dose.Kerma KERMA, that stands for
kinetic energy released per unitmass, right?
Correct. It's formally defined as the sum
of the initial kinetic energies of all the charged particles set
in motion by uncharged particleslike our photons, within a
(05:42):
little volume element divided bythe mass of that volume.
Units are joules per kilogram, same as Gray.
Same units joules per kilogram, which we call the Gray, and
kerma itself can be it. Total Kerma K is divided into
collision kerma K sub coal and radiative kerma K sub rad.
Collision kerma is the part transferred to electrons that
they then lose through collisions, ionization and
excitation. That's the part we care about
(06:04):
for biological effects. And collision kerma is related
to that G factor again. Yep, collision kerma K sub coal
equals total kerma K * 1 -, g. It's the locally deposited
fraction. Radiative kerma is the part lost
as brim. OK.
So Kerma is fundamentally about the energy transferred to the
electrons at the point of interaction.
(06:25):
Absorb dose D is about the energy that's actually absorbed
locally from those electrons as they slow down and travel
through the tissue. That's the critical distinction.
Think about a mega voltage photon beam entering a patient
right at the surface. The number of socons hitting is
highest, so the photon Fluence is maximum.
Therefore, Kerma is maximum right at the surface.
(06:47):
But the absorbed dose isn't maximum there.
No, it's actually quite low. Those electrons that get knocked
loose are mostly traveling forward deeper into the tissue.
They haven't had the distance yet to deposit much of their
energy right there in that superficial layer.
This difference is the physical basis for the skin sparing
effect we value so much in MV therapy.
OK. So as the beam goes deeper.
(07:07):
As the beam penetrates, 2 thingshappen.
Kerma starts to decrease becausethe photons are being
attenuated, but the electrons set in motion near the surface
are now reaching deeper depths and depositing their energy.
Plus, new electrons are being generated deeper down, so the
absorb Joe's actually builds up with depth.
Until it reaches a pick D Max. Exactly absorb dose peaks at the
(07:30):
depth of maximum dose D Max. The depth of D Max depends on
the beam energy. Higher energy beings have longer
range electrons, so D Max is deeper.
And beyond D Max. Beyond D Max, the absorb dose
starts to decrease as well, mainly because the Kerma, the
production of new electrons, is falling off due to photon
attenuation. The dose curve roughly parallels
(07:51):
the Kerma curve beyond D Max, but it shifted deeper because of
the forward range of those electrons.
And it's in this region beyond DMax that we approach something
called transient electronic equilibrium.
That's the term. It means that for any small
volume, the number of electrons carrying energy into that volume
is roughly balanced by the number carrying energy out plus
(08:11):
the energy that's actually deposited within the volume by
electrons slowing down. South dose and karma are related
there. In this region, under transient
electronic equilibrium, the absorbed dose D becomes
proportional to the collision kerma K sub Col.
If you had perfect electronic equilibrium where electrons
(08:31):
stopped exactly where they were created, dose would equal
collision kerma. But for MV beams, it's usually
transient equilibrium. OK, so the key take away here,
Kerma is highest at the surface,always decreasing with depth.
Absorbed dose starts low, buildsup to D Max, then decreases, and
crucially before D Max Kerma is greater than absorbed dose.
(08:52):
You got it. That relationship is
fundamental. So now we know about energy
transfer and dose, but how do the photons actually interact to
kick off this whole process? You mentioned 5 main ways,
right? Let's get into the mechanisms.
There are 5 main interaction processes photons can undergo.
1st at very, very low energies, there's Rayleigh scattering,
sometimes called coherent scattering.
(09:12):
OK, what's on? It's pretty simple actually.
The photon interacts with the whole atom, makes the electron
cloud kind of wiggle for a moment, and then the atom
reemits A photon with the exact same energy, just going off in a
different direction. So no energy is actually
deposited. Nope, no energy loss, no
ionization, it just changes direction.
It's really only significant at very low energies and doesn't
(09:35):
play much role in therapy or even diagnostic imaging.
Frankly, pretty negligible for us.
OK, negligible. What's next?
And more important, especially at lower like diagnostic
energies. That would be the photoelectric
effect. This one is dominant at the
lower end of the kilovoltage range.
Think typical diagnostic X-ray energies.
And how does this one work? This is a true absorption event.
(09:56):
The incoming photon hits a tightly bound inner shell
electron, often from the K or L shell.
The photon completely disappears, vanishes.
And all its energy goes where? All of its energy is transferred
to that electron minus the energy that was holding the
electron in its shell, its binding energy.
That electron gets ejected from the atom with kinetic energy.
We call it a photoelectron. OK.
(10:17):
And the probability of this happening, remember you said it
depends strongly on Z and energy.
Very, very strongly. This is critical.
The probability shoots up rapidly with the atomic number
of the atom. Roughly is Z ^3 so high Z
materials are much more likely to have photoelectric
interactions Z. Cubed.
Wow. Yeah, huge dependence.
And it drops off just as dramatically as the photon
(10:39):
energy increases, roughly as 1 /e ^3.
Also, the photon has to have enough energy to kick the
electron out more than its binding energy.
This leads to sharp jumps in attenuation called absorption
edges. What happens in the atom after
the electron is gone? You've got a vacancy, an empty
spot in an inner electron shell.An electron from a higher outer
(11:00):
shell will quickly drop down to fill that hole.
When it does, it releases energyeither as a characteristic X-ray
photon specific to that element,or by kicking out another
electron, which we call an Augerelectron.
And the energetic photoelectron itself.
That photoelectron, plus any characteristic X-rays or Auger
electrons produced, deposits itsenergy very locally, right near
the site of the original interaction.
(11:22):
Relevance. You mentioned diagnostic X-rays.
Absolutely dominant for diagnostic X-rays, typically in
the 20 to 100 key range. Yeah, and that Z cube dependence
is why we get such great contrast between bone, which has
a high effect of Z and soft tissue, which has a low Z on our
X-ray images. It's all photoelectric effect.
OK. Super important for imaging now
(11:43):
moving up in energy into the therapy range.
What takes over? Now we get to the king of
therapeutic interactions, Compton scattering.
This is the dominant process from around say 30 key all the
way up to maybe 25 or 30 MEVI. What happens in Compton
scattering? Here the incoming photon
interacts with a loosely bound outer shell electron.
(12:06):
These electrons are bound so loosely compared to the photons
energy that we can basically treat them as free electrons
just sitting there at rest. OK, so it hits a free electron.
Right. The photon transfers some of its
energy to this electron, knocking it out of the atom.
This ejected electron is called a Compton electron, or sometimes
a recoil electron. The photon itself doesn't
disappear, but it loses energy and scatters off in a different
(12:27):
direction. So unlike photoelectric, the
photon survives, just with less energy.
Exactly. Energy and momentum are
conserved in the whole interaction distributed between
the scattered photon and the Compton electron.
Now the probability dependence here is key and very different
from photoelectric, right? Hugely different, and this is a
massive point for therapy. Yes, the probability of Compton
(12:49):
scattering does decrease as photon energy increases, but
much more slowly than photoelectric, roughly as 1 / e.
Okay, 1 / E but the dependence on Z.
This is the kicker. It's almost independent of the
atomic number Z per gram of material.
Bone and soft tissue attenuate photons almost identically via
the Compton effect. Wait really?
(13:10):
So why does bone look different on an MV port film then just
density? Mostly density, yeah.
Compton does depend on the electron density, the number of
electrons available per gram of material.
Most materials like water, soft tissue, even bone have very
similar numbers of electrons pergram.
The exception is hydrogen. Hydrogen.
Why? Hydrogen has no neutrons, just a
(13:30):
proton and an electron O for itsmass.
It has roughly double the numberof electrons per gram compared
to most other elements found in tissue.
Fat has more hydrogen, so slightly higher electron
density, but overall the Z dependence is minimal compared
to photoelectric. Interesting.
And the energy sharing depends on the angle.
It does. The photon just has a glancing
(13:52):
collision. It scatters at a small angle and
loses very little energy. If it scatters at 90°, its
energy drops towards .511 mevi. The electron rest mass energy.
Exactly, and if it scatters straight back 180°, the photon
energy gets even lower, approaching .255 MEVI or half
the electron rest mass energy. And the electron, where does it
(14:15):
go? To conserve momentum, the
Compton electron always gets ejected in a generally forward
direction relative to the incoming photon.
OK. Relevance.
Clinically, this is our main dose mechanism.
This is absolutely how our therapeutic mega voltage photon
beams deposit the vast majority of their dose and soft tissue.
Those Compton electrons are the primary agents causing
ionization and excitation withinthe tumor volume.
(14:36):
It's also, unfortunately, a major source of scattered
radiation that fogs up our images, both diagnostic and MV
ports. Got it.
Compton is king for therapy dose.
What happens if we go even higher in energy like above 10
or 20 mevi? Then we start to see pair
production become significant. This interaction has a definite
energy threshold. The incoming photon must have at
(14:58):
least 1.02 wave of energy. Why that specific?
Number because that's exactly twice the rest mass energy of an
electron .511 mere V Compare reduction.
A high energy photon passes close to the strong electric
field of an atomic nucleus. The nucleus itself.
Yes, the field of the nucleus and the photons energy
spontaneously converts into mass.
Specifically, it creates a pair of particles, an electron, and
(15:20):
it's antiparticle, a positron. Wow, matter creation.
Pretty much EMC squared in action.
Any energy the photon had above the 1.022 and go D threshold is
shared between the electron and positron as kinetic energy,
making them fly off. And the probability of this
happening. It increases with photon energy
once you're above the threshold,and importantly, it increases
(15:40):
significantly with the atomic number of the nucleus involved,
roughly as Z ^2. So high Z materials are much
more prone to pair production. Z ^2 this time.
OK, what happens to that positron?
It's antimatter. It is.
It travels a short distance, losing its kinetic energy like
an electron does, but once it slows down enough, it inevitably
encounters an electron. Matter meets antimatter.
(16:02):
Annihilation. Annihilation.
They both disappear and their combined rest mass energy is
converted back into pure energy,specifically to 0.511 ME photons
that fly off in opposite directions 180° part to conserve
momentum. And those 2.511 mevi photons,
that's the basis for PT imaging,right?
Detecting those pairs. Exactly that annihilation
(16:25):
radiation is what PT scanners detect.
There's also a related process called triplet production, where
the interaction happens near an orbital electron instead of the
nucleus. It has a higher threshold 2.044
MEVI and is less common. So, relevance of pair production
and therapy. It becomes an important
contributor to attenuation and dose deposition for beams with
(16:47):
energies significantly above, say, 10 MEVI.
It starts to compete with and eventually overtake Compton
scattering at very high energies, especially in high Z
materials like shielding. It's why higher energy beans can
sometimes have slightly better penetration, but it also
complicates things. OK, one more interaction to
cover. The last one, generally
requiring the highest energies is photo disintegration.
(17:08):
Photo disintegration sounds dramatic.
It is here a very high energy photon doesn't just interact
with the electrons of the nucleus's field, it gets
absorbed directly by the nucleusitself.
Into the nucleus? Yep.
This dumps a lot of energy into the nucleus, making it highly
unstable. To get back to a more stable
state, the nucleus immediately ejects A particle, most commonly
(17:30):
A neutron, but sometimes a proton or even an alpha
particle. What kind of energy does this
take? The threshold energy depends on
the specific nucleus, but for the materials typically found in
a linear accelerator treatment head, like tungsten in the
target or lead tungsten in the collimators, the threshold is
usually around 8 to 10 mega electron volts. 8 to 10 MV That
(17:50):
sounds familiar. It should.
This is the primary reason why we get unwanted neutron
contamination from Linux. Operating above about 10 MV,
those high energy photons from the beam hit the high Z
components in the head. And knock neutrons out.
Photo Neutrons. Exactly.
Photo neutrons. This is a really big deal for
radiation protection. Neutrons are hard to shield,
(18:11):
requires special door to vines of materials like polyethylene
or concrete, and they contributean unwanted whole body dose to
the patient, which has implications for secondary
cancer risk. Wow OK so operating above 10 MV
really changes the game regarding shielding and safety
because of photo disintegration.It really does.
It's a major consideration. So let's pull this all together.
We've got these five interactions, each dominant in
(18:34):
different energy ranges and depending differently on Z.
How does knowing all this actually influence our clinical
decisions? Well, it dictates our imaging
choices. For starters, we use kilovoltage
beams for diagnostic CTS and X-rays because we want that Z ^3
dependence of the photoelectric effect to give a sharp contrast
between bone and soft tissue. And conversely, MV portal images
(18:57):
look washed out. Because they're formed mainly by
Compton scattering, which barelycares about Z.
The little contrast you see is mostly due to differences in
density and thickness. Makes sense And for planning
therapy? Knowing Compton is dominant for
our typical MV beams and tissue simplifies dose calculation in a
way we don't need super complex corrections for the Z of bone
versus tissue like we would withKV beams.
(19:20):
The main thing we need to account for accurately is the
difference in electron density, which is closely related to
physical density. So density corrections are
paramount for MV planning systems.
Absolutely. And the choice of beam energy
itself involves these trade-offs.
Going to higher MV, say 15 or 18MV, might give slightly deeper
penetration, partly due to the onset of pair production, which
(19:42):
can be good for deep tumors. But you cross that photo
disintegration threshold around 10 MV, introducing neutrons.
So you need much more shielding in the room, have your doors
maybe a maze, and you have that extra patient dose component to
consider. It's a balance. 6 MB is often a
sweet spot. Good skin sparing, decent
penetration, no neutrons. And shielding design itself must
be tailored to the dominant interaction.
(20:04):
Right, totally. For a diagnostic X-ray room, low
energy KV lead is fantastic. It's high Z maximizes the
photoelectric effect, efficiently absorbing those
photons. But for a high energy linac
vault? Lead isn't enough.
Well, for the primary beam you need sheer mass to handle
Compton scattering, so thick concrete is the main barrier.
Lead might be used strategically, but concrete
(20:25):
provides the bulk. And if you're above 10 MV you
also need neutron shielding. That's where the hydrogen and
concrete helps. Or adding layers of polyethylene
inside the door comes in. You have to stop photons and
neutrons. It all comes back to the physics
of these interactions. OK, let's crystallize some of
this time for some clinical pearls.
The absolute must remember points for boards and practice.
(20:47):
All right, Pearl, one photoelectric effect.
Think kilovoltage energies. Think Z cube dependence.
Think inverse E cube dependence.Think imaging contrast.
Pearl 2 Compton scattering. Think mega voltage therapy
energies. Think largely independent of Z
Think dependent on electron density.
Think roughly 1 / E dependence. Think dose deposition in tissue.
(21:07):
Pearl three pair production. Think threshold energy greater
than 1.022 MV becomes important at high MV, especially above
1015 MV. Probability increases with
energy and with Z ^2. Think high energy, high Z.
Pearl 4 Photo Disintegration. Think threshold energy around 8
to 10 MEVI Think neutron contamination in Linux operating
(21:29):
above 10 MV. Think shielding implications and
patient safety. And don't forget that rough 5050
rule for water, Photoelectric and Compton crossover around 30.
Cave, Compton and pair production crossover somewhere
around 24 to 30. Mebi gives you a quick sanity
check for which process dominates.
Excellent pearls, ready for a quick board blitz?
Let's test these concepts. Question one which photon
(21:50):
interaction process is most responsible for the difference
in image contrast between bone and soft tissue on a diagnostic
kilovoltage X-ray? OK, diagnostic KV contrast
between bone high Z and tissue low Z.
That's got to be the photoelectric effect because of
its Z cube dependence. Correct explanation.
The probability of the photoelectric effect is roughly
proportional to Z ^3 bones. Higher Z causes much greater
(22:13):
attenuation of KB photons via this effect, creating contrast
Question 2A10 mega electron Voltphoton beam is incident on
alleged shield. Lead has zeal 82.
Which photon interaction processis likely dominant within the
lead at this energy? 10 Mevi is getting up there and
lead is high Z Compton dominatesin water up to maybe 25 Mevi,
(22:34):
but pair production increases with Z ^2.
So in high Z lead pair production probably takes over
much earlier. I'll say pair production.
You got it. Explanation.
Pair production Z ^2 dependence makes it dominant over Compton
at lower energies in high Z materials compared to low Z
materials by 10. Nevian lead pair production is
likely dominant. Question 3.
Compton's scattering probabilityis primarily dependent on a
(22:57):
atomic number. ZB mass number, AC electron
density, electrons per gram, D photon energy only.
Compton, we said it's almost Z independent, but depends on how
many electrons are available to hit so C electron density.
Perfect explanation. Compton probability is largely
independent of Z but directly proportional to the number of
electrons per unit mass. Question 4.
(23:18):
Neutron contamination becomes a significant concern for linear
accelerators operating above what approximate photon energy?
A1 Mevio. B6 mevi.
C10 mevi. D25 Mevi.
That's the photo disintegration threshold we talked about.
Around 8 to 10 Bev is where it starts to matter in linac
material. So C10 maybe?
Excellent explanation. Photo disintegration creating
(23:40):
photo neutrons generally has a threshold around 810 mevi in
relevant linac materials like the target and collimators.
OK, so let's quickly summarize what we covered today.
We established that photons are indirectly ionizing.
They interact via processes thatremove them from the beam
attenuation, which follows that exponential equation.
And we made that key distinctionbetween Kermo, the kinetic
(24:02):
energy released to electrons andabsorb dose, the energy actually
deposited locally. That explained the dose buildup
effect D Max and the concept of electronic equilibrium.
Then we walked through the five key interactions.
The almost negligible Rayleigh scattering.
The photoelectric effect, dominant at KV energies with its
strong Z cube dependence, crucial for imaging.
(24:24):
Compton scattering, dominant at therapy.
MV energies depending on electron density and responsible
for dose and tissue pair production needing over 1.022
MEVI and depending on Z ^2 important at high energies.
And finally, photo disintegration kicking in above
810 mevi and causing neutron contamination.
And understanding how these interactions compete and
dominate based on photon energy and the Z of the material, well,
(24:48):
it really does underpin just about everything, doesn't it?
From why our images have contrast to how TPS algorithms
calculate dose, to how we safelydesign treatment faults.
It really does. Thinking about how these
microscopic events, these individual photon interactions,
add up to create the macroscopicdose distributions we shape and
deliver to patients, it's prettyincredible when you step back
(25:11):
and think about it, a real reminder of the fundamental
physics driving our clinical practice.
Absolutely. And building right on this, next
time we'll get into how we actually go about measuring that
absorbed dose accurately in the clinic.
We'll talk about different typesof detectors and the theories
behind them, like Brag Ray cavity theory.
Sounds great. Remember, you can find
completepracticeoralboardsover@radonsmartlearn.com.Be sure to subscribe to Radon
(25:36):
Smart Review for our next session.
Thanks for tuning in.
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