RadPhysics E16a: Radiation Protection & Regulations

Welcome back to the RadOnc Smart Review Physics Series! We've explored how radiation is produced, how it interacts, how we measure it, plan treatments, and ensure quality. Now, we circle back to a fundamental responsibility: radiation safety. In Episode P16a: Radiation Protection & Regulations, we'll review the types of radiation effects, the quantities used to measure risk (Dose Equivalent, Effective Dose), the ALARA principle, the critical regulatory dose limits for workers and the public, and touch upon managing dose from internal radioactivity.

Episode Transcript

(00:00):
All right, let's jump right in. Today, we're tackling a really
crucial area for anyone working in radiation oncology,
especially if you're navigating residency right now.
We're talking about radiation protection and the regulations
surrounding it. Absolutely fundamental stuff.
Think of this session as really digging deep to pull out the

(00:21):
absolute essentials you need to know.
Not just for passing boards, though that's important, but,
you know, for the day-to-day safety for you, your colleagues
and obviously your patients. Right, This isn't just
theoretical physics, it's the practical foundation for safe
practice. We'll cover quite a bit.
We'll look at the basic ways radiation effects tissue, how we
quantify dose for protection purposes, what happens with

(00:42):
internal emitters, the. Actual regulations, shielding
design basics, how we detect radiation.
And then really importantly, howwe handle things when they don't
go perfectly, like incidents andthe ethical framework that
guides us. Yeah, big landscape, but it's
all interconnected. This knowledge really is the
bedrock, as you said. Couldn't agree more.
OK, so let's start at the very beginning.

(01:03):
What actually happens biologically when ionizing
radiation interacts with living tissue?
I know we break it down into like 2 main categories.
That's right. Broadly speaking, we think about
stochastic effects and deterministic effects.
Let's impact stochastic first. My understanding is that the
keyword here is random or probabilistic.
Exactly. Stochastic means governed by

(01:25):
chance. These effects don't have a
definite dose threshold below which they absolutely won't
occur. Instead, the probability of the
effect occurring increases as the radiation dose increases.
And the main examples we worry about here are things like
radiation induced cancer, right?And also hereditary effects.
Precisely, cancer induction and genetic effects passed to

(01:46):
offspring are the primary stochastic risks we consider in
radiation protection. OK.
So probability goes up of dose, but what about the severity?
If someone unfortunately develops a radiation induced
cancer, is it worse if the dose was higher?
That's a critical point for stochastic effects.
The severity of the defect, if it actually happens, is
generally considered to be independent of the dose that

(02:07):
initiated it. You know, a cancer is a cancer,
pathologically speaking. Regardless of whether the
initiating dose was 10 millise Vor 100 millise V, the chance of
getting it was higher with the higher dose.
But the disease itself isn't necessarily graded by the
initial dose in that way. That's counterintuitive, but
really important. And because of that
probabilistic nature and the lack of a definite threshold.

(02:29):
For radiation protection purposes, we make a very
conservative assumption. We assume that there is no safe
dose threshold for stochastic effects.
Meaning any dose, no matter how tiny, carries some theoretical
risk. Correct.
It might be an incredibly small risk at low doses, practically
negligible compared to other life risks, but we assume it's
non 0. This linear no threshold or L&T

(02:51):
model is the basis for LRI and our dose limits for occupational
and public exposure. We're managing probability.
OK, that makes sense. For stochastic probability, no
threshold assumed severity independent.
Now, what about the other category deterministic effects?
Right deterministic effects, which you might also see called
non stochastic effects, are fundamentally different.

(03:13):
These effects do have a clear dose threshold.
Meaning you need to receive a certain minimum dose before the
effect even appears. Exactly below that specific
threshold dose for a given effect in a given tissue, you
simply won't see that clinical effect manifest.
But what happens if you do go above that threshold?
Then, unlike sarcastic effects, the severity of the

(03:33):
deterministic effect is directlyrelated to the dose received
above the threshold. The higher the dose, the more
severe the damage. You give some examples of these.
Sure, think about acute effects like skin erythema, reddening of
the skin, or disclamation, whichis skin peeling.
These only happen above certain doses and get worse with higher
doses. Longer term examples include

(03:55):
cataract formation in the lens of the eye, tissue fibrosis or
scarring, organ dysfunction or failure if enough cells are
killed, and effects on fertilitylike temporary or permanent
sterility. So these are effects where we
can clearly see a dose response relationship in terms of
severity, but only after crossing that initial threshold.
Precisely. It's usually related to

(04:15):
significant cell killing or damage impairing tissue
function, and because these effects have thresholds are dose
limits for specific organs like the lens of the eye or the skin
are set specifically to prevent these deterministic effects from
occurring. OK, So 2 very different
mechanisms and dose response relationships.
Stochastic probability, no threshold for protection

(04:37):
severity, independent deterministic threshold required
severity increases with dose above threshold.
That distinction is key. Absolutely fundamental.
Getting that straight helps understand why we have different
types of dose limits. Which brings us nicely to the
next point. How do we actually quantify
these doses and risks in a consistent way, especially when

(04:57):
dealing with different types of radiation or exposing different
parts of the body? Right.
We need specific quantities for radiation protection that go
beyond just the physical absorbed dose.
We start with absorbed dose D, which you know is the energy
deposited per unit mass. The unit is the Gray 1, Gray 1.
Gray is 1 Joule per kilogram. But a Gray of alpha particles
does more biological damage thana Gray of X-rays, right?

(05:19):
Especially for those stochastic effects.
Exactly. That's where dose equivalent H
comes in. It's designed to put different
types of radiation on an equivalent footing in terms of
their potential to cause stochastic harm.
We calculate it by taking the absorbed dose D in Gray and
multiplying it by a radiation weighting factor which we denote
as W sub R. So the formula is H equals DW

(05:42):
sub R. And the unit for dose equivalent
isn't Gray anymore, it's. Sievert SV.
That's the SI unit. You'll still sometimes see the
older unit, the R.E.M. Especially in US regulations.
The conversion is easy. One Sievert equals 100 R.E.M.
SO50 Milliseverts is five R.E.M.OK, so this W sub R factor
adjusts for the biological effectiveness.
What are some typical values? They're standardized in reports

(06:04):
like NCRP 116 for photons like X-rays and gamma rays, and for
our electrons and beta particlesW sub R is set to 1.
They are the reference radiation.
Makes sense? What about particles?
For protons with energy greater than two, maybe the West sub R
used for protection is typically2, although the actual relative
biological effectiveness or RBE can vary and sometimes be higher

(06:27):
up to around 5:00 depending on energy and endpoint.
Neutrons are more complicated. Their W sub R depends strongly
on their energy. It ranges from about 5 for low
energy thermal neutrons up to 20for high energy fast neutrons.
And the really damaging ones like alpha particles.
Alpha particles and other heavy ions, because they create very
dense ionization tracks, are given the highest W sub R value

(06:51):
of 20. They're considered 20 times more
effective at inducing stochasticeffects per Gray than photons
are. So dose equivalent helps us
compare apples and oranges. Radiation wise, one sievert of
neutrons is considered to carry the same stochastic risk as one
sievert of photons. That's the idea.
It's a protection quantity, normalizing risk across
radiation types. OK, but what if the exposure

(07:12):
isn't uniform? Like maybe just the hands get a
dose during a procedure or one organ gets irradiated in nuclear
medicine. How do we estimate the overall
risk to the whole person in those situations?
Dose equivalent is tissue specific, right?
Exactly. That's where effective dose E
comes into play. Effective dose is a calculated
quantity designed to estimate the overall stochastic risk to

(07:34):
the entire body from a non uniform radiation exposure.
How does it do that? It takes into account that
different organs and tissues have different sensitivities to
radiation induced stochastic effects, Primarily fatal cancer
risk, but also considering non fatal cancer in heredity
effects. The calculation involves summing
up the dose equivalents receivedby all the significant tissues

(07:55):
and organs in the body, but eachtissues dose equivalent is
multiplied by a specific tissue weighting factor W sub T before
summing. So the formula looks like E
equals the sum of H for each tissue times W sub T for that
tissue. Precisely, E equals the HTWOT.
The sum of all the tissue weighting factors WT is
normalized to equal 1 for the whole body.

(08:16):
What tissues are considered mostsensitive?
What gets the highest W sub T? According to the current
recommendations, the gonads testisovares have the highest
weighting factor, reflecting theconcern for hereditary effects.
Then you have a group of tissueswith relatively high
sensitivity, including red bone marrow, risk of leukemia, colon,

(08:37):
lung, stomach and breast. Have a least sensitive?
Tissues like the bone surface and the skin have much lower
tissue weighting factors. They contribute less to the
overall fatal cancer risk compared to say the Lumb or
colon for the same dose equivalent.
So by using these weighting factors, effective dose gives us
a single number also in sieverts.

(08:58):
Yes, also in Sieverts. That represents the equivalent
uniform whole body dose that would carry the same overall
stochastic risk as the actual non uniform exposure.
That's a perfect way to put it. It allows us to compare risks
from vastly different exposure scenarios, like comparing the
risk from inhaling A radionuclide that concentrates
in the lungs versus the risk from an external exposure

(09:18):
primarily to the chest. It's a risk related quantity,
not a direct physical measurement.
OK. That clarifies those key
protection quantities, dose equivalent H for radiation type,
effective dose for tissue sensitivity and non uniform
exposure. Now let's shift focus a bit.
What about situations where the radioactive material actually

(09:40):
gets inside the body? Internal dosimetry.
Right. That's a whole different
challenge because the source is now within the person,
potentially irradiating tissues continuously until it decays or
is eliminated. We talked about the amount of
radioactivity inside someone. What's the term for that?
That's called body burden. It's simply the quantity of a
specific radio nuclear present within the body at any given

(10:02):
time. In the units would be activity
units like. Exactly, micro curries or mega
becquerels MBQ I. Remember hearing about something
called the Maximal Permissible Body Burden or MPBB?
Is that still used? That's an older concept related
to earlier dose limits. It represented the activity of a
radionuclide continuously present in the body that would
result in the maximum permissible dose rate to a

(10:25):
critical organ or the whole body.
While you might encounter the term, current regulations are
generally framed around conceptslike the annual limit on intake
Ali, which is derived from the primary dose limits like the 50
milli C effective dose limit. Ali is the amount you couldn't
take in a year that would lead to that limit, but understanding
body burden as the amount present is still relevant.

(10:48):
When a radionuclide is inside the body, its activity decreases
not just from radioactive decay,but also because the body gets
rid of it. Right.
Absolutely. We have to consider both
physical decay and biological clearance.
This combined effect is described by the effect of half
life T effective so. It's different from the physical
half life T physical listed in charts.
Yes. T physical is just how long it

(11:09):
takes for half the atoms to decay radioactively biological
half life. T biological is how long it
takes for the body to eliminate half of the substance through
biological processes like excretion or metabolism.
Assuming it wasn't radioactive. The effective half life combines
these two processes. The formula that relates them is

(11:30):
1 / T effective equals 1 / T physical plus 1 / T biological.
1 / t effective equals 1 / t physical plus 1 / t biological.
OK. And looking at that formula,
since you're adding rates of removal.
The effective half life T effective will always be shorter
than either T physical or T biological alone.
Both processes are working together to reduce the amount of

(11:52):
radionuclide present in the bodyor a specific organ.
Can you give an example? Sure.
A classic one is iodine 131, used for thyroid treatments.
Its physical half life is about 8 days if it gets taken up by
the thyroid gland. The thyroid has a biological
half life for iodine of let's say around 24 days on average,
though it varies. So plugging 8 days and 24 days
into the Formula, 1 / T effective equals 18 + 124 + 124

(12:16):
+ 124 = 16. So T effective is 6 days.
So the radioactivity in the thyroid decreases with a half
life of six days faster than theeight day physical decay because
the thyroid is also clearing iodine biologically.
Exactly this is crucial for calculating the total dose
delivered over time from an internal emitter.
And how do we actually calculatethat dose to specific organs

(12:39):
from these internal sources? It seems complicated.
It can be the standard approach used especially in nuclear
medicine. Dosemetry is the Air Rd.
formalism. Air Rd.
Stands for the Medical Internal Radiation Dose Committee, which
developed the system. What's the basic idea behind
ARD? It provides A structured method
to calculate the absorbed dose. It involves identifying source

(12:59):
organs, that's where the radionuclide is actually located
or distributed, and target organs, which are the organs for
which you want to calculate the absorbed dose.
An organ can be both a source and a target, receiving dose
from the activity within itself,self dose, as well as dose from
activity and other source organsacross dose.
So it accounts for the radiationtraveling from one organ to
another. Yes, it uses things called S

(13:20):
factors or absorbed fractions that represent the fraction of
energy emitted by the source organ that gets absorbed by the
target organ. Considering the type and energy
of radiation and the geometry ofthe organs, it's quite
sophisticated but provides standardized dose estimates.
OK, switching gears slightly back to practical safety.
What if someone gets radioactivematerial on them like a skin

(13:41):
contamination? Any quick tips?
Yes, a key practical point. A clinical Pearl if you will.
If dealing with skin contamination, the standard
advice is to wash the area gently with soap and lukewarm or
cool water, not hot water. Why not hot water?
Hot water can increase blood flow to the skin surface,
potentially enhancing the systemic absorption of the

(14:02):
radioactive material through theskin into the bloodstream.
Cool or lukewarm water minimizesthis risk while still helping to
remove the contaminant. Good tip.
And you mentioned iodine 131 earlier.
Is there anything specific for potential internal exposure to
radioactive iodine? Yes, another important practical
measure. If there's a risk of inhaling

(14:23):
and ingesting significant amounts of radioactive iodine,
administering stable non radioactive potassium iodide Ki
promptly can be very effective. How does that work?
The stable iodide floods the thyroid gland, effectively
blocking the uptake of the radioactive iodine that might
enter the bloodstream. The thyroid can only take up so
much iodine at once, so if it's saturated with stable iodine,

(14:47):
the radioactive form is less likely to be absorbed and will
be excreted more quickly, dramatically reducing the dose
to the thyroid. Timing is critical, though it
needs to be given before or veryshortly after exposure.
OK, so we've covered the basic science, the dose quantities,
internal dissymmetry concepts. Now let's get into the nitty
gritty of the rules and regulations that govern our

(15:08):
daily practice. Right, the overarching
philosophy here is a alara. As low as reasonably achievable.
Exactly. It's not just about staying
below the legal limits, it's a mindset.
We have an obligation to actively minimize radiation
doses whenever possible, withoutcompromising the necessary
medical information or therapeutic effect, of course.
And the practical ways we achieve ALRI usually boil down

(15:30):
to three things. The three coordinal principles,
time, distance, and shielding. Minimize the time you spend near
a source of radiation. Maximize your distance from the
source. Remember the inverse square law?
Doubling your distance cuts the dose rate to 1/4.
It's incredibly effective. And use appropriate shielding,
placing absorbing materials likelead or concrete between you and

(15:52):
the source. Those three are the cornerstones
of practical radiation safety day-to-day.
To put the regulatory limits we're about to discuss into
context, it's probably helpful to remember how much radiation
we get just from living on Earth.
What's the typical background radiation level?
Yeah, that's a good benchmark. The average annual effective
dose from background radiation in the United States is

(16:12):
estimated to be around six milliseverts, or 600 millerram
per year. Wow, 6 milliseverts and where
does that come from? It's roughly a 5050 split
between natural sources and man made sources.
Natural sources be. The biggest single contributor
by far is radon gas, which comesfrom the natural decay of
uranium in soil and rock. Radon inhalation accounts for

(16:33):
something like 56% of the natural background dose, or well
over a third of the total background dose.
Other natural sources include cosmic radiation from space,
terrestrial radiation from radioactive elements in the
ground itself like uranium, thorium, potassium and
radionuclides naturally present inside our bodies, mainly
potassium 40. OK, so radon is the big one,

(16:55):
naturally. What about the man made half?
The largest contributor there ismedical procedures.
Diagnostic X-rays account for about 11% of the total average
background dose and nuclear medicine procedures contribute
another 4% or so. Other man made sources like
consumer products, industrial uses and fallout are much
smaller contributions nowadays. So around 3 mill SV natural, 3

(17:16):
mill SV man made mostly medical,for a total of about 6 mill SV
per year on average. Good to keep in mind when we
hear the occupational limits. Exactly, it provides
perspective. All right, let's dive into those
critical regulatory dose limits.These are the numbers set by
bodies like the NCRP and enforced by regulators like the
NRC in the US. And importantly, these limits

(17:37):
are in addition to background radiation and any medical doses
you receive yourself, right? Correct.
These limits apply specifically to dose received as a result of
licensed activities, basically your occupational exposure or
exposure to the public from those activities.
OK, first the limits for occupational limits for adults
working with radiation, what's the main annual limit?

(17:59):
The annual limit for the total effective dose equivalent TD is
50 milliseverts, 50 milliseede. That's equivalent to 5.
The R.E.M. And T includes both external
dose and internal dose from any intake.
Yes. TD is the sum of the deep dose
equivalent from external sourcesand the committed effective dose
equivalent from internal uptakesover a year.
It's the primary limit for overall stochastic risk.

(18:20):
What about specific parts of thebody like the eyes?
The annual limit for the dose equivalent to the lens of the
eye is also 50 millisieverts, 50mil SV or five R.E.M.
Per year, same as the TD limit under current USNRC regs.
Although it's worth noting some international bodies now
recommend a lower limit of 20 Millis a year averaged over five

(18:40):
years, based on newer data suggesting cataracts might occur
at lower cumulative doses than previously thought.
But the US legal limit is currently 50 Millis fee.
OK, 50 for teeth, 50 for the lens.
What about hands, feet, skin or other individual organs?
So if I'm handling sources my hands might get more dose.
Right. For deterministic effects in

(19:00):
specific tissues the limits are higher.
The annual limit for the total organ dose equivalent Tod to any
individual organ or tissue otherthan the lens, and also
specifically for the skin and extremities, hands, forearms,
feet, ankles is 500 milliseve, roots 500 milliseve or 50 R.E.M.
Per year. Wow, 10 times higher than the
effective dose limit. Yes, because those limits are

(19:21):
primarily named at preventing deterministic effects in those
tissues which have higher thresholds, rather than limiting
overall stochastic risk which drives the T to limit.
Is there a lifetime limit too? Yes, there's a cumulative limit
intended as a check. The cumulative effective dose
equivalent over a worker's lifetime should ideally not
exceed 10 millisieverts times the worker's age in years, 10

(19:41):
millisie age, or in old units one R.E.M.
Times age in years. OK, those are the occupational
limits. 50 Millisie Cat TD, 50 Millis feet lens, 500 Millis
feet organ skin extremities and 10 millisie times age
cumulative. What about members of the
public? People in areas outside the
radiation controlled zones. The limits for the public are

(20:01):
much lower, reflecting the involuntary nature of their
exposure and the larger population size.
The annual effective dose equivalent limit for continuous
or frequent public exposure is 1millisievert, 1 millisie, or 100
millisie per year. So 50 times lower than the
occupational limit. Correct, and there's also a
constraint on the dose rate. In unrestricted areas.
The dose in any one hour must not exceed 0.02 millisieverts,

(20:24):
.02 millisie and two millisiex. This prevents higher short term
exposures even if the annual total might be met.
What about pregnant workers? Once a worker declares their
pregnancy in writing, special limits apply to protect the
developing embryo or fetus. Yes, the fetus is considered
particularly sensitive to radiation.
The regulatory limit on the total dose equivalent to the
embryo fetus over the entire gestation period is 5

(20:48):
millisieverts, 5 Millis fee, or 500 R.E.M. 5 millisieverts for
the whole pregnancy. Right.
And to help ensure this limit ismet smoothly, there's also a
regulatory guidance, not a strict limit, but strong
guidance that the dose should not exceed 0 point, 5
millisieverts .5 millisie or 50 millim in any given month of the
pregnancy. OK, 5 milis fee total term limit

(21:09):
with .5 milis fee monthly guidance for declared pregnant
workers. These dose limits are absolutely
crucial to know. You absolutely have to know
these numbers. They come up constantly in
practice and on exams. 50 occupational one public, 5 for
the fierce. Those are the big annual term
effective dose numbers to nail down.
To help enforce these limits andcommunicate hazards, we use

(21:29):
specific signs. How are radiation areas defined
and signposted? The signage requirements are
based on the potential dose ratesomeone could receive if they
were in that area. The measurements are typically
specified at 30 centimeters fromthe source or surface.
OK, So what defines a radiation area?
A radiation area requires a sign, usually magenta or black
trefoil on yellow background with the words caution radiation

(21:51):
area. If the dose rate exceeds 0.05
millisieverts .05 Millis fees inone hour.
At 30 centimeters, that's five milliram per hour.
So relatively low levels, but enough that access should be
controlled. What's the next level up?
A high radiation area. This requires a sign saying
caution, high radiation area or danger high radiation area.

(22:12):
If the dose rate exceeds 1 millisieverd 1 millisie in one
hour At 30 centimeters, that's 100 milli R.E.M.
Per hour. And these areas usually have
extra controls. Right Yes.
Entry into high radiation areas typically requires specific
procedures, often involving interlocks, alarms, or direct
surveillance, to prevent unauthorized or accidental entry
when the radiation levels are high.
Is there anything beyond high radiation area?

(22:34):
Yes, the most hazardous level isa very high radiation area.
The sign usually says grave danger, very high radiation
area. This is defined based on the
absorbed dose rate, specificallywhere an individual could
receive an absorbed dose greaterthan 5 Gray 5 GI in one hour at
1m from the source or from any surface the radiation beam

(22:55):
passes through. 5 Grays in an hour?
That's an enormous, potentially lethal dose.
Actually, these areas are extremely dangerous and require
very stringent controls, usuallymultiple independent interlocks
and safety features to prevent anyone from being present when
the source is exposed. Think inside the treatment head
path or beam path directly. And before all that, we have

(23:17):
controlled areas which are areaswhere occupational exposure is
supervised and kept below limits, typically aiming for
less than one milliseed year perweek or .1 milliseed week
actually is the occupational monitored limit.
Right access is controlled, but it doesn't automatically require
a specific radiation area sign unless those dose rate
thresholds are exceeded. OK, beyond those limits and
signs, there are tons of administrative requirements for

(23:40):
handling radioactive materials safely.
What about written directives? When are they absolutely
required? Written directives are like
super prescriptions required forspecific high risk therapeutic
procedures. To ensure accuracy and prevent
errors, the NRC mandates a written directive for all Bracha
therapy procedures, regardless of dose.
Every single Brachy case needs one.

(24:01):
What about unsealed sources likeradiopharmaceuticals?
A written directive is required for the therapeutic
administration of unsealed radionuclides if the
administered activity exceeds certain thresholds, generally
defined as greater than 1.01 megabek rolls MBQ, which is 30
micro curies or if the dose delivered is considered
therapeutic. So this definitely applies to
common therapies like iodine 131for thyroid cancer or

(24:24):
hyperthyroidism, Radium 22 to 3,Zofigo, ytrium 90 microspheres,
litigium 177 therapies like Lutathera.
But probably not for standard diagnostic scans, even PTT with
F18 FTG. Correct standard diagnostic
doses like for F18 FDGPET scans typically do not require a
written directive because they fall below the therapeutic

(24:46):
threshold. Activity or considered
diagnostic, not therapeutic intent.
And what information absolutely has to be on that written
directive? It needs to be very specific.
The patient's name, the radionuclide, the prescribed
dose or dose range, the route ofadministration, the treatment
site for bracket therapy. It also includes details like
the dose per fraction, number offractions, total dose and

(25:09):
specifics about the sources likestrength and treatment time.
It has to be dated and signed byan authorized user physician
before the administration. And record keeping.
How long do we generally need tokeep records like calibrations
or leak tests? The general rule for many
routine records required by the NRC, like survey meter
calibrations, sealed source leaktests and inventories,

(25:29):
radioactive material receipt anddisposal records, is that they
must be maintained for three years.
Some records, like patient dose records or personnel dosimetry
records, often need to be kept much longer, sometimes
indefinitely or for the durationof the license.
What about the instruments we use to measure radiation?
The survey meters? How often do they need
calibration? Survey instruments must be

(25:50):
calibrated before their first use, annually thereafter, and
also following any repair that might affect their calibration.
You need documentation proving they were calibrated correctly
against a traceable standard. How do we verify the activity of
sources we use? It depends on the type.
For unsealed sources, like that therapeutic dose of I 131 liquid

(26:11):
or capsules, the activity must be directly measured in a dose
calibrator just before administration, and that
measured activity must be compared to the prescribed
activity on the written directive within specific
tolerances, usually plus -20%. For sealed sources like HDR
bracket therapy sources or teletherapy sources, the

(26:31):
activity is typically certified by the manufacturer.
The facility verifies this upon receipt.
For subsequent uses, the activity is calculated based on
radioactive decay from the initial certified value using
the known half life. And machine calibrations like
for an HDR after loader. HDR after loaders required dose
symmetric calibration quarterly,every three months and also
after every source exchange. Older teletherapy units like

(26:54):
Cobalt 60 machines require annual calibration.
Lanix, of course, have very extensive daily, monthly and
annual QA procedures mandated byregulations and professional
guidelines, including regular output calibration checks.
You mentioned sealed sources. What are the routine checks
needed for them? Two key things done
periodically, leak testing and physical inventory, both need to

(27:16):
be performed every six months for most sealed sources used in
medicine. Leak testing is to make sure
they aren't leaking radioactive material.
Exactly. A wipe test is done on the
source or its housing and the wipe is counted.
The source is considered to be leaking if the test reveals the
presence of greater than or equal to .005 micro curious CI
of removable radioactive contamination.

(27:36):
That's equivalent to 100 and 85 Beck rolls BQ. 0.005
microcarries. That's the magic number for a
failed leak test. That's the one to remember.
And the physical inventory, alsoevery six months, is just what
it sounds like physically verifying that every licensed
sealed source is accounted for and in its assigned location.

(27:56):
When can patients who've had permanent radioactive implants
like prostate seeds be released?The regulation states that a
patient with a permanent implantcan be released if the total
effective dose equivalent to anyother individual from exposure
to that patient is not likely toexceed 5 millisieverts. 5
Millisie. So it's based on the potential
dose to others, not just the activity remaining in the

(28:19):
patient. Correct.
It involves calculations based on the remaining activity, the
radionuclides, emissions and half life, and assumptions about
how close other people might be and for how long.
Often specific instructions are given to the patient to limit
close contact for a period. One more regulatory detail.
Those shipping labels on packages with radioactive
materials. White eye yellow 2, yellow 3.

(28:42):
How are they determined? They're based on the measured
radiation levels at 2 points thepackage surface and at 1m from
the surface. OK, what's white ironing?
White R is for packages with very low external radiation
levels. The dose rate at the surface
must be less than or equal to .5Milliker R each R which is blam
005 Milliker RE R&D. The radiation level at 1m must

(29:04):
be essentially undetectable or background.
Got it. Low level.
What about yellow 2? Yellow 2 is the intermediate
level. The surface dose rate must be
less than or equal to 50. Miller RER 0.5.
Miller RERER. And the dose rate at 1M must be
less than or equal to 1 LR 0.01.Miller R.
OK surface equal 5081, meter equal 1 and yellow three.

(29:26):
UL 3 is for packages exceeding either of the yellow 2 limits.
So if the surface dose rate is greater than 50 Miller RDR, the
dose rate at 1M is greater than 1 Miller ER.
It requires a yellow three label.
And that dose rate at 1M has a special name.
Yes. The dose rate in MRR at 1m is
called the transportation index TI.
For yellow 2 and yellow three packages, the TI value must be
written on the label. It helps freight handlers

(29:47):
determine safe storage distances.
That's a classic board question scenario.
Knowing those thresholds is key.Yeah, OK, that's a dense but
critical overview of the regulatory landscape.
Let's shift gears slightly againand talk about designing the
rooms themselves. Treatment room shielding.
How do we figure out how thick the walls need to be?

(30:07):
Right shielding calculations area core part of medical physics.
We need to ensure that the dose rates outside the treatment room
in occupied areas are below those public or occupational
limits we just discussed. What factors go into that
calculation? There are several key parameters
often remembered by the acronym WUTPDD.
W is for workload. This quantifies how much the

(30:30):
machine is used, typically in terms of dose delivered at a
reference point per week, PG Gray per week at 1M, or
sometimes beam on time or MU delivered per week.
It reflects the machine's usage intensity.
U is the use factor. This is the fraction of the
total workload for which the primary beam is likely to be
pointed towards a specific barrier.
Wall, floor, ceiling for secondary barriers, shielding

(30:51):
against scatter and leakage. U is always taken as one.
T is the occupancy factor. This represents the fraction of
time that the area outside the barrier being calculated is
actually occupied by people. A control console or office
would have T or one full occupancy, whereas a rooftop or
a rarely used storage closet might have a much lower T, like
1:20 or 1:40. P is the permissible dose limit.

(31:14):
This is the designed dose rate goal for the occupied area
outside the Bay Area, based on whether it's a controlled area,
typically .1 milliseie Week, or an unrestricted public area,
typically Point O2 Milliseie Week, though often designed to
be even lower. And finally, D is the distance
from the radiation source like the linac, ISIS center and
target to the point where you'recalculating the dose just
outside the barrier. So WTPD workload use occupancy,

(31:39):
permissible limit, distance. How do they fit together?
They're used to calculate the required barrier transmission
factor B. This B represents how much the
radiation intensity needs to be reduced by the barrier.
Conceptually, for a primary barrier, the formula looks
something like BPD 2 WTB. Equals PD squared over WT.
Right, the PD two term represents the dose rate limit

(32:00):
scaled by distance squared inverse square and the WT term
represents the effect of radiation output directed
towards that point. So you calculate the need of
transmission B and then use datafor concrete or lead to figure
out how many centimeters thick the barrier needs to be to
achieve that transmission. Exactly.
You use attenuation data, often expressed in terms of half value

(32:21):
layers HVLS or 10 value layers TVLS for the specific radiation
energy and shielding material tofind the thickness corresponding
to the calculated B value. What about secondary barriers
shielding from scatter and leakage?
For secondary barriers, you calculate the required thickness
separately for the leakage radiation component coming
through the lenak head and the scattered radiation component

(32:43):
scattering off the patient. Then you compare the two
required thicknesses. A common conservative rule of
thumb is if the thickness required for scatter and the
thickness required for leakage are within one TVL of each
other, you should take the thicker of the two and add 1 HVL
of extra shielding material to it.
If they differ by more than a TVL, you generally just use the

(33:03):
thicker of the two calculations.OK, so calculate both, compare,
maybe add an HVL. Now what about higher energy
machines? You mentioned neutrons earlier.
When do we need specific neutronshielding?
Neutron production becomes significant in the LENAC head
components like the target flattening filter jaws when the
accelerating electron energy exceeds about 10 megavolts 10 MV

(33:25):
due to giant dipole resonance photonuclear reactions.
So for machines running at 15 MV, 18 MV, neutron shielding is
a must. Yes, and neutrons are tricky to
shield. You need materials containing
light nuclei, especially hydrogen, to thermalize.
Slow down the fast neutrons and then a material with a high

(33:45):
neutron capture cross section like boron 10 to absorb the
thermal neutrons without producing too many high energy
capture gamma rays. What materials do we typically
use? Borated polyethylene BPE is
common. The polyethylene provides the
hydrogen for thermalization and the boron provides the neutron
capture. Sometimes concrete itself
provides sufficient shielding ifthick enough or specialized

(34:06):
concrete formulations are used. Lead is also often incorporated
mainly for the photon shielding component and to absorb capture
gammas. Does the order of materials
matter like in the maze door? It can for the main vault door,
often at the end of a maze entrance.
The typical design is BPE first on the maze side, followed by
lead on the outside. The idea is that neutrons

(34:27):
scattering down the maze get thermalized by the BPE first,
then captured. The lead then attenuates the
capture gamma rays produced in the BPE and any primary photon
scattering down the maze. BPE then lead for the door.
What about if the primary beam could hit a wall at 10 MV?
That's less common design wise, but if a primary barrier needs
neutron fielding, the order might be reversed.

(34:49):
Lead first to significantly attenuate the primary high
energy photons, followed by BPE to deal with the photo neutrons
generated in the lead and targetthe photon shielding is often
the dominant need for the primary barrier thickness
itself. Does using techniques like IMRT
which use a lot more monitor units and MU for the same
delivered dose, affect shieldingdesign?

(35:11):
Absolutely high MU techniques like IMRT or VMAT significantly
increase the beam on time for a given patient dose compared to
conventional 3D conformal treatments.
This means the leakage radiationcomponent from the linac head
becomes much more dominant in the secondary barrier
calculations. The primary barrier calculation
WT part isn't directly affected by MU per SE if the dose per

(35:33):
fraction is the same, but the secondary barrier shielding,
especially for leakage, often needs to be substantially
thicker for rooms heavily used for IMRT compared to older
designs. Workload calculations must
properly account for this increased leakage contribution.
And where exactly are we calculating the dose to when we
do these calculations? The point of interest?
The standard protection point for calculation is usually taken

(35:55):
to be 0.3 meters, 0.3 meters, orabout 1 foot beyond the finished
outer surface of the barrier being assessed.
OK, shielding design is complex but crucial.
Now, how do we actually know if radiation is present?
How do we detect and measure it?Let's talk radiation detection.
The workhorses for general survey and measurement in

(36:16):
radiation oncology are often gasfilled detectors.
How do they work fundamentally? The basic principle is
straightforward. They have a volume of gas
enclosed between two electrodes.When ionizing radiation passes
through the gas, it knocks electrons off the gas molecules,
creating pairs of positive ions and free electrons, and
electrical voltage is applied across the electrodes.

(36:37):
This electric field causes the electrons to drift towards the
positive electrode anode and thepositive ions towards the
negative electrode capode. And that movement of charge is
the signal. Exactly.
The collection of these charges creates a very small electrical
current or a pulse which can then be amplified and measured.
The key thing is that the detectors response.

(36:57):
How big the signal is for a given amount of radiation
depends dramatically on the voltage applied across those
electrodes. Right.
I remember seeing a graph of detector response versus voltage
with different regions. Can you walk us through those?
Sure. It's often called the six region
curve. At very low voltages, region 1
recombination region, the electric field is too weak.

(37:18):
The electrons and ions tend to just recombine back into neutral
molecules before they reach the electrodes.
You collect very little charge, so it's useless for detection.
OK, need more voltage, what's next?
Increase the voltage into the ion chamber region regions
again, typically 103 hundred volts.
Here, the voltage is strong enough to collect virtually all
the primary electron ion pairs created by the radiation before

(37:40):
they can recombine. Importantly, there's no
multiplication of charge in thisregion.
The collected charge is directlyproportional to the total
ionization produced by the radiation in the gas volume.
So the signal is proportional tothe energy deposited where the
dose rate. Exactly.
This makes ion chambers ideal for quantitative dose
measurements. The survey meters we use for

(38:02):
accurate dose rate readings and the thimble chambers used for
calibrating linac output all operate in this stable ion
chamber region. OK.
Ion chamber region for accurate dose?
What if we crank up the voltage more?
Now we enter the proportional region, Region 3rd, 305 hundred
volts. Here the electric field is
strong enough that the primary electrons, as they drift towards

(38:23):
the anode, gain enough energy toknock off additional electrons
from other gas molecules. This is called gas amplification
or the Townsend avalanche. The collected charge is now
multiplied, giving a larger signal.
Crucially, the total charge collected is stay proportional
to the initial number of ion pairs created by the incident
radiation event. So bigger signal, still

(38:44):
proportional. Any damages?
Yes, the larger signal makes it easier to detect low energy
radiation, and because the output is proportional to the
initial ionization, proportionalcounters can actually
distinguish between different types of radiation that create
different amounts of initial ionization, like alpha particles
versus beta particles. They're used more in laboratory

(39:05):
settings. Interesting, what's after
proportional? Keep increasing the voltage and
you hit the Geiger Mueller GM region.
Region V, 515 hundred volts. Note the outline mentions region
4 for GM, but typically proportional is third limited.
Proportional is 4 and GM is V I'll follow the common V
convention. In the GM region, the voltage is

(39:25):
so high that even a single primary ionization event
triggers a massive cascade or avalanche of ionization that
spreads along the entire length of the anode wire.
The result is a very large, easily detectable electrical
pulse. Importantly, the size of this
pulse is independent of the energy or type of the initial
radiation event that triggered it.
The whole detector volume essentially discharges.

(39:46):
So any event big or small give the same big pulse.
Pretty much. This makes GM counters extremely
sensitive for detecting the merepresence of radiation, even very
low levels. That's why they are great as
survey meters for contamination checks or finding small sources.
But if every pulse is the same size, they can't measure dose
rate accurately, right? Correct, they are not good for
quantitative dose rate measurements.

(40:08):
They basically just count events.
Also, after each pulse the detector needs a short time to
recover the dead time or refractory period during which
it can't detect another event. At high radiation levels they
can become saturated and give erroneously low readings or even
paralyze. So great for detection, poor for
measurement. OK.

(40:28):
GM for sensitive detection? Is there a region after GM?
Yes, if you keep increasing the voltage region 6 continuous
discharge region, the electric field becomes so intense that
the gas breaks down and conductscontinuously like a spark.
Even without any external radiation.
The detector is useless, possibly damaged.
So the key operating regions areion chamber for measurement and

(40:51):
GM for detection. Those are the 2 main types of
gas filled survey meters you'll encounter daily.
Are there other important detector types besides gas
filled ones? Oh, definitely.
Scintillation detectors are veryimportant, especially in nuclear
medicine and radiation monitoring.
How do they work? They use special materials
called scintillators. Common 1 is a crystal of sodium

(41:12):
iodide doped with thallium, written Nai TL.
When radiation interacts with the scintillator material, it
excites the atoms and they then de excite by emitting tiny
flashes of visible light scintillations.
This light is then detected usually by a sensitive device
called a photomultiplier tube PMT, which converts the light

(41:33):
flash into an electrical pulse and amplifies it significantly.
Are they sensitive? Extremely sensitive, especially
for detecting gamma rays. They form the basis of gamma
cameras, put ET scanners, well counters used for sample
analysis and some sensitive survey meters.
What about detecting neutrons? We talked about needing neutron
shielding. How do we detect them?
Neutron detection is trickier. Because neutrons are uncharged,

(41:56):
we usually detect them indirectly.
A common type of neutron detector uses a gas filled
counter, often operating in the proportional or GM region, but
the gas is something like boron trifluoride, VF3, or the walls
are coated with boron 10. When a thermal slow neutron is
captured by a boron 10 nucleus, it causes a nuclear reaction in

(42:17):
alpha that releases an energeticalpha particle and a lithium
nucleus. It's this charged alpha particle
that then ionizes the gas and gets detected by the counter
mechanism. So you're detecting the
secondary charged particle produced by the neutron
interaction. Other methods use proton recoil
and hydrogenous scintillators. OK, so we know how to detect
radiation. Let's move into the final
section, what happens when things go wrong and the ethical

(42:40):
principles that guide our profession.
Let's talk incidents, ethics andprofessionalism.
This is incredibly important. First, understanding what
constitutes A reportable medicalevent, sometimes called a
misadministration, according to the NRC or state regulations.
It's not just any error, right? There are specific thresholds.
Correct. For an error to be classified as
a reportable medical event, it generally needs to meet criteria

(43:02):
related to both the dose delivered and the nature of the
error. Specifically, a key threshold is
if the dose delivered differs from the prescribed dose by more
than 20% for the total dose, or if a single fraction dose is off
by more than 50%, or if the dosedelivered results in a total
organ dose equivalent Tod exceeding .5 SV 50 R.E.M.

(43:25):
To an organ or tissue, or an effective dose equivalent
exceeding .05 SCA, 5 R.E.M. And there has to be a
significant delivery error, suchas delivering the dose to the
wrong patient, using the wrong radiopharmaceutical, delivering
to the wrong body site if the difference is significant, or
using the wrong mode of treatment.
It often requires both a dose consequence and a process error.

(43:45):
So a dose error over a certain limit PLUSA specific type of
mistake. Yes, and if a medical event
occurs, the reporting requirements are strict.
The NRC must be notified by telephone no later than the next
calendar day after discovery anda detailed written report must
be submitted within 15 days. Next day phone call, 15 day
written report. Got it.
How do we learn from the stakes or near misses to prevent future

(44:09):
incidents? That's where incident learning
systems come in. A crucial one in our field is
ROILS Radiation Oncology Incident Learning System
sponsored by Astro and AAPM. What is ROILS?
It's a confidential, voluntary web-based system where radiation
oncology practices can report errors, near misses, and unsafe

(44:30):
conditions without fear of punitive action.
The data is anonymized and aggregated nationally.
And their purpose is. To identify patterns,
vulnerabilities, and common failure modes across the field.
By analyzing these events, ROILScan share lessons learned and
best practice recommendations toimprove safety for everyone.
Does the data show common causesof errors?
Yes, the material mentions that miscommunication between team

(44:52):
members is frequently identifiedas a contributing factor or root
cause in reported events. This really highlights the
importance of clear, unambiguouscommunication protocols like
read backs, standardized handoffs, and pretreatment
timeouts or huddles. Where are most potential errors
caught? Encouragingly, analysis often
shows that a significant number of potential errors are
intercepted during routine pretreatment, quality assurance

(45:15):
QA procedures like chart checks,physics plan reviews, and
machine QA. This underscores just how vital
those QA steps are as safety Nets.
And if an event does happen, a thorough analysis is needed.
Absolutely a good root cause analysis.
RCA investigates not just what happened, but why it happened,
looking at systems, factors, processes, human factors, et

(45:37):
cetera. A comprehensive RCA report
typically includes a clear definition of the event, an
analysis of causality, identifying contributing factors
and root causes, an assessment of the actual or potential
severity, process maps to visualize the workflow and
relevant data elements. Beyond incidents, there's the
broader context of professionalism and ethics.

(45:58):
What are the guiding principles?Professional societies like AAPM
have codes of ethics, and reports like TG109 address
ethics for medical physicists. The absolute cornerstone, the
prime directive, is that the patient's best interest is
primary. Everything flows from that.
Yes, all professional judgments,decisions, and actions must
prioritize the well-being and safety of the patient above all

(46:20):
other considerations, including personal convenience, financial
gain, or institutional pressures.
What about conflicts of interest?
Professionals have an ethical obligation to identify, disclose
and manage any potential conflicts of interest.
This could be financial relationships with vendors,
research funding sources, or anyother situation where personal
interests might reasonably appear to influence professional

(46:42):
judgement. Transparency is key.
And what about our scope of practice?
This is critical. We have an ethical duty to
practice only within our areas of competence based on our
education, training and experience.
Furthermore, we must ensure we have the necessary resources,
equipment, staffing, protocols to perform procedures safely and
effectively according to established standards of care.

(47:05):
So if you feel the resources areinadequate for a particular
procedure. You have an ethical obligation
to speak up. You should advocate for the
necessary resources if they aren't available.
You need to define the limits ofwhat can be done safely under
the current circumstances, or potentially recommend that the
patient be referred to another center that is adequately
equipped. You cannot ethically proceed if

(47:25):
you believe it's unsafe. That takes courage sometimes.
It does, but it's a fundamental professional responsibility.
Patient safety cannot be compromised.
What about ethics and research? Research involving human
subjects is governed by strict ethical principles, historically
codified in documents like the Nuremberg Code, the Declaration
of Helsinki, and the Belmont Report in the US.

(47:48):
Key principles include informed consent, minimizing risks,
ensuring benefits outweigh risks, and equitable subject
selection. Institutional review boards Irbs
oversee research ethics. For research involving animals,
the principle of the three Rs applies.
Replacement Use non animal alternatives whenever possible.
Reduction Use the minimum numberof animals necessary to obtain

(48:09):
valid results and refinement Minimize any pain, suffering or
distress to the animals. OK, that ties together the
technical, regulatory, and ethical dimensions.
Let's try to distill this down. If you had to pick the absolute
key takeaways from all this material, the stuff residents
really need burned into their memory for boards and practice,
what would they be? All right, rapid fire key

(48:29):
points. Let's start with dose limits.
Quick recap. Occupational Annual. 50 mill SV
TD. 50 mill SV Lens. 500 mill SVOrgan skin extremities.
Public annual 1 Mill SV Effective dose hourly .02 mill
SV in unrestricted area. Declared pregnant worker 5 mill
SV Total term to fetus .5 mill SV per month guidance.
Got it. Medical event definition and

(48:50):
reporting. Dose threshold UG .5 SV organ or
20% dose error PLUS delivery error, wrong patient, site
modality, etcetera. Report phone by next calendar
day, written within 15 days. Shielding factors and neutron
needs. Remember WTPD workload use
occupancy permissible limit distance, neutrons need
shielding above 10 MV often using boride polyethylene, BPE,

(49:12):
maybe with lead order matters for doors, BPE then LED.
Key regulations, frequencies, limits.
Sealed source leak tests and inventories every six months.
Leak test failure limit. ADFEO SL2 removable
contamination. Detectors the main difference.
Ion chambers. Quantitative doses, Dose rate
measurement, GM counters, sensitive qualitative detection.

(49:33):
Yes, No radiation. And the overarching ethics
principle. Patients best interest first
disclose conflicts. Practice within scope.
Be accountable. Perfect.
That's a great high yield summary.
Now let's try to apply some of this with a quick board review.
Blitz ready. Let's do it.
Hit me. Question one.
According to USNRC regulations, what is the annual occupational
dose limit for the total organ dose equivalent TODE to the

(49:56):
skin? A50 MLCB 150 Millis VB, 500
Millis VD one SD. OK, skin is considered an
individual organ tissue for thispurpose.
The limit for two AD to skin, hands, feet, or individual
organs is higher than the T It's500 milliseconds per year.
So that's answer C. Correct 500 Millis V or 50
R.E.M. Question 2A radioactive package

(50:17):
is monitored. The maximum reading on the
surface is 60mm AR. The reading at 1m is 2L.
Where RARR? What shipping label is required?
A white IB, Yellow 2C, Yellow 3Drequires special permit and no
label applies. All right, let's check the
thresholds. White eye needs surface .5 and
undetectable at 1m. No yellow 2 needs surface 50

(50:38):
mill RHRAND 1m yours one MRRHR. The surface reading here is 60
mill Rs, which is greater than 50.
The one you're reading is 2 LRRSwhich is greater than 1.
So it fails both conditions for yellow 2.
Therefore it must be yellow 3 because either the surface 50 OR
the 1 meter one condition is met.
Both are met here. Actually answer C.
Exactly yellow three that is thetransportation index would be
two question 3 for shilling calculator for a standard HDR

(50:59):
bracket therapy treatment room. What use factor U is typically
applied to the primary barrier calculation for the ceiling?
A1B12C14D. OK, HDR involves placing the
source inside the patient, oftennear the center of the room.
From the sources perspective, during treatment it radiates
isotropically, meaning radiationgoes out in all directions
there. So all surrounding barriers,

(51:19):
walls, floor, ceiling are potentially irradiated by the
primary source radiation, attenuated by the patient, but
still considered primary. So the use factor for all these
barriers in HD room design is typically taken as U = 1 answer
A. Perfect U equal 1 for HDR
barriers. Last question.
This one involves professionalism.

(51:39):
You are checking the treatment plan for a patient about to be
treated. You notice the prescription
signed by the physician specifies treating the left
lung, but the patient's signed consent form clearly indicates
consent was given for treating the right lung.
The planning system reflects theprescription left lung.
What is the most appropriate immediate action?
A Proceed with treatment as prescribed, as a prescription is
the legal order. B Stop the treatment process

(52:02):
immediately and notify the attending radiation oncologist
of the discrepancy. C Ask the patient to quickly
sign a new consent form for the left lung.
D Inform the radiation therapistto treat, but make a note in the
chart about the consent issue. Wow, that's a major discrepancy
and a huge safety concern. Treating the wrong site is a
serious medical event. The consent form is a critical

(52:22):
document reflecting the patient's agreement.
You absolutely cannot proceed with treatment when there's such
a fundamental conflict between the consent and the prescription
plan. Trying to get a quick new
consent under pressure is coercive and inappropriate.
Ignoring it is negligent. The only correct and ethical
action is to stop everything immediately and bring the
discrepancy to the attention of the attending position so it

(52:45):
could be resolved definitively before any treatment is
delivered. Patient safety first.
So the answer is B. Absolutely.
B Stop and resolve. That's a critical safety check
function. Well, those examples really show
how intertwined all this information is.
The physics numbers, the regulations, the ethical
decision making, it all comes together in clinical practice.
It really does, and mastering these fundamentals is just non

(53:07):
negotiable for being a safe and competent radiation oncology
professional. It's a lot to absorb, I know.
It definitely feels like a huge amount, especially when you're
studying for boards. You spend weeks cramming dose
limits, half lives W sub R factors, signage rules.
And then six months after the exam, you find yourself
thinking, wait, was the public limit one or five millisieverts?

(53:31):
And what was that leak test limit again?
Exactly. You have to keep refreshing it.
It's not just learn it once and forget it kind of stuff, you use
it or at least need access to itconstantly.
It's true. It's like building muscle memory
for safety. But you know that constant
vigilance, that habit of double checking, questioning things
that don't seem right, knowing when to stop the line, That's

(53:51):
what really builds a strong safety culture.
The regulations are the minimum standard.
The culture of safety is what truly protects patients.
Well said. This has been a really
comprehensive run through of these essential radiation
protection and regulation topics.
Hopefully breaking it down this way helps solidify some of those
key concepts. That was the goal, to provide a
focus review, hitting the highlights and the critical

(54:13):
details based on the core knowledge base in this area.
Thanks for joining us for this deep dive into radiation safety.
Stay safe, keep learning, and keep asking questions.