June 29, 2025 • 38 mins
Today, we're stepping into the treatment planning suite. We'll walk through the entire process, from patient education and simulation setup to the meticulous art of target volume delineation and the critical science of dose selection and OAR constraint application. This episode is your hands-on guide to building a high-quality rectal cancer radiation plan.
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(00:00):
Welcome to the Radonk Smart Review GI Cancer series.
Great to be here. Today we're tackling a really
fundamental topic, but one with so much nuance.
You know, rectal cancer specifically focusing on getting
from simulation to a safe treatment plan.
Yeah, it's crucial. We've laid a lot of groundwork
in previous sessions. Exactly.
(00:20):
We've talked anatomy, staging, the surgical side, those big
trials like. The total neoadjuvant therapy or
TNT shift. Right.
And all the talk about organ preservation, de escalation,
that kind of thing. So today is about putting it all
together, taking that knowledge and actually, you know, building
the plan, a high quality plan. That's the mission.
(00:41):
How do we translate understanding into practical
steps? Precisely and it's one thing to
know the rules, another to applythem so let's let's use a case
to make it real. Good idea.
OK picture Mr. Chen. He's 68.
He's got a clinical T3 N 1 mid rectal adenocarcinoma.
It's located say 8 centimeters from the anal verge.
And importantly, his MRI, it shows a clear circumferential
(01:02):
resection margin, The CRM. So he's lined up for neoadjuvant
long course chemo radiation. Standard scenario.
Very standard. So our job starting now is
designing his plan safe, effective and it begins really
before he even gets on the table.
It starts with simulation setup.Yes, the simulation.
It really is the foundation, isn't it?
Get that wrong and everything that follows is compromised.
(01:23):
You got it. And that foundation starts well,
it starts with patient prep and education even before the SIM
appointment. Making sure Mr. Chen does
exactly what to do. For reproducibility, right and
OAR Sperry. Absolutely.
Reproducibility is the name of the game here.
So for Mr. Chen and really for all our pelvic radiation
patients we have these specific protocols, bladder and rectal
(01:45):
let's. Breakdown the bladder one first.
OK, so one hour before the simulation appointment and then
one hour before every single treatment session.
The instruction is first urinate, empty the bladder
completely, then right after that, drink about two cups of
water. That's around 20 oz.
Got it. Urinate, then drink.
Yep, and then this is the key part.
(02:06):
Do not urinate again until afterthe scan or the treatment is
done. Hold it.
And there's a very specific reason for that, isn't there?
It's not arbitrary. Not at all.
A consistently full bladder. It acts like an internal spacer.
Think of it like like a water balloon inflating in the pelvis,
OK, It gently pushes the small bowel loops, which are really
(02:29):
mobile. You know, it pushes them
superiorly kind of up and out ofthe main radiation field.
Oh, so it protects the small bowel?
Exactly, it reduces the volume of small bowel getting high
dose. Super important for cutting down
side effects, both the acute ones and the late ones.
OK, so that handles the bladder,the internal spacer.
What about the rectum itself? That needs to be consistent too,
right? This is important so the rectal
(02:50):
protocol we ask patients to try and have a bowel move treatment
within say 4 hours before simulation and each treatment.
Empty the rectum. Try to, Yeah.
And then about an hour before they come in, try to pass any
gas, any flattus, get the air out.
Makes sense? Now here's a really important
point. We have distressed to patients
no prophylactic enemas. Right, I've heard residents ask
(03:11):
about that. Why specifically no enemas if
the goal is an empty rectum? Seems counterintuitive.
It does seem that way, but enemas can be unpredictable.
They can sometimes cause the rectum to get overly distended
or cause spasms, or just fill inconsistently.
I see our goal isn't just an empty rectum, it's a
consistently empty, or at least consistently filled rectum.
(03:32):
We want it to be in the same position, same shape every
single day. Natural emptying is usually more
consistent than an enema induced1.
Because if the rectum is distended differently each day.
Then the target, the tumor itself, can shift position.
You could end up under treating the tumor or over treating
normal tissue. It's a setup uncertainty we can
minimize with good prep. Geographic miss is a real
(03:53):
concern. Clear, so bladder full, rectum
empty and consistent. What about the chemo side if Mr.
Chen's on oral capacitibine withhis radiation?
Any special instructions here? Good point.
Yeah. For capacitabine, we tell him
take it only on the days you getradiation, usually Monday
through Friday. No chemo on the weekends.
OK. And this is a big clinical
(04:15):
Pearl. Avoid taking propon pump
inhibitors, PPI's like omeprazole at the same time.
Why is that? There's some concern and data
suggesting PPI's can interfere with how well capacitabine gets
absorbed. So it might reduce the chemo's
effectiveness. We want that chemo working
alongside the radiation. Important interaction to catch
and labs. Oh yeah, standard weekly labs
(04:36):
during long course, a CBC, complete blood count and a CMP
comprehensive metabolic panel just to monitor for any
hematologic or metabolic issues from the treatment.
Keep an eye on things. OK, patient prep nailed down.
Now let's get Mr. Chen on the table.
Positioning, imaging choices. These really matter.
What's the go to for him? So for most rectal cancer
(04:57):
patients like Mr. Chen, we strongly prefer simulating them
prone, face down and specifically on a belly board.
Explain the Belly Board rationale.
Right, so the belly board has anopening.
When the patient lies prone, their abdomen sags down into
that opening and gravity becomesyour friend.
It helps pull those mobile smallbowel loops anteriorly away from
(05:19):
the back and also superiorly up towards the head.
So again, moving the bowel out of the target zone.
Exactly. Compared to lying supine face
up, the prone belly board position consistently reduces
the amount of small bowel that gets hit with higher radiation
doses. It's a really effective OAR
sparing technique built right into the setup.
(05:40):
But what if someone just can't tolerate being prone?
It happens. Is supine OK?
Yeah, absolutely. Supine is definitely an
acceptable alternative if prone isn't tolerated.
OK. And there's one situation where
supine is actually preferred. That's if you need to cover the
inguinal lymph nodes. Which you wouldn't typically
need for Mr. Chen's mid rectal tumor.
Correct, but if you had, say, a very low tumor involving the
(06:02):
anal canal, you might need inguinal coverage.
In that case supine is better, often with the legs slightly
apart. Kind of a frog leg position to
open up the groin area for the radiation fields.
Got it. So prone, preferred, supine,
acceptable or needed for infidelnodes.
Now the scan itself was the protocol.
We get a CT scan and we use intravenous contrast.
Helps delineate vessels in tissues.
(06:25):
And how far down do you scan? Critically important, you have
to scan low enough to clearly include the anal verge, the
absolute bottom of the target area.
Do you ever mark The Verge externally?
Yeah, sometimes placing a small external marker like a
radiopaque wire right at the anal verge can be really
helpful. Just ensures you can pinpoint it
accurately on the planning scan.OK.
And for image guidance during the actual treatments.
(06:47):
Standard practice now is daily kilovolt imaging, KV imaging
like orthogonal X-rays that verifies the Bony alignment is
correct each day. Simple quick setup check.
And cone beam CT we. Typically do weekly cone beam CT
or CBCT. This gives us a 3D picture right
before treatment. It's crucial for checking things
that KV imaging can't see well, like.
(07:09):
Bladder filling rectal distension.
Exactly. Is the bladder still full like
it was at SIM? Is the rectum still relatively
empty? If we see significant changes
week to week on the CBCT, we might need to adapt the plan.
Makes sense? What about fiducials?
Sometimes we hear about implanting markers.
Yeah, that's usually for more specialized situations if you're
doing something like SBRT, stereotactic body radiation
(07:31):
therapy with very tight margins or perhaps in an organ
preservation case where precision is absolutely
paramount. Then fiducials help.
Then implanting small fiducial markers near the tumor before
the simulation can be really beneficial.
They act like tiny GPS beacons, allowing for incredibly precise
daily targeting using the CBCT or even KV imaging.
(07:53):
Better accuracy for those highlyconformal plans.
OK, so Mr. Chen is prepped, positioned, scanned.
Now we move to the planning computer.
The art of contouring, defining the targets.
You mentioned a wedding cake approach.
Yeah, I like that analogy. You start with the smallest,
most definite layer, the known disease, and then you build
outwards, adding layers for areas at microscopic risk.
(08:14):
It's systematic. So layer one, the gross tumor
volume GTV for Mr. Jeff, right? The GTV has two parts for him.
First, the GTV primary. This is the tumor you can
actually see. We define it using everything we
have, the physical exam findings, what the endoscopist
saw, and critically, the diagnostic imaging.
And this is where the MRI fusioncomes in.
Absolutely essential. We strongly, strongly recommend
(08:36):
fusing the diagnostic pelvic MRI, particularly the T2
weighted sequences, to the simulation CT scan.
Why T2 specifically? T2 imaging gives you fantastic
soft tissue contrast in the pelvis.
It's the best way to see the extent of the tumor within the
rectal wall, whether it's threatening that circumferential
margin, and helps identify suspicious nodes.
(08:56):
Fusing it gives you the anatomical truth laid over your
planning scan optimal GTV definition.
GTV primary? What's the other GTV part?
GTV nodes. So any lymph nodes that look
frankly involved on the imaging,maybe they're large, have
suspicious features on MRI. Those get contoured individually
as GTV nodes. Got it.
Innermost layer done. Now the next layer out the
(09:18):
clinical target volume standard risks CTVSR.
This is a bigger volume, gettingtypically 45 Gray.
Exactly. The CTVSR is designed to cover
the entire primary tumor region,plus areas at risk for
microscopic spread that we can'tnecessarily see.
It includes the entire rectum itself, the mesorectum, and
those key regional lymphatic bases for long course like Mr.
(09:40):
Chen's. Yeah, 45 grey in 25 fractions is
the standard dose to this volume.
OK, so walk us through contouring that CTVSR, what are
the key structures and boundaries for his mid rectal
tumor? First you contour the entire
mesorectum. That's the fatty envelope
surrounding the rectum. It contains the primary tumor,
yes, but also all those perico lymph nodes.
(10:00):
You got to cover the whole thing.
OK, the whole mesorectum and fear your border.
For a mid rectal tumor, you typically go down to the pelvic
floor muscles. Anatomically you can often see
it as where the mesorectal fat kind of disappears or tapers.
As a rule of thumb, you want to be at least 2cm below the lowest
extent of the gross tumor, whichever boundary is more
inferior. And if the tumor was much lower
(10:21):
down, say involving the sphincter, or if an APR and a
domino peroneal resection was planned.
Good point. Then the inferior border changes
significantly if an APR is planned or the sphincter is
involved. You absolutely must include the
entire anal sphincter complex. The tissues around the anus and
those dyskirectal phosse, the fatty spaces on either side much
(10:44):
broader coverage inferiorly. OK, superior border from Mr.
Chen. Superiorly, we generally extend
up to the L5S1 inner space, the start of the sacrum, or again,
make sure you're at least 2cm above the highest extent of the
gross tumor, whichever is more superior.
Anterior border towards the bladder.
You include all of the mesorectum anteriorly.
(11:04):
And here's a practical tip, a clinical Pearl for planning.
Always add about a 1 to 1.5cm margin into the posterior aspect
of the bladder wall. Why end of the bladder?
It accounts for day-to-day variations.
Bladder filling isn't always perfect.
The rectum might shift slightly.That little margin into the
bladder ensures your anterior mesorectal target remains
(11:24):
covered even with those small shifts.
It's a safety margin. Makes sense laterally.
Laterally you generally extend just a few millimeters beyond
the levator anti muscles and crucially you must include the
internal iliac lymph node basin.And the elective nodal volumes
within that CTVSR, Which ones are standard?
OK. For elective coverage in the
(11:46):
CTVSR, definitely the pre sacralnodes.
You cover that entire pre sacralspace.
Think of it as about a 1 centimeter rind of tissue just
anterior to the sacral bone. Very important area for
recurrence, right? Then the internal iliac nodes,
we typically cover these by drawing about a 7mm margin
around the internal iliac vessels, starting where they
(12:06):
branch off the common iliacs andfollowing them down.
And the obturator nodes which are down lower in that same
region are also included in thisstandard volume.
So meserectum, precicle space, internal iliacs, opterators,
that's the core CTVSR. That's the standard package for
a tumor like Mr. Chen's. Now you mentioned earlier
special or conditional nodal considerations.
These aren't included from Mr. Chen, but when do they get?
(12:28):
Right, good distinction. These are exceptions, not the
rule. External iliac notes you only
include those if you have AT4 tumor that's directly invading
into anterior pelvic organs likethe bladder or the prostate.
The tumor has to be growing forward significantly.
OK, and inguinal nodes. Inguinal nodes are only included
if the tumor extends below the dentate line in the anal canal
(12:51):
or significantly involves the anal canal itself.
Usually this means the tumor is very low, maybe less than 4
centimeters from the animal purge.
It's about the drainage pathwaysfor those very low tumors.
So those are conditional based on specific invasion patterns.
Got it. Now back to the wedding cake.
We've got the GTV, the CTVSR. What's the next layer?
The boost volume? Yep, the final clinical target
(13:12):
volume layer, the CTV high risk or CTVHR.
This is the volume that gets thehigher dose the boost.
How do we define that? At a minimum, the CTVHR includes
the GTV primary and any GTV nodes, plus you add a margin of
about 1.5 to 2cm in the superiorand inferior directions.
And within those superior inferior boundaries, it should
(13:33):
encompass the entire rectum and the the rectum at those levels.
And don't forget the presacral space or that sacral hollow
behind the rectum at those levels that needs to be in the
boost volume to. What if it's AT4?
Tumor invading something, ah. Yes, for AT4 tumor you'd also
add a 1 to 2cm margin into whatever adjacent organ is being
(13:55):
invaded as part of that high risk volume.
I've also heard some experts advocate for a slightly
different, maybe more comprehensive boost approach.
Yes, there's definitely a schoolof thought, and I tend to agree
with it, that strongly considersboosting the entire mesorectum
and the adjacent presacral spaceto the full boost dose.
The whole thing. The whole mesorectum from top to
bottom is defined in the esser volume plus the presacral space.
(14:18):
What this often looks like is a volume pretty similar in shape
to the CTVSR, but importantly you don't include the elective
superior nodal coverage like upper internal iliacs or the
wider lateral margins in that boost.
Why do that? The rationale is to deliver the
highest dose to the entire anatomical compartment where the
primary tumor arose and its mostimmediate drainage, really
(14:41):
maximizing the chance of local control in the highest risk area
without unnecessarily boosting more distant elective nodes.
Interesting approach. OK, So we have our CT VS
defined. The final step is the planning
target volume PTV. What's the purpose of that
margin? The PTV margin is that final
expansion around the CT VS. It's purely A geometric margin
(15:02):
to account for uncertainties. Things like small errors and
daily patient setup, organ motion during treatment, maybe
slight breathing effects. It's the safety net.
Exactly. It ensures that despite these
small, unavoidable day-to-day variations, the prescribed dose
actually covers the intended clinical target volume.
For modern setups using daily image guidance like KV or
(15:23):
CBCTAPTV, margin of about 0.5 to1 centimeter around the CT VS is
pretty typical. Great volumes defined.
Now let's talk dose and fractionation.
Highly individualized, you said,but there are common schemes.
Back to Mr. Chen, Neo adjuvant, Long course chemo radiation.
What's the standard prescription?
OK. For preoperative long course,
the standard is the CTVSR. That larger volume gets 45 Gray
(15:48):
delivered in 1.8 Gray fractions.That's 25 fractions total.
OK. Then the CTVHR, the boost volume
is typically taken to a total dose of 50.4 Gray, also using
1.8 Gray fractions. So that's an additional 3
fractions for 28 fractions total.
Standard for T3 or no positive disease like his. 50.4 and 28
(16:08):
fractions. That's the classic long course
dose. That's the classic.
But you mentioned IMRT earlier and in this simultaneous
integrated boost or SIB approach, that's becoming really
common, isn't it? How does that work?
SIB is a very elegant and efficient way to do it,
especially with IMRT or VMAT techniques, instead of treating
the 45 grey volume first, then doing a separate plan and
(16:29):
treatment phase for the boost. You do it all at once.
You deliver different doses to different target volumes
simultaneously in the same fraction using a single plan.
So for Mr. Chen an example sub approach might be deliver 50
grey total dose at 2 greys per fraction to the PTVHR the high
risk boost volume. OK.
Higher dose per fraction there. Right.
And at the same time in the sametreatment session deliver 45
(16:53):
grey total dose maybe at 1.8 grey per fraction to the PTVSR,
the standard risk volume excluding the HR volume, all
done in say 25 fractions total. So you finish treatment faster
than the sequential boost method.
Exactly. That's one of the big
advantages. Can you give us that analogy
again? The oven one?
(17:13):
It was helpful. Oh right.
Think of Sib like a really advanced multi zone oven.
You know, instead of baking yourmain course like a roast chicken
at 1 temp, taking it out, then cranking up the heat to bake
potatoes separately. Yeah, with SIB, it's like
putting both the chicken and thepotatoes in the oven at the same
time. The oven is smart enough to
deliver precisely the right amount of heat to each zone, so
(17:35):
the chicken cooks perfectly at its temperature and the potatoes
cook perfectly at their slightlyhigher temperature, all
simultaneously. Finishing together.
More efficient. Way more efficient, faster, more
precise heat distribution. Same idea with sieve for
radiation doses. And the benefits go beyond just
saving time, right? Oh definitely, reducing the
overall treatment time, like going from 28 down to maybe 25
(17:57):
fractions is great for patient convenience and compliance.
Last time coming in, Fewer trips.
Fewer trips plus reducing treatment time might have a
biological advantage, potentially reducing the chance
for tumor cells to repopulate during treatment.
Interesting. And from a planning standpoint,
optimizing 1 SIP plan can often lead to more conformal dose
distributions compared to summing to separate plans.
(18:19):
You can potentially achieve better OAR sparing and get a
smoother, more controlled dose fall off between the high dose
and low dose regions. It's just a cleaner, more
integrated approach. OK, SIB makes a lot of sense for
a long course. What about the other major
neoadjuvant approach, short course radiation?
How does that compare? Totally different beast.
(18:40):
Short course radiation therapy, sometimes called the Swedish
style, involves delivering 25 grey total dose but in only 5
fractions. Wow, 5 grey per fraction.
Exactly. A much higher dose per fraction
delivered over just one week. It targets the CTVSR, that whole
elective pelvic volume. And concurrent chemo with short
course. Typically no.
(19:01):
Short course is usually radiation alone and another key
difference, there's generally noprimary tumor boost incorporated
into the short course regimen. It's just 25 and five to the
elective volume. Very different philosophy.
OK, shifting gears to post operative radiation.
Different scenario entirely. When do we use this?
Post op radiation comes into play mainly if a patient had
(19:24):
surgery upfront, maybe because their initial staging didn't
seem to warrant neoadjuvant therapy.
But then the final pathology report comes back with some
adverse features. Like what kind of features?
Things like positive surgical margins, that's a big one.
Or maybe close margins, extensive lymphovascular
invasion, or perhaps a higher T or end stage than expected, like
finding positive nodes, pathologic N1 or N2 even if the
(19:47):
margins were negative. OK.
So if margins are negative but there are other adverse
features, say PT 3 and 0 with LVI or maybe PT 1-2 and one,
what's the dose? In that situation, negative
margins but other risk factors. The standard is PTVSR gets 45
Gray at 1.8 Gray per fraction. Then you add a boost to the
(20:09):
PTVHR which is typically the surgical bed area deemed at
highest risk. That boost usually goes to 54 to
55.8 Gray. Total dose using 1.8 Gray
fractions. So a higher boost than typical
neoadjuvant if margins were negative?
A bit higher, yeah, reflecting the post operative setting.
But what if the margins were positive or if there was
(20:30):
actually gross residual disease left behind after surgery that
needs more dose, right? Absolutely critical difference.
If you have positive margins or gross residual disease, you need
to be much more aggressive with the dose to that area.
The PTVSR still gets 45 Grays at1.8 Gray per fraction, but the
boost to the PTVHR, the area of positive margin or gross disease
(20:50):
needs to go significantly higher.
We're talking 59.4 to 60 Gray total dose. 60 Gray, How do you
deliver that? You can use conventional 1.8
Gray fractions which would take you up to around 59.4 Gray or
increasingly common use in SIB approach or slightly larger
fractions like 2 Grays per fraction for the boost phase to
(21:10):
get you up to 60 Gray, maybe even a bit higher in select
cases. The goal is maximizing local
control where you know disease was left behind.
That's a big dose difference. Now for patients who've had an
APR that abdominoperineal resection, what's the special
consideration at simulation and for planning?
Key thing for APR, you must wirethe perineal scar at the time of
(21:32):
simulation. Plays a radiopaque wire along
the external scar. Why?
Because that scar represents theinferior extent of the surgical
bed in the perineum, an area at risk for recurrence, you need to
reliably identify it on the planning scan.
Then that scar area, usually with about a 1.5cm margin, needs
to be included in the target volume, typically receiving
(21:54):
around 45 Gray. And technique wise for post APR
cases. IMRT is generally preferred in
the post operative setting, especially after an APR.
The reason is that after removing the rectum, the small
bowel often falls down into the empty pelvic space.
Right into the radiation field. Exactly, IMRT gives you the
conformality you need to treat the target volumes while
(22:16):
carefully sculpting the dose away from those sensitive small
bowel loops. Much harder to do well with
older 3D conformal techniques inthis situation.
Makes sense. One last dose question.
Lateral nodes. Sometimes they need a boost.
Yes, this is a specific situation if a patient has
enlarged lateral pelvic lymph nodes, say bigger than 7mm
(22:36):
initially or maybe still enlarged like over 4mm even
after neo event chemiradiation and if those nodes are not going
to be surgically removed. Then you boost.
Them then you'd consider a radiation boost specifically to
those persistent unresected lateral nodes.
The dose target is usually around 60 greys delivered using
conformal techniques like IMRT or SBRT.
(22:57):
It's targeting known bulky nodaldisease that hasn't been
cleared. OK.
We've covered targeting the tumor extensively, but just as
important, maybe more so sometimes, is protecting the
normal tissues safety. First, let's talk organ at risk
constraints. The small bowel was always the
big one in pelvic radiation, right?
Absolutely. Minimizing toxicity is
paramount, and small bowel toxicity, both acute and late,
(23:21):
can significantly impact qualityof life.
So when we talk constraints, we're usually looking at
individual small bowel loops or sometimes a composite structure
called the bowel bag. Let's focus on the loops for
now, OK? What are the key numbers for
loops? To limit acute and late grade
two to three GI toxicity, we aimto keep the V-35 Gray.
That's the volume of small bowelloops receiving 35 Gray or more
(23:43):
less than about 150 cubic centimeters CCS.
Some references say even lower, maybe less than 40 a hundred CCS
if possible. OK V 35 less than 150CC.
What about higher doses? The V45 Gray volume getting 45
Gray or more should be even lower, ideally less than 60 CCS,
maybe even less than 20 CCS. If you can achieve it, keep high
dose off the bowel. Now, wasn't there a specific
(24:06):
Pearl about the V30 from a studyrelated to late toxicity?
Yes, excellent point to bring upfrom the Abraham PRO paper in
2020. A really useful guideline for
late grade 3 or higher small bowel toxicity, again looking at
individual loops, is to keep theV30 Gray volume getting 30 Gray
or more below 200 cubic centimeters.
(24:27):
V30 less than 200CC for late grade 3 plus spread.
Exactly, and for late grade 2 orhigher toxicity, the
recommendation was even tighter.Keep the V30 Gray below 100
cubic centimeters. These V30 constraints are really
valuable for long term bowel health.
Great numbers to remember. What about the bladder?
We use it as a spacer, but we don't want to irritate it too
much. Constraints.
(24:48):
Right. For acute cystitis prevention,
we look at dose volume parameters, the D 50%, meaning
the dose received by 50% of the bladder volume should generally
be kept less than 35 to 45 grey.D50 Less than 45 grey.
And the D 5% dose to 5% of the volume, which reflects a small
high dose region should be less than 50 to 56 grey.
(25:09):
Trying to avoid hotspots. Yeah, femoral heads, risk of
fracture later on. Yeah, We need to protect those
for late femoral head fracture risk.
The D 50% should be less than 30to 45 grey and the D 5% less
than 44 to 55 grey. Keep the dose down, especially
the mean dose if possible. Bone marrow next.
We're treating a lot of pelvic bone.
(25:31):
Hematologic toxicity is a concern.
Big concern, especially with concurrent chemo.
We look at the entire pelvic bone marrow sacrum, iliac
crests, the acetabula, proximal femurs to limit acute grade 3 or
higher hematologic toxicity. The goal is a mean dose to this
a whole volume of less than 20 to 30 Gray.
Mean dose less than 30 J Any volume constraints?
(25:53):
Yes, the V10 Gray, the volume getting 10 Gray or more should
ideally be less than 70 to 90%. Basically try to keep some
marrow spared from even low doseradiation to maintain blood
production. Got it.
What about genitalia? Skin toxicity.
Yeah, for the external genitalia, penis and scrotum in
men, vulva in women to limit acute grade 2 or higher skin
reactions, the D 50% should generally be less than 20 to 35
(26:16):
Gray. And the gluteal cleft, that skin
can get quite irritated. It sure can.
We often contour the gluteal cleft specifically, usually as
about a 5mm rind of skin in the intergluteal fold.
Same goal as genitalia D 50% less than 20 to 35 Gray to
minimize grade 2 plus skin toxicity.
(26:36):
And finally, for female patients, vaginal stenosis is a
potential lead effect, any constraint for the vaginal.
Yes, preserving sexual function is important.
We often contour the vaginal wall, sometimes as a 5mm rind
around where a vaginal dilator would be placed to reduce the
risk of late vaginal stenosis. The D 50% to this structure
should be kept less than 30 to 40 grey.
(26:58):
OK, a lot of constraints to juggle, but all crucial for
safety. It's a balancing act.
Maximize tumor dose, minimize normal tissue dose.
That's the core challenge. All right, let's shift gears and
distill this down. Time for some clinical pearls.
These are the absolute must knowhigh yield facts to lock away
for exams and practice. Love it Pearl #1 simulation
prep. Remember the mantra full
(27:20):
bladder, empty rectum BFER? We'll come back to that.
It displaces small bowel, improves reproducibility, and
remember, a prone on a belly board is generally preferred.
Pearl 2 CCVSR components for that standard mid rectal T3 and
one like Mr. Chen. Your 45 grey CTVSR must include
these three things. The entire mesorectum, the
(27:41):
entire pre sequel space, and theinternal iliac lymph nodes.
Don't forget any of them. Pearl 3.
New adjuvant dose fractionation Know the standards Long course
typically 50.4 grey and 28 fractions, short course 25 Gray
5 fractions and remember simultaneous integrated boost
SIB is a common efficient way todeliver long course often
(28:01):
reducing treatment time. Pearl 4.
Post operative boost dose is keyafter the initial 45 Gray to the
pelvis. Post op, if you have positive
margins or gross residual disease, you must boost that
area to 59.4 to 60 Gray for adequate local control.
Don't underdose positive margins.
Pearl 5 The small bowel V30 constraint super important for
(28:22):
late toxicity To minimize late grade 3 or higher small bowel
problems, keep the volume of individual small bowel loops
getting 30 Gray V30 Gray below 200 cubic centimeters.
Drill that number in V30 less than 200CC.
And Pearl 6 conditional nodal coverage don't automatically
include everything. External iliacs are only for T4
tumors invading anteriorly. Inguinols are only for tumors
(28:45):
invading the anal canal or belowthe dentate line.
Know when to add them and when not to.
Excellent pearls. And for that simulation prep,
let's try a mnemonic. How about BFER?
BFER bladder full empty rectum. Exactly BFER simple reminder for
just placing small bowel and getting that consistent set up.
BFER lock it in. Nice.
(29:05):
OK, Now let's move into applyingthis knowledge in the clinical
decision making segment. We've got the facts, the
constraints, how do we use them Back to Mr. Chen T3 in one mid
rectal tumor, clear CRM, gettingneoadjuvant long course from a
planning perspective, what drives that choice and what
flexibility or restrictions doesit give us?
Great question. So the T3 in one status itself
(29:26):
is the primary driver for neoadjuvant therapy.
Even with a clear CRM on MRI, the node positivity puts him at
high risk for recurrence withoutpreoperative treatment.
Long course chemo RT is the standard approach in North
America for this stage, aiming to maximize tumor shrinkage and
sterilize microscopic disease before surgery.
(29:46):
OK. So the staging dictates the
neoadjuvant approach. How does choosing long course
impact our planning choices? Choosing long course gives us
well time. We have five to six weeks of
daily treatments. This duration makes techniques
like IMRT and SIB very feasible and advantageous.
We can really leverage the technology to deliver highly
conformal dose distributions, carefully sparing the OA Rs over
(30:08):
many fractions. This one enables more
sophisticated planning. It does.
If you were getting short course, for instance, the
planning would be simpler, likely 3D conformal, because the
biological effect relies more onthe high dose per fraction over
just five days. Long course allows for and often
demands more intricate IMRT planning to safely deliver that
50.4 gichi or equivalent SIB dose.
(30:29):
Does it restrict us in any way? The main restriction is simply
the complexity and the need for meticulous adherence to those
OAR constraints we just discussed.
You have to meet those small bowel, bladder, femoral head
limits while ensuring full target coverage.
The flexibility comes in how youachieve that sequential boost
versus SIB, IMRT, beam angles, optimization parameters.
(30:50):
Lots of tools in the toolbox with long course IMRT.
And zooming out, how do these specific planning decisions take
part? The technique, The precision
meeting constraints directly impact Mr. Chen, the overall
outcome and his potential futuremanagement pathways.
It's absolutely fundamental. The goal of neoadjuvant therapy
(31:11):
isn't just to tick a box. It's to achieve the best
possible pathologic response, ideally downstaging the tumor,
maybe even achieving a complete response while minimizing
treatment toxicity. Which opens up options later.
Exactly. Meticulous planning that
maximizes tumor dose while rigorously sparing OA Rs sets
the stage for better surgical outcomes, potentially allowing
for sphincter preservation if the tumor shrinks away from it,
(31:32):
or just resulting in an easier, safer operation with fewer
complications. And what about organ
preservation? We hear so much about that now.
That's where precision planning becomes even more critical.
If Mr. Chen has a fantastic response to this long course
chemo, RT, maybe even a clinicalcomplete response, he might
become a candidate for a wash and wait or organ preservation
(31:52):
strategy, potentially avoiding major surgery altogether.
But that's only really feasible if the radiation was delivered
safely, minimizing long term bowel, bladder and sexual
dysfunction. The quality of the radiation
plan directly enables or disables these less invasive
future options. It underpins the entire modern
philosophy of rectal cancer management.
(32:13):
Hit the target hard. Protect the patient.
Maximize their future prospect for both cure and quality of
life. Excellent connection planning
isn't just technical, it's strategic for the patient's
entire journey. OK, time to test ourselves.
Let's hit the board blitz. Ready case 150 year old woman
T3N1 mid rectal adenocarcinoma 8centimeters from anal verge
(32:34):
getting long course neoadjuvant chemo RT.
Which of these is an essential component of her CTVSR?
The 45 Gray volume. A Entire pararectal fat, pre
sacral space and common iliac nodes.
B Entire mesorectum, pre sacral space and internal iliac nodes.
C only the primary tumor plus 2cm margin and external iliac
(32:54):
nodes, or D only the primary tumor and inguinal nodes.
OK, thinking back to our CVSR pearls.
For a standard mid rectal T3 N 1you absolutely need the
mesorectum, the presacral space,and the internal iliac nodes.
Common iliacs are usually too high, External iliacs and
inguinals are conditional. So the best answer has to be.
B is correct. Entire mesorectum, presacral
space and internal iliac nodes are the standard components.
(33:16):
Well reasoned case. 27 year old man gets postoperative radiation
to 45. Gray pathology showed a positive
circumferential resection margin.
CRM plus what's the most appropriate boost dose to the
tumor bed for this positive margin?
A 5.4 guy total 50.4 guy B 10.8 guy Total 55.8 GI C 14.4 to 15
(33:37):
point O guy total 59.4 to 60 GI or D20 GI total 65 GI.
OK. Positive margin post op perle
force that we need to boost higher.
For positive margins, 50.4 is standard neoadjuvant, 55.8 is
more. For negative margins post op, we
need to get close to 60 Grays, so 14.4 to 15 Gray additional
dose sounds right. That gets us at 59.4 to 60 grade
(33:58):
total answer CC. Is absolutely correct, you need
that higher dose 59.4 to 60 grade eye total to adequately
address a positive surgical margin in the postoperative
setting. Excellent recall, fine feeling
good final one case 3 During daily image guidance for long
course pelvic RT, the CBCT consistently shows a
significantly distended rectum despite the patient following
(34:19):
bowel prep instructions. What potential consequence is
this most likely to cause? A increased dose to bladder, B
Increased dose to small bowel. C Increased dose to femoral
heads or. D Geographic miss of the primary
tumor. Distended rectum and we talked
about this during SIM prep. If the rectum shape and position
(34:40):
change drastically day-to-day because it's full of stool or
gas, it's going to push the target, the rectal wall and
tumor around. While it might slightly shift
the bladder bowel, the biggest direct risk is that the tumor
itself moves out of the high dose region.
So geographic miss answer DD. Is the answer.
While dose to OA Rs might changeslightly, the primary concern
(35:00):
with inconsistent rectal fillingor significant distension is the
potential for a geographic miss of the rectal target itself due
to its variable position. That's why consistent prep is so
vital. You nailed the blitz all right.
Now stepping back again, we focus heavily on the how to of
planning today, but you mentioned controversies in
advance dances earlier. We know the landscape of rectal
cancer is shifting with TNT organ preservation.
(35:23):
How does this meticulous plan building we've detailed fit into
those bigger picture philosophies and ongoing
discussions? It's the absolute linchpin.
You can't seriously talk about advanced strategies like TNT
followed by watch and wait or deescalating surgery or using SPRT
for boost or primary treatment in select cases without having
the ability to deliver radiationwith extreme precision and
(35:44):
conformality. The.
Plan enables the strategy. Precisely, think about organ
preservation. The whole premise relies on
achieving a complete clinical response with chemoradiation and
ensuring that the long term functional outcome, bowel
function, bladder function, sexual function will be
acceptable. Hopefully better than after
radical surgery that requires minimizing dose to those
(36:04):
critical organs which demands advanced planning techniques
like IMRT, VMAT, SIB, and rigorous adherence to OAR
constraints. So the precision allows for
better functional outcomes, which makes de escalation more
palatable. Exactly the controversy might
be, you know, how much dose is needed.
Can we shorten the course even more?
Can we use highly focused radiation like SBRT more
(36:25):
broadly? But underpinning all those
questions is the assumption thatwe can actually deliver the
intended dose distribution accurately and safely.
Advances in imaging, motion management and adaptive
radiotherapy, where we adjust the plan during the treatment
course based on changes in anatomy, are, while pushing this
forward, building directly on the meticulous planning
(36:45):
principles we discussed. It's about refining our ability
to hit the target harder while being even kinder to the
surrounding tissues. It really highlights how
planning isn't just a technical exercise, it's integral to the
evolving therapeutic strategy for rectal cancer.
Couldn't have said it better myself.
OK, let's try and wrap this up with a quick summary of the key
takeaways from today. I'll start optimal rectal cancer
(37:08):
treatment planning absolutely begins way before treatment with
meticulous simulation and thorough patient preparation
that BFER protocol. And then accurate target volume
delineation following that wedding cake approach from GTV
up through CTVSR&CTVHR is just critical for ensuring you're
treating the right areas to achieve local control.
(37:30):
Next, remember that dose and fractionation are always
tailored to the specific clinical situation.
Neoadjuvant long course versus short course post operative
boost for adverse features or positive margins and techniques
like SIB can offer real advantages in efficiency and
conformality for long course. And overarching all of this
rigorous adherence to organ at risk constraints is absolutely
(37:53):
essential. Protecting the small bowel,
bladder, bone marrow and other structures is paramount for
minimizing toxicity and preserving quality of life.
Safety first, always. A fantastic summary.
It covers the journey from SIM to safety checks.
And remember, this detailed planning is the engine driving
many of the exciting advances we're seeing in rectal cancer
care. Well said.
To keep sharpening your skills, you can complete practice oral
(38:15):
boards over at radonsmartlearn.com.
That's radonsmartlearn.com. And be sure to subscribe to
Radon Smart Review so you catch our next session.
Thanks so much for tuning in. Thanks everyone.
Welcome to the Radonk Smart Review GI Cancer series.
Great to be here. Today we're tackling a really
fundamental topic, but one with so much nuance.
You know, rectal cancer specifically focusing on getting
from simulation to a safe treatment plan.
Yeah, it's crucial. We've laid a lot of groundwork
in previous sessions. Exactly.
(00:20):
We've talked anatomy, staging, the surgical side, those big
trials like. The total neoadjuvant therapy or
TNT shift. Right.
And all the talk about organ preservation, de escalation,
that kind of thing. So today is about putting it all
together, taking that knowledge and actually, you know, building
the plan, a high quality plan. That's the mission.
(00:41):
How do we translate understanding into practical
steps? Precisely and it's one thing to
know the rules, another to applythem so let's let's use a case
to make it real. Good idea.
OK picture Mr. Chen. He's 68.
He's got a clinical T3 N 1 mid rectal adenocarcinoma.
It's located say 8 centimeters from the anal verge.
And importantly, his MRI, it shows a clear circumferential
(01:02):
resection margin, The CRM. So he's lined up for neoadjuvant
long course chemo radiation. Standard scenario.
Very standard. So our job starting now is
designing his plan safe, effective and it begins really
before he even gets on the table.
It starts with simulation setup.Yes, the simulation.
It really is the foundation, isn't it?
Get that wrong and everything that follows is compromised.
(01:23):
You got it. And that foundation starts well,
it starts with patient prep and education even before the SIM
appointment. Making sure Mr. Chen does
exactly what to do. For reproducibility, right and
OAR Sperry. Absolutely.
Reproducibility is the name of the game here.
So for Mr. Chen and really for all our pelvic radiation
patients we have these specific protocols, bladder and rectal
(01:45):
let's. Breakdown the bladder one first.
OK, so one hour before the simulation appointment and then
one hour before every single treatment session.
The instruction is first urinate, empty the bladder
completely, then right after that, drink about two cups of
water. That's around 20 oz.
Got it. Urinate, then drink.
Yep, and then this is the key part.
(02:06):
Do not urinate again until afterthe scan or the treatment is
done. Hold it.
And there's a very specific reason for that, isn't there?
It's not arbitrary. Not at all.
A consistently full bladder. It acts like an internal spacer.
Think of it like like a water balloon inflating in the pelvis,
OK, It gently pushes the small bowel loops, which are really
(02:29):
mobile. You know, it pushes them
superiorly kind of up and out ofthe main radiation field.
Oh, so it protects the small bowel?
Exactly, it reduces the volume of small bowel getting high
dose. Super important for cutting down
side effects, both the acute ones and the late ones.
OK, so that handles the bladder,the internal spacer.
What about the rectum itself? That needs to be consistent too,
right? This is important so the rectal
(02:50):
protocol we ask patients to try and have a bowel move treatment
within say 4 hours before simulation and each treatment.
Empty the rectum. Try to, Yeah.
And then about an hour before they come in, try to pass any
gas, any flattus, get the air out.
Makes sense? Now here's a really important
point. We have distressed to patients
no prophylactic enemas. Right, I've heard residents ask
(03:11):
about that. Why specifically no enemas if
the goal is an empty rectum? Seems counterintuitive.
It does seem that way, but enemas can be unpredictable.
They can sometimes cause the rectum to get overly distended
or cause spasms, or just fill inconsistently.
I see our goal isn't just an empty rectum, it's a
consistently empty, or at least consistently filled rectum.
(03:32):
We want it to be in the same position, same shape every
single day. Natural emptying is usually more
consistent than an enema induced1.
Because if the rectum is distended differently each day.
Then the target, the tumor itself, can shift position.
You could end up under treating the tumor or over treating
normal tissue. It's a setup uncertainty we can
minimize with good prep. Geographic miss is a real
(03:53):
concern. Clear, so bladder full, rectum
empty and consistent. What about the chemo side if Mr.
Chen's on oral capacitibine withhis radiation?
Any special instructions here? Good point.
Yeah. For capacitabine, we tell him
take it only on the days you getradiation, usually Monday
through Friday. No chemo on the weekends.
OK. And this is a big clinical
(04:15):
Pearl. Avoid taking propon pump
inhibitors, PPI's like omeprazole at the same time.
Why is that? There's some concern and data
suggesting PPI's can interfere with how well capacitabine gets
absorbed. So it might reduce the chemo's
effectiveness. We want that chemo working
alongside the radiation. Important interaction to catch
and labs. Oh yeah, standard weekly labs
(04:36):
during long course, a CBC, complete blood count and a CMP
comprehensive metabolic panel just to monitor for any
hematologic or metabolic issues from the treatment.
Keep an eye on things. OK, patient prep nailed down.
Now let's get Mr. Chen on the table.
Positioning, imaging choices. These really matter.
What's the go to for him? So for most rectal cancer
(04:57):
patients like Mr. Chen, we strongly prefer simulating them
prone, face down and specifically on a belly board.
Explain the Belly Board rationale.
Right, so the belly board has anopening.
When the patient lies prone, their abdomen sags down into
that opening and gravity becomesyour friend.
It helps pull those mobile smallbowel loops anteriorly away from
(05:19):
the back and also superiorly up towards the head.
So again, moving the bowel out of the target zone.
Exactly. Compared to lying supine face
up, the prone belly board position consistently reduces
the amount of small bowel that gets hit with higher radiation
doses. It's a really effective OAR
sparing technique built right into the setup.
(05:40):
But what if someone just can't tolerate being prone?
It happens. Is supine OK?
Yeah, absolutely. Supine is definitely an
acceptable alternative if prone isn't tolerated.
OK. And there's one situation where
supine is actually preferred. That's if you need to cover the
inguinal lymph nodes. Which you wouldn't typically
need for Mr. Chen's mid rectal tumor.
Correct, but if you had, say, a very low tumor involving the
(06:02):
anal canal, you might need inguinal coverage.
In that case supine is better, often with the legs slightly
apart. Kind of a frog leg position to
open up the groin area for the radiation fields.
Got it. So prone, preferred, supine,
acceptable or needed for infidelnodes.
Now the scan itself was the protocol.
We get a CT scan and we use intravenous contrast.
Helps delineate vessels in tissues.
(06:25):
And how far down do you scan? Critically important, you have
to scan low enough to clearly include the anal verge, the
absolute bottom of the target area.
Do you ever mark The Verge externally?
Yeah, sometimes placing a small external marker like a
radiopaque wire right at the anal verge can be really
helpful. Just ensures you can pinpoint it
accurately on the planning scan.OK.
And for image guidance during the actual treatments.
(06:47):
Standard practice now is daily kilovolt imaging, KV imaging
like orthogonal X-rays that verifies the Bony alignment is
correct each day. Simple quick setup check.
And cone beam CT we. Typically do weekly cone beam CT
or CBCT. This gives us a 3D picture right
before treatment. It's crucial for checking things
that KV imaging can't see well, like.
(07:09):
Bladder filling rectal distension.
Exactly. Is the bladder still full like
it was at SIM? Is the rectum still relatively
empty? If we see significant changes
week to week on the CBCT, we might need to adapt the plan.
Makes sense? What about fiducials?
Sometimes we hear about implanting markers.
Yeah, that's usually for more specialized situations if you're
doing something like SBRT, stereotactic body radiation
(07:31):
therapy with very tight margins or perhaps in an organ
preservation case where precision is absolutely
paramount. Then fiducials help.
Then implanting small fiducial markers near the tumor before
the simulation can be really beneficial.
They act like tiny GPS beacons, allowing for incredibly precise
daily targeting using the CBCT or even KV imaging.
(07:53):
Better accuracy for those highlyconformal plans.
OK, so Mr. Chen is prepped, positioned, scanned.
Now we move to the planning computer.
The art of contouring, defining the targets.
You mentioned a wedding cake approach.
Yeah, I like that analogy. You start with the smallest,
most definite layer, the known disease, and then you build
outwards, adding layers for areas at microscopic risk.
(08:14):
It's systematic. So layer one, the gross tumor
volume GTV for Mr. Jeff, right? The GTV has two parts for him.
First, the GTV primary. This is the tumor you can
actually see. We define it using everything we
have, the physical exam findings, what the endoscopist
saw, and critically, the diagnostic imaging.
And this is where the MRI fusioncomes in.
Absolutely essential. We strongly, strongly recommend
(08:36):
fusing the diagnostic pelvic MRI, particularly the T2
weighted sequences, to the simulation CT scan.
Why T2 specifically? T2 imaging gives you fantastic
soft tissue contrast in the pelvis.
It's the best way to see the extent of the tumor within the
rectal wall, whether it's threatening that circumferential
margin, and helps identify suspicious nodes.
(08:56):
Fusing it gives you the anatomical truth laid over your
planning scan optimal GTV definition.
GTV primary? What's the other GTV part?
GTV nodes. So any lymph nodes that look
frankly involved on the imaging,maybe they're large, have
suspicious features on MRI. Those get contoured individually
as GTV nodes. Got it.
Innermost layer done. Now the next layer out the
(09:18):
clinical target volume standard risks CTVSR.
This is a bigger volume, gettingtypically 45 Gray.
Exactly. The CTVSR is designed to cover
the entire primary tumor region,plus areas at risk for
microscopic spread that we can'tnecessarily see.
It includes the entire rectum itself, the mesorectum, and
those key regional lymphatic bases for long course like Mr.
(09:40):
Chen's. Yeah, 45 grey in 25 fractions is
the standard dose to this volume.
OK, so walk us through contouring that CTVSR, what are
the key structures and boundaries for his mid rectal
tumor? First you contour the entire
mesorectum. That's the fatty envelope
surrounding the rectum. It contains the primary tumor,
yes, but also all those perico lymph nodes.
(10:00):
You got to cover the whole thing.
OK, the whole mesorectum and fear your border.
For a mid rectal tumor, you typically go down to the pelvic
floor muscles. Anatomically you can often see
it as where the mesorectal fat kind of disappears or tapers.
As a rule of thumb, you want to be at least 2cm below the lowest
extent of the gross tumor, whichever boundary is more
inferior. And if the tumor was much lower
(10:21):
down, say involving the sphincter, or if an APR and a
domino peroneal resection was planned.
Good point. Then the inferior border changes
significantly if an APR is planned or the sphincter is
involved. You absolutely must include the
entire anal sphincter complex. The tissues around the anus and
those dyskirectal phosse, the fatty spaces on either side much
(10:44):
broader coverage inferiorly. OK, superior border from Mr.
Chen. Superiorly, we generally extend
up to the L5S1 inner space, the start of the sacrum, or again,
make sure you're at least 2cm above the highest extent of the
gross tumor, whichever is more superior.
Anterior border towards the bladder.
You include all of the mesorectum anteriorly.
(11:04):
And here's a practical tip, a clinical Pearl for planning.
Always add about a 1 to 1.5cm margin into the posterior aspect
of the bladder wall. Why end of the bladder?
It accounts for day-to-day variations.
Bladder filling isn't always perfect.
The rectum might shift slightly.That little margin into the
bladder ensures your anterior mesorectal target remains
(11:24):
covered even with those small shifts.
It's a safety margin. Makes sense laterally.
Laterally you generally extend just a few millimeters beyond
the levator anti muscles and crucially you must include the
internal iliac lymph node basin.And the elective nodal volumes
within that CTVSR, Which ones are standard?
OK. For elective coverage in the
(11:46):
CTVSR, definitely the pre sacralnodes.
You cover that entire pre sacralspace.
Think of it as about a 1 centimeter rind of tissue just
anterior to the sacral bone. Very important area for
recurrence, right? Then the internal iliac nodes,
we typically cover these by drawing about a 7mm margin
around the internal iliac vessels, starting where they
(12:06):
branch off the common iliacs andfollowing them down.
And the obturator nodes which are down lower in that same
region are also included in thisstandard volume.
So meserectum, precicle space, internal iliacs, opterators,
that's the core CTVSR. That's the standard package for
a tumor like Mr. Chen's. Now you mentioned earlier
special or conditional nodal considerations.
These aren't included from Mr. Chen, but when do they get?
(12:28):
Right, good distinction. These are exceptions, not the
rule. External iliac notes you only
include those if you have AT4 tumor that's directly invading
into anterior pelvic organs likethe bladder or the prostate.
The tumor has to be growing forward significantly.
OK, and inguinal nodes. Inguinal nodes are only included
if the tumor extends below the dentate line in the anal canal
(12:51):
or significantly involves the anal canal itself.
Usually this means the tumor is very low, maybe less than 4
centimeters from the animal purge.
It's about the drainage pathwaysfor those very low tumors.
So those are conditional based on specific invasion patterns.
Got it. Now back to the wedding cake.
We've got the GTV, the CTVSR. What's the next layer?
The boost volume? Yep, the final clinical target
(13:12):
volume layer, the CTV high risk or CTVHR.
This is the volume that gets thehigher dose the boost.
How do we define that? At a minimum, the CTVHR includes
the GTV primary and any GTV nodes, plus you add a margin of
about 1.5 to 2cm in the superiorand inferior directions.
And within those superior inferior boundaries, it should
(13:33):
encompass the entire rectum and the the rectum at those levels.
And don't forget the presacral space or that sacral hollow
behind the rectum at those levels that needs to be in the
boost volume to. What if it's AT4?
Tumor invading something, ah. Yes, for AT4 tumor you'd also
add a 1 to 2cm margin into whatever adjacent organ is being
(13:55):
invaded as part of that high risk volume.
I've also heard some experts advocate for a slightly
different, maybe more comprehensive boost approach.
Yes, there's definitely a schoolof thought, and I tend to agree
with it, that strongly considersboosting the entire mesorectum
and the adjacent presacral spaceto the full boost dose.
The whole thing. The whole mesorectum from top to
bottom is defined in the esser volume plus the presacral space.
(14:18):
What this often looks like is a volume pretty similar in shape
to the CTVSR, but importantly you don't include the elective
superior nodal coverage like upper internal iliacs or the
wider lateral margins in that boost.
Why do that? The rationale is to deliver the
highest dose to the entire anatomical compartment where the
primary tumor arose and its mostimmediate drainage, really
(14:41):
maximizing the chance of local control in the highest risk area
without unnecessarily boosting more distant elective nodes.
Interesting approach. OK, So we have our CT VS
defined. The final step is the planning
target volume PTV. What's the purpose of that
margin? The PTV margin is that final
expansion around the CT VS. It's purely A geometric margin
(15:02):
to account for uncertainties. Things like small errors and
daily patient setup, organ motion during treatment, maybe
slight breathing effects. It's the safety net.
Exactly. It ensures that despite these
small, unavoidable day-to-day variations, the prescribed dose
actually covers the intended clinical target volume.
For modern setups using daily image guidance like KV or
(15:23):
CBCTAPTV, margin of about 0.5 to1 centimeter around the CT VS is
pretty typical. Great volumes defined.
Now let's talk dose and fractionation.
Highly individualized, you said,but there are common schemes.
Back to Mr. Chen, Neo adjuvant, Long course chemo radiation.
What's the standard prescription?
OK. For preoperative long course,
the standard is the CTVSR. That larger volume gets 45 Gray
(15:48):
delivered in 1.8 Gray fractions.That's 25 fractions total.
OK. Then the CTVHR, the boost volume
is typically taken to a total dose of 50.4 Gray, also using
1.8 Gray fractions. So that's an additional 3
fractions for 28 fractions total.
Standard for T3 or no positive disease like his. 50.4 and 28
(16:08):
fractions. That's the classic long course
dose. That's the classic.
But you mentioned IMRT earlier and in this simultaneous
integrated boost or SIB approach, that's becoming really
common, isn't it? How does that work?
SIB is a very elegant and efficient way to do it,
especially with IMRT or VMAT techniques, instead of treating
the 45 grey volume first, then doing a separate plan and
(16:29):
treatment phase for the boost. You do it all at once.
You deliver different doses to different target volumes
simultaneously in the same fraction using a single plan.
So for Mr. Chen an example sub approach might be deliver 50
grey total dose at 2 greys per fraction to the PTVHR the high
risk boost volume. OK.
Higher dose per fraction there. Right.
And at the same time in the sametreatment session deliver 45
(16:53):
grey total dose maybe at 1.8 grey per fraction to the PTVSR,
the standard risk volume excluding the HR volume, all
done in say 25 fractions total. So you finish treatment faster
than the sequential boost method.
Exactly. That's one of the big
advantages. Can you give us that analogy
again? The oven one?
(17:13):
It was helpful. Oh right.
Think of Sib like a really advanced multi zone oven.
You know, instead of baking yourmain course like a roast chicken
at 1 temp, taking it out, then cranking up the heat to bake
potatoes separately. Yeah, with SIB, it's like
putting both the chicken and thepotatoes in the oven at the same
time. The oven is smart enough to
deliver precisely the right amount of heat to each zone, so
(17:35):
the chicken cooks perfectly at its temperature and the potatoes
cook perfectly at their slightlyhigher temperature, all
simultaneously. Finishing together.
More efficient. Way more efficient, faster, more
precise heat distribution. Same idea with sieve for
radiation doses. And the benefits go beyond just
saving time, right? Oh definitely, reducing the
overall treatment time, like going from 28 down to maybe 25
(17:57):
fractions is great for patient convenience and compliance.
Last time coming in, Fewer trips.
Fewer trips plus reducing treatment time might have a
biological advantage, potentially reducing the chance
for tumor cells to repopulate during treatment.
Interesting. And from a planning standpoint,
optimizing 1 SIP plan can often lead to more conformal dose
distributions compared to summing to separate plans.
(18:19):
You can potentially achieve better OAR sparing and get a
smoother, more controlled dose fall off between the high dose
and low dose regions. It's just a cleaner, more
integrated approach. OK, SIB makes a lot of sense for
a long course. What about the other major
neoadjuvant approach, short course radiation?
How does that compare? Totally different beast.
(18:40):
Short course radiation therapy, sometimes called the Swedish
style, involves delivering 25 grey total dose but in only 5
fractions. Wow, 5 grey per fraction.
Exactly. A much higher dose per fraction
delivered over just one week. It targets the CTVSR, that whole
elective pelvic volume. And concurrent chemo with short
course. Typically no.
(19:01):
Short course is usually radiation alone and another key
difference, there's generally noprimary tumor boost incorporated
into the short course regimen. It's just 25 and five to the
elective volume. Very different philosophy.
OK, shifting gears to post operative radiation.
Different scenario entirely. When do we use this?
Post op radiation comes into play mainly if a patient had
(19:24):
surgery upfront, maybe because their initial staging didn't
seem to warrant neoadjuvant therapy.
But then the final pathology report comes back with some
adverse features. Like what kind of features?
Things like positive surgical margins, that's a big one.
Or maybe close margins, extensive lymphovascular
invasion, or perhaps a higher T or end stage than expected, like
finding positive nodes, pathologic N1 or N2 even if the
(19:47):
margins were negative. OK.
So if margins are negative but there are other adverse
features, say PT 3 and 0 with LVI or maybe PT 1-2 and one,
what's the dose? In that situation, negative
margins but other risk factors. The standard is PTVSR gets 45
Gray at 1.8 Gray per fraction. Then you add a boost to the
(20:09):
PTVHR which is typically the surgical bed area deemed at
highest risk. That boost usually goes to 54 to
55.8 Gray. Total dose using 1.8 Gray
fractions. So a higher boost than typical
neoadjuvant if margins were negative?
A bit higher, yeah, reflecting the post operative setting.
But what if the margins were positive or if there was
(20:30):
actually gross residual disease left behind after surgery that
needs more dose, right? Absolutely critical difference.
If you have positive margins or gross residual disease, you need
to be much more aggressive with the dose to that area.
The PTVSR still gets 45 Grays at1.8 Gray per fraction, but the
boost to the PTVHR, the area of positive margin or gross disease
(20:50):
needs to go significantly higher.
We're talking 59.4 to 60 Gray total dose. 60 Gray, How do you
deliver that? You can use conventional 1.8
Gray fractions which would take you up to around 59.4 Gray or
increasingly common use in SIB approach or slightly larger
fractions like 2 Grays per fraction for the boost phase to
(21:10):
get you up to 60 Gray, maybe even a bit higher in select
cases. The goal is maximizing local
control where you know disease was left behind.
That's a big dose difference. Now for patients who've had an
APR that abdominoperineal resection, what's the special
consideration at simulation and for planning?
Key thing for APR, you must wirethe perineal scar at the time of
(21:32):
simulation. Plays a radiopaque wire along
the external scar. Why?
Because that scar represents theinferior extent of the surgical
bed in the perineum, an area at risk for recurrence, you need to
reliably identify it on the planning scan.
Then that scar area, usually with about a 1.5cm margin, needs
to be included in the target volume, typically receiving
(21:54):
around 45 Gray. And technique wise for post APR
cases. IMRT is generally preferred in
the post operative setting, especially after an APR.
The reason is that after removing the rectum, the small
bowel often falls down into the empty pelvic space.
Right into the radiation field. Exactly, IMRT gives you the
conformality you need to treat the target volumes while
(22:16):
carefully sculpting the dose away from those sensitive small
bowel loops. Much harder to do well with
older 3D conformal techniques inthis situation.
Makes sense. One last dose question.
Lateral nodes. Sometimes they need a boost.
Yes, this is a specific situation if a patient has
enlarged lateral pelvic lymph nodes, say bigger than 7mm
(22:36):
initially or maybe still enlarged like over 4mm even
after neo event chemiradiation and if those nodes are not going
to be surgically removed. Then you boost.
Them then you'd consider a radiation boost specifically to
those persistent unresected lateral nodes.
The dose target is usually around 60 greys delivered using
conformal techniques like IMRT or SBRT.
(22:57):
It's targeting known bulky nodaldisease that hasn't been
cleared. OK.
We've covered targeting the tumor extensively, but just as
important, maybe more so sometimes, is protecting the
normal tissues safety. First, let's talk organ at risk
constraints. The small bowel was always the
big one in pelvic radiation, right?
Absolutely. Minimizing toxicity is
paramount, and small bowel toxicity, both acute and late,
(23:21):
can significantly impact qualityof life.
So when we talk constraints, we're usually looking at
individual small bowel loops or sometimes a composite structure
called the bowel bag. Let's focus on the loops for
now, OK? What are the key numbers for
loops? To limit acute and late grade
two to three GI toxicity, we aimto keep the V-35 Gray.
That's the volume of small bowelloops receiving 35 Gray or more
(23:43):
less than about 150 cubic centimeters CCS.
Some references say even lower, maybe less than 40 a hundred CCS
if possible. OK V 35 less than 150CC.
What about higher doses? The V45 Gray volume getting 45
Gray or more should be even lower, ideally less than 60 CCS,
maybe even less than 20 CCS. If you can achieve it, keep high
dose off the bowel. Now, wasn't there a specific
(24:06):
Pearl about the V30 from a studyrelated to late toxicity?
Yes, excellent point to bring upfrom the Abraham PRO paper in
2020. A really useful guideline for
late grade 3 or higher small bowel toxicity, again looking at
individual loops, is to keep theV30 Gray volume getting 30 Gray
or more below 200 cubic centimeters.
(24:27):
V30 less than 200CC for late grade 3 plus spread.
Exactly, and for late grade 2 orhigher toxicity, the
recommendation was even tighter.Keep the V30 Gray below 100
cubic centimeters. These V30 constraints are really
valuable for long term bowel health.
Great numbers to remember. What about the bladder?
We use it as a spacer, but we don't want to irritate it too
much. Constraints.
(24:48):
Right. For acute cystitis prevention,
we look at dose volume parameters, the D 50%, meaning
the dose received by 50% of the bladder volume should generally
be kept less than 35 to 45 grey.D50 Less than 45 grey.
And the D 5% dose to 5% of the volume, which reflects a small
high dose region should be less than 50 to 56 grey.
(25:09):
Trying to avoid hotspots. Yeah, femoral heads, risk of
fracture later on. Yeah, We need to protect those
for late femoral head fracture risk.
The D 50% should be less than 30to 45 grey and the D 5% less
than 44 to 55 grey. Keep the dose down, especially
the mean dose if possible. Bone marrow next.
We're treating a lot of pelvic bone.
(25:31):
Hematologic toxicity is a concern.
Big concern, especially with concurrent chemo.
We look at the entire pelvic bone marrow sacrum, iliac
crests, the acetabula, proximal femurs to limit acute grade 3 or
higher hematologic toxicity. The goal is a mean dose to this
a whole volume of less than 20 to 30 Gray.
Mean dose less than 30 J Any volume constraints?
(25:53):
Yes, the V10 Gray, the volume getting 10 Gray or more should
ideally be less than 70 to 90%. Basically try to keep some
marrow spared from even low doseradiation to maintain blood
production. Got it.
What about genitalia? Skin toxicity.
Yeah, for the external genitalia, penis and scrotum in
men, vulva in women to limit acute grade 2 or higher skin
reactions, the D 50% should generally be less than 20 to 35
(26:16):
Gray. And the gluteal cleft, that skin
can get quite irritated. It sure can.
We often contour the gluteal cleft specifically, usually as
about a 5mm rind of skin in the intergluteal fold.
Same goal as genitalia D 50% less than 20 to 35 Gray to
minimize grade 2 plus skin toxicity.
(26:36):
And finally, for female patients, vaginal stenosis is a
potential lead effect, any constraint for the vaginal.
Yes, preserving sexual function is important.
We often contour the vaginal wall, sometimes as a 5mm rind
around where a vaginal dilator would be placed to reduce the
risk of late vaginal stenosis. The D 50% to this structure
should be kept less than 30 to 40 grey.
(26:58):
OK, a lot of constraints to juggle, but all crucial for
safety. It's a balancing act.
Maximize tumor dose, minimize normal tissue dose.
That's the core challenge. All right, let's shift gears and
distill this down. Time for some clinical pearls.
These are the absolute must knowhigh yield facts to lock away
for exams and practice. Love it Pearl #1 simulation
prep. Remember the mantra full
(27:20):
bladder, empty rectum BFER? We'll come back to that.
It displaces small bowel, improves reproducibility, and
remember, a prone on a belly board is generally preferred.
Pearl 2 CCVSR components for that standard mid rectal T3 and
one like Mr. Chen. Your 45 grey CTVSR must include
these three things. The entire mesorectum, the
(27:41):
entire pre sequel space, and theinternal iliac lymph nodes.
Don't forget any of them. Pearl 3.
New adjuvant dose fractionation Know the standards Long course
typically 50.4 grey and 28 fractions, short course 25 Gray
5 fractions and remember simultaneous integrated boost
SIB is a common efficient way todeliver long course often
(28:01):
reducing treatment time. Pearl 4.
Post operative boost dose is keyafter the initial 45 Gray to the
pelvis. Post op, if you have positive
margins or gross residual disease, you must boost that
area to 59.4 to 60 Gray for adequate local control.
Don't underdose positive margins.
Pearl 5 The small bowel V30 constraint super important for
(28:22):
late toxicity To minimize late grade 3 or higher small bowel
problems, keep the volume of individual small bowel loops
getting 30 Gray V30 Gray below 200 cubic centimeters.
Drill that number in V30 less than 200CC.
And Pearl 6 conditional nodal coverage don't automatically
include everything. External iliacs are only for T4
tumors invading anteriorly. Inguinols are only for tumors
(28:45):
invading the anal canal or belowthe dentate line.
Know when to add them and when not to.
Excellent pearls. And for that simulation prep,
let's try a mnemonic. How about BFER?
BFER bladder full empty rectum. Exactly BFER simple reminder for
just placing small bowel and getting that consistent set up.
BFER lock it in. Nice.
(29:05):
OK, Now let's move into applyingthis knowledge in the clinical
decision making segment. We've got the facts, the
constraints, how do we use them Back to Mr. Chen T3 in one mid
rectal tumor, clear CRM, gettingneoadjuvant long course from a
planning perspective, what drives that choice and what
flexibility or restrictions doesit give us?
Great question. So the T3 in one status itself
(29:26):
is the primary driver for neoadjuvant therapy.
Even with a clear CRM on MRI, the node positivity puts him at
high risk for recurrence withoutpreoperative treatment.
Long course chemo RT is the standard approach in North
America for this stage, aiming to maximize tumor shrinkage and
sterilize microscopic disease before surgery.
(29:46):
OK. So the staging dictates the
neoadjuvant approach. How does choosing long course
impact our planning choices? Choosing long course gives us
well time. We have five to six weeks of
daily treatments. This duration makes techniques
like IMRT and SIB very feasible and advantageous.
We can really leverage the technology to deliver highly
conformal dose distributions, carefully sparing the OA Rs over
(30:08):
many fractions. This one enables more
sophisticated planning. It does.
If you were getting short course, for instance, the
planning would be simpler, likely 3D conformal, because the
biological effect relies more onthe high dose per fraction over
just five days. Long course allows for and often
demands more intricate IMRT planning to safely deliver that
50.4 gichi or equivalent SIB dose.
(30:29):
Does it restrict us in any way? The main restriction is simply
the complexity and the need for meticulous adherence to those
OAR constraints we just discussed.
You have to meet those small bowel, bladder, femoral head
limits while ensuring full target coverage.
The flexibility comes in how youachieve that sequential boost
versus SIB, IMRT, beam angles, optimization parameters.
(30:50):
Lots of tools in the toolbox with long course IMRT.
And zooming out, how do these specific planning decisions take
part? The technique, The precision
meeting constraints directly impact Mr. Chen, the overall
outcome and his potential futuremanagement pathways.
It's absolutely fundamental. The goal of neoadjuvant therapy
(31:11):
isn't just to tick a box. It's to achieve the best
possible pathologic response, ideally downstaging the tumor,
maybe even achieving a complete response while minimizing
treatment toxicity. Which opens up options later.
Exactly. Meticulous planning that
maximizes tumor dose while rigorously sparing OA Rs sets
the stage for better surgical outcomes, potentially allowing
for sphincter preservation if the tumor shrinks away from it,
(31:32):
or just resulting in an easier, safer operation with fewer
complications. And what about organ
preservation? We hear so much about that now.
That's where precision planning becomes even more critical.
If Mr. Chen has a fantastic response to this long course
chemo, RT, maybe even a clinicalcomplete response, he might
become a candidate for a wash and wait or organ preservation
(31:52):
strategy, potentially avoiding major surgery altogether.
But that's only really feasible if the radiation was delivered
safely, minimizing long term bowel, bladder and sexual
dysfunction. The quality of the radiation
plan directly enables or disables these less invasive
future options. It underpins the entire modern
philosophy of rectal cancer management.
(32:13):
Hit the target hard. Protect the patient.
Maximize their future prospect for both cure and quality of
life. Excellent connection planning
isn't just technical, it's strategic for the patient's
entire journey. OK, time to test ourselves.
Let's hit the board blitz. Ready case 150 year old woman
T3N1 mid rectal adenocarcinoma 8centimeters from anal verge
(32:34):
getting long course neoadjuvant chemo RT.
Which of these is an essential component of her CTVSR?
The 45 Gray volume. A Entire pararectal fat, pre
sacral space and common iliac nodes.
B Entire mesorectum, pre sacral space and internal iliac nodes.
C only the primary tumor plus 2cm margin and external iliac
(32:54):
nodes, or D only the primary tumor and inguinal nodes.
OK, thinking back to our CVSR pearls.
For a standard mid rectal T3 N 1you absolutely need the
mesorectum, the presacral space,and the internal iliac nodes.
Common iliacs are usually too high, External iliacs and
inguinals are conditional. So the best answer has to be.
B is correct. Entire mesorectum, presacral
space and internal iliac nodes are the standard components.
(33:16):
Well reasoned case. 27 year old man gets postoperative radiation
to 45. Gray pathology showed a positive
circumferential resection margin.
CRM plus what's the most appropriate boost dose to the
tumor bed for this positive margin?
A 5.4 guy total 50.4 guy B 10.8 guy Total 55.8 GI C 14.4 to 15
(33:37):
point O guy total 59.4 to 60 GI or D20 GI total 65 GI.
OK. Positive margin post op perle
force that we need to boost higher.
For positive margins, 50.4 is standard neoadjuvant, 55.8 is
more. For negative margins post op, we
need to get close to 60 Grays, so 14.4 to 15 Gray additional
dose sounds right. That gets us at 59.4 to 60 grade
(33:58):
total answer CC. Is absolutely correct, you need
that higher dose 59.4 to 60 grade eye total to adequately
address a positive surgical margin in the postoperative
setting. Excellent recall, fine feeling
good final one case 3 During daily image guidance for long
course pelvic RT, the CBCT consistently shows a
significantly distended rectum despite the patient following
(34:19):
bowel prep instructions. What potential consequence is
this most likely to cause? A increased dose to bladder, B
Increased dose to small bowel. C Increased dose to femoral
heads or. D Geographic miss of the primary
tumor. Distended rectum and we talked
about this during SIM prep. If the rectum shape and position
(34:40):
change drastically day-to-day because it's full of stool or
gas, it's going to push the target, the rectal wall and
tumor around. While it might slightly shift
the bladder bowel, the biggest direct risk is that the tumor
itself moves out of the high dose region.
So geographic miss answer DD. Is the answer.
While dose to OA Rs might changeslightly, the primary concern
(35:00):
with inconsistent rectal fillingor significant distension is the
potential for a geographic miss of the rectal target itself due
to its variable position. That's why consistent prep is so
vital. You nailed the blitz all right.
Now stepping back again, we focus heavily on the how to of
planning today, but you mentioned controversies in
advance dances earlier. We know the landscape of rectal
cancer is shifting with TNT organ preservation.
(35:23):
How does this meticulous plan building we've detailed fit into
those bigger picture philosophies and ongoing
discussions? It's the absolute linchpin.
You can't seriously talk about advanced strategies like TNT
followed by watch and wait or deescalating surgery or using SPRT
for boost or primary treatment in select cases without having
the ability to deliver radiationwith extreme precision and
(35:44):
conformality. The.
Plan enables the strategy. Precisely, think about organ
preservation. The whole premise relies on
achieving a complete clinical response with chemoradiation and
ensuring that the long term functional outcome, bowel
function, bladder function, sexual function will be
acceptable. Hopefully better than after
radical surgery that requires minimizing dose to those
(36:04):
critical organs which demands advanced planning techniques
like IMRT, VMAT, SIB, and rigorous adherence to OAR
constraints. So the precision allows for
better functional outcomes, which makes de escalation more
palatable. Exactly the controversy might
be, you know, how much dose is needed.
Can we shorten the course even more?
Can we use highly focused radiation like SBRT more
(36:25):
broadly? But underpinning all those
questions is the assumption thatwe can actually deliver the
intended dose distribution accurately and safely.
Advances in imaging, motion management and adaptive
radiotherapy, where we adjust the plan during the treatment
course based on changes in anatomy, are, while pushing this
forward, building directly on the meticulous planning
(36:45):
principles we discussed. It's about refining our ability
to hit the target harder while being even kinder to the
surrounding tissues. It really highlights how
planning isn't just a technical exercise, it's integral to the
evolving therapeutic strategy for rectal cancer.
Couldn't have said it better myself.
OK, let's try and wrap this up with a quick summary of the key
takeaways from today. I'll start optimal rectal cancer
(37:08):
treatment planning absolutely begins way before treatment with
meticulous simulation and thorough patient preparation
that BFER protocol. And then accurate target volume
delineation following that wedding cake approach from GTV
up through CTVSR&CTVHR is just critical for ensuring you're
treating the right areas to achieve local control.
(37:30):
Next, remember that dose and fractionation are always
tailored to the specific clinical situation.
Neoadjuvant long course versus short course post operative
boost for adverse features or positive margins and techniques
like SIB can offer real advantages in efficiency and
conformality for long course. And overarching all of this
rigorous adherence to organ at risk constraints is absolutely
(37:53):
essential. Protecting the small bowel,
bladder, bone marrow and other structures is paramount for
minimizing toxicity and preserving quality of life.
Safety first, always. A fantastic summary.
It covers the journey from SIM to safety checks.
And remember, this detailed planning is the engine driving
many of the exciting advances we're seeing in rectal cancer
care. Well said.
To keep sharpening your skills, you can complete practice oral
(38:15):
boards over at radonsmartlearn.com.
That's radonsmartlearn.com. And be sure to subscribe to
Radon Smart Review so you catch our next session.
Thanks so much for tuning in. Thanks everyone.
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