CNS E05: HGG Treatment Planning Workshop

In our first four episodes, we've established the "what" and the "why" of high-grade glioma management. Today, we get practical and move to the "how." Treatment planning is a core competency for any radiation oncologist, and for high-grade gliomas, it's a discipline of millimeters and nuanced judgments.

Episode Transcript

(00:00):
Welcome to the Radox Smart Review CNS Cancer series.
We're really glad you could joinus again as we get into some
detailed neuro oncology topics. Yes, welcome.
Today we're moving into Episode 5, High Grade Glioma Treatment
Planning workshop. So in our previous talks we've,
you know, we really laid the groundwork, the what and the why

(00:20):
of managing high grade gliomas. We covered the pathology, the
EPI presentation, even the systemic therapies, right?
Exactly. All that foundational stuff.
But now we're shifting gears to the really practical side, the
essential how How do you actually plan the radiation for
these complex cases? That's right.
And treatment planning, it's, well, it's more than just
drawing lines, isn't it? It's a core skill, a fundamental

(00:42):
competency for any radiation oncologist.
Absolutely. And for high grade gliomas, it
demands incredible precision. I mean, we're talking
millimeters, really nuanced judgments.
The stakes are just incredibly high.
It directly impacts tumor control, obviously, but also,
crucially, the patient's qualityof life down the road.
Definitely. So to make this really

(01:03):
practical, we're going to walk you through the process using
our two main cases again, the ones we discussed before.
We'll bring back our 64 year oldgentleman with the resected
glioblastoma, the GBM, that classic aggressive tumor and
also our 38 year old woman, the one with the anaplastic
astrocytoma. It's a different beast

(01:25):
biologically, and that means different planning
considerations. Yeah, good comparison.
These aren't just theoretical examples, are they?
They really represent the kind of dilemmas, the decisions you
face every day in clinic precisely.
So using these cases, we'll giveyou a kind of hands on guide to
modern target delineation. We'll compare the the classic
treatment ideas. The other ways, yeah.

(01:46):
So the contemporary evidence based approaches that we use now
and we're going to really nail down the critical rules for
target editing. That's where the precision truly
matters, right? Making sure we cover the tumor,
protect the healthy stuff. Meticulously.
And of course, the organette risk constraints, the OA Rs.
These are absolutely essential for safe practice and let's be

(02:07):
honest, for your board exams too.
Oh. Definitely you need to know
those numbers called, but more than just the numbers,
understand why they matter. OK, so before we even like think
about opening the planning software, let's quickly cover
the setup, the crucial first steps for these patients.
Good idea, It's foundational, isn't it?
Sometimes overlooked, but it really sets the stage for

(02:28):
everything else. Bad setup, bad plan.
It's like building a house on shaky ground.
Yeah. Exactly.
So our patients, they undergo ACT simulation, A computed
tomography SIM. Right.
And this isn't just any CT. No, no, it's specialized.
The patient is carefully positioned and mobilized in a
custom thermoplastic mask. Can you just grab that mask a
bit? Sure.

(02:49):
Think of it like a rigid shell, perfectly molded to the
patient's head and shoulders. It ensures they're in the exact
same position for every single treatment, day after day.
Which is critical for reproducibility, especially over
six or seven weeks. Absolutely.
Then we get a contrast enhanced thin slice CT scan, usually
vertex down to the upper cervical spine.

(03:10):
And thin slice is key here. Very cute.
We're talking one maybe 2mm thick.
Gives us that high res anatomical detail we need for
dose calculation and initial contouring.
It's the blueprint. But the CT on its own isn't
really enough for these brain cases, is it?
No, definitely not. The CT gives great bone detail,

(03:30):
good for DOS calc, but it's well, it's pretty poor for soft
tissue contrast. Seeing the tumor boundaries,
that's tough on CT alone. Which is where the MRI comes in.
Exactly. The absolute key to a
successful, accurate plan in neuro oncology is image fusion.
You simply must fuse that planning CT with the patient's
diagnostic MRI scans. Because the MRI gives you that

(03:52):
superior soft tissue contrast. Indispensable for visualizing
those subtle tumor edges, seeinghow close it is to critical
structures, checking for any residual bits left after
surgery. You need the MRI.
So OK, the MRI is the secret weapon.
What specific MRI sequences are like non negotiable for you?
The ones you need every single time.
Good question. Bare minimum, you absolutely

(04:13):
need the post operative T1 weighted post contrast sequence.
That's the workhorse for enhancement, right?
That's your workhorse for seeingresidual enhancing disease.
Yeah, the active aggressive stuff usually lights up there
and you also need the T2 Folir sequences.
And the flare shows. The flare is crucial for picking
up non enhancing tumor infiltration and also the
surrounding edema the swelling. Understanding how T1 post

(04:37):
contrast and T2 flyer look together is fundamental.
OK, T1 postcon and T2 flyer are minimum, but you mentioned
something else, something that really distinguishes an expert
planner. Ah yes, and this is where it
gets really interesting. Best practice, absolutely
essential in my view, is to alsofuse the preoperative MRI scan.
The pre op wow why go back to before the surgery?

(04:59):
Seems counterintuitive. It does initially, but think
about it. It's like detective work.
Surgery, while necessary, can drastically change the brain's
anatomy. Things shift around.
Exactly. Brain shifts, fluid collects in
the cavity. The post op scans might not show
you where the tumor originally was or its full extent before
they remove parts of it. OK, so the pre op is the only

(05:20):
true map of the original tumor footprint?
Precisely its full extent, its relationship to vital structures
before anything was disturbed. This is critical for accurately
defining your initial gross tumor volume, your GTV.
Especially for infiltrated tumors like glioma, you need to
know where it was, not just what's left.
That makes perfect sense. So having captured the anatomy

(05:41):
meticulously with CT infused Mris, including the pre op,
let's go back to our 64 year oldGBM patient.
The core question where do you draw the line?
This has been debated for years.Decades really, and
understanding this history is key for GBM.
There have basically been 2 competing philosophies on
defining the target volume. OK.
The first is the classic RTOG approach, radiation therapy

(06:03):
oncology group and you can summarize it simply as treat the
edema, treat the. Edema.
That sounds broad. Where did that come from?
It came from early autopsy studies way back.
They found glioma cells, especially GBM cells, had
infiltrated way beyond the enhancing part you saw on scans.
They were hiding in the edema. Microscopic disease, yeah.
Right, which led them to think you needed a much bigger

(06:25):
treatment volume. So they developed a 2 volume
technique, sometimes called a cone down.
How did that work? OK.
So the initial volume CTV one got 46 grey.
The GTV for this phase was huge surgical cavity, any leftover T1
enhancement and the entire surrounding T2 Fla or edema, all
of it. And then they added a generous

(06:45):
margin, 2 to 2.5 centimeters forthe CTV expansion really wide
field. Trying to catch every last cell
and the booth. After that wide field, the boost
volume CTV two took the dose up to 60 grey that typically
included just the T1 enhancementand the cavity again with a
large 2 to 2.5 centimeters CTV margin.
So lower dose the wider field including edema, higher dose to

(07:06):
the core. That was the idea.
Catch the microscopic spread wide.
Hit the main tumor hard. OK, so that was the classic RTO
way. What's changed?
How does that compare to modern practice?
Seems like there's been a big shift A.
Massive shift. And this is really the crux of
modern GBM planning. The current approach, championed
by groups like STRO in Europe and ERTC, follows a totally

(07:30):
different philosophy. Treat the recurrence.
Treat the recurrence. What does that mean?
It means we learn from experience, from lots and lots
of studies, that over 80%, maybeeven higher of GBM recurrences
happened centrally. Centrally meaning.
Meaning close to the original tumor, specifically within about
2cm of the original enhancing lesion.

(07:51):
So even when they treated that huge edema field.
Exactly. Even treating that vast T2A
filler edema field didn't stop these central failures, but what
it did do was increase toxicity,more side effects, more
cognitive problems because you were irradiating so much healthy
brain unnecessarily. That's a huge realization.
So the bigger field wasn't better, it was potentially

(08:12):
worse. It was a game changer in our
thinking. It LED directly to the modern
single volume approach, which isnow the standard.
OK, walk us through that. How does the single volume
approach work? It's much more focused.
The GTB is defined simply as thesurgical cavity plus any
residual T1 post contrast enhancement.
That's it. Much smaller.
Much smaller then the CTV is created by a uniform 1.5

(08:34):
centimeter expansion on that GTV, and this entire single
volume gets treated to the full 60 Gray, usually in 30
fractions. So 1 volume 60 Gray and the key
difference is? The crucial difference worth
repeating is that the surrounding vasogenic T2 and FLR
edema it's intentionally excluded from the GTV and
therefore from that high dose 60Gray volume.

(08:56):
Because treating it didn't help prevent recurrence where it
usually happens and it caused harm.
Precisely. It reflects our current
understanding. The edema is there, sure, but
targeting it broadly doesn't improve outcomes and adds
toxicity. And this isn't just theory,
right? This is baked into guidelines
now. Absolutely.
The 2023 ESTRO and Eno guidelines have formally adopted
this as the standard approach for GBM planning.

(09:18):
It shows how we evolve based on evidence, always aiming for
better control with less harm. Now you mentioned a quick Pearl
earlier about a sort of hybrid GTV.
What's that about? Right.
So while the standard is clear cavity plus T1 enhancement, some
experts in very specific cases might advocate for a slight
modification. This involves including not just

(09:40):
the cavity and T1 enhancement, but also any areas of nodular
non linear T2 signal that look really suspicious for non
enhancing tumor infiltration. So not the diffuse finger like
edema. Exactly.
You still exclude the diffuse visogenic edema, but if you see
a distinct, maybe lumpy looking T2 brightness right next to the

(10:01):
enhancing part, something that doesn't look like typical edema.
You might consider adding just that specific nodule to the GTV.
You might it takes a keen eye, careful review of all the
imaging, maybe advanced sequences.
It's nuanced, requires judgment,but it's something to consider
if you have a high suspicion of infiltration just beyond the
enhancement itself. Interesting.
Requires real radiological expertise.

(10:24):
OK, let's switch gears now to our other case, the 38 year old
woman with the anaplastic astrocytoma.
The planning philosophy here is quite different, you said.
Distinctly different, yes, and it all comes down to the
underlying biology. What's the key biological
difference we need to remember? The main thing for anaplastic
astrocytomas, these are grade 3 gliomas, is that they are often
non enhancing on that T1 post contrast MRI.

(10:47):
Unlike GBM where enhancement is key, right?
For many anaplastic astrocytomas, the T2 and the
Fella are hyperintense, that bright signal that is a tumor
itself. It's not just edema surrounding
an enhancing core like in GBM. So the T2 FLIR signal represents
the actual tumor cells infiltrating.
Largely, yes. These tumors often don't disrupt

(11:07):
the blood brain barrier enough to 'cause that T1 enhancement,
so if you only contoured the enhancement, you'd miss most of
the tumor. OK, so that obviously changes
how you define the GTV completely.
Completely for an anaplastic astrocytoma, the GTV is defined
as the entire T2 and flare abnormality plus the surgical
cavity if one exists. You treat the whole T2 FL air
our signal area as gross disease.

(11:28):
Essentially, yes, because that'swhere the bulk of the tumor
cells are presumed to be. Then we create the CTV with a 1
to 1.5cm margin around that entire T2 FL Air GTV.
And that margin comes from? That was the standard used in
the CAT Non trial, which was thelandmark study for these grade 3
tumors, established the standardof care.

(11:49):
And the dose is different too. Slightly different, yes.
The prescription is 59.4 Gray in33 fractions.
Different fractionation. The GBM reflecting the specific
trial data and biology. A good reminder that high grade
glioma isn't monolithic. The specifics matter.
Definitely so for both GBM and anaplastic astrocytoma.

(12:09):
Once you've made that initial CTV expansion 1.5cm or one 1.5
centimeters, the next critical step is manually editing that
CTV. Yes, crucial step.
And it's not just cosmetic, it'sabout respecting an anatomy,
minimizing dose where it's not needed, while still covering
your target. So what's the guiding principle?
Is there an easy way to rememberwhere to trim?
There is, actually. It's quite elegant.

(12:30):
Just remember this. Clamas generally respect bone,
dura and the tantorium. OK, bone dura, tantorium.
They act like barriers. Natural dense anatomical
barriers. They tend to stop the
microscopic spread. Think of them as walls.
So for boards, lock this in gliomas respect bone dura and
the tantorium. OK, so applying that.

(12:52):
You should trim the CTV right off the inner table of the skull
because these are brain tumors, parenchymal tumors, they
typically don't invade bone. Similarly, you edit your CTV to
stop right at the Falk cerebri, that dural fold between the
hemispheres, and the tentorium cerebelli, the dural fold
separating cerebrum from cerebellum.
These dense structures are strong barriers.

(13:14):
Allows us to safely reduce the volume near them.
Exactly. But, and this is the huge
exception, the one that can really get you out, the corpus
callosum, where the the fox meets the corpus callosum, that
big bundle of white matter connecting the hemispheres.
You must not trim the CTV there.Why not if the fox is a barrier?
Because the corpus callosum itself is like.
Think of it as the superhighway for tumor cells crossing the

(13:34):
midline. A direct pathway across.
It's a dense tract of fibers specifically designed to connect
the two sides of the brain. And unfortunately, tumor cells
exploit that highway really efficiently.
They track right along those fibers.
Wow, that's a powerful analogy. The Super highway and the
implication is serious. Very serious.

(13:55):
Failing to adequately cover the tumor interface near the corpus
callosum is a classic reason formarginal recurrences.
Tumors coming back right at the edge of your field, often on the
other side. So just to be clear for everyone
listening, why is that spot so vulnerable?
It's those organized white matter tracks letting cells
sneak across, right? Bypassing the faux barrier right

(14:16):
there. Exactly.
So you deliberately don't trim to the dura right at that
specific interface because the tumor biology overrides the
general rule. Critical exception.
OK, so we've edited the CTV. What about the PTV?
The planning target volume? Right.
The PTV, this is a purely geometric concept.
It's not tumor, it's an expansion added to account for
uncertainties. Uncertainties like.

(14:37):
Like small day-to-day variationsin patient setup, even with the
mask and tiny patient movements during treatment, A slight head
turn, even breeding effects. So it's a safety margin.
It's a safety buffer, exactly anengineering solution.
Usually a three to 5mm uniform expansion on your final edited
CTV. It ensures that even with those

(14:58):
small shifts, the entire CTV gets the dose you prescribed.
And the rule for editing the PTVis.
The rule is simple. You never trim the PTV to
anatomical structures. Never.
Never. It's a planning margin, A
geometric buffer, not an anatomical contour.
You don't trim it off the skin or bone or an OAR.
Why not? What happens if you do?
Trimming the PTV means you're deliberately creating a

(15:20):
potential cold spot right where you intended to treat.
You risk underdosing the edge ofyour CTV, your microscopic
disease. It defeats the whole purpose of
the PTV margin. It's a major planning no, no.
OK. That's a really important
distinction. Don't trim the PTV, got it.
So we've defined our targets accounted and for motion.
Now how do we keep patients safe?
We have to talk about OA Rs. Absolutely.
Organs at risk. After all the work defining the

(15:43):
target, you have to make sure you're respecting the tolerance
of nearby critical structures. These are the sensitive bits
that, if you overdose them, can cause serious irreversible
problems. Devastating problems sometimes.
It's always a balancing act. Maximize tumor coverage.
Minimize OAR dose. And for board exams you
absolutely have to know these OAR constraints cold.

(16:05):
They're non negotiable. For safe planning.
The numbers from protocols like RTOG 08/2 and five are the
standard. They are.
Let's run through the big ones. OK, where do we start?
Brain stem. The maximum point dose must be
kept strictly below 60 Gray. 60 And why is that so critical?
Exceeding 60 Gray risks catastrophic permanent
neurological damage, swallowing problems, cranial nerve issues,

(16:28):
breathing control, potentially life threatening.
It's often the hardest OAR to spare, especially for deep
tumors. You really have to scrutinize
the plane if you're getting close to 60.
OK, brainstem Max less than 60. What about vision?
Visual pathways are next. The optic chiasm Max dose needs
to be less than 56 grey. 56 for the chiasm.
And the optic nerves left and right.

(16:49):
Max dose less than 55 grey. 55 for the nerves, a little lower.
Slightly lower for the nerves themselves.
Go above these and you risk significant vision loss,
potentially bilateral. Preserving sight is paramount.
You often need advanced techniques like IMRT or VMAT to
sculpt the dose away from these if the tumor is close right.

(17:10):
What else? Eyes and ears.
Yep, the lenses of the eyes keepthe dose below 10 Gray if
possible. That's mainly to minimize the
long term risk of radiation induced cataracts.
Annoying, but usually fixable. Years of fixable is a surgery
but prevention is better and then hearing the mean dose to at
least one cochlea. The inner ear should ideally be
kept below 45 grade to preserve hearing. 45 mean to 1 cochlea.

(17:34):
Hearing loss impacts quality of life and communication so these
are really the non negotiable limits you have to respect in
your planning. Brain stem 60 Max Chasm 56 Max.
Nerves 55 Max. Lens 10 Max Cochlea 45 mean.
Got it. OK, so treatment is done.
The plan was great, but then comes the follow up MRI and
things can get tricky again. Let's talk about progression

(17:56):
versus pseudo progression. Yes, the bane of post treatment
surveillance, especially after chem radiation for GBM, a real
challenge. So what is pseudo progression
exactly? It's basically treatment related
inflammation and some necrosis cell death in the area that got
radiation. And the frustrating part is on a
standard T1 post contrast MRI, it looks identical to true tumor

(18:20):
growth. It enhances.
It looks bigger. Exactly.
It enhances, it can expand. It mimics active tumor
recurrence perfectly, causes huge anxiety for patients and
clinicians. You might think the treatment
failed when it hasn't. Leading to potentially wrong
decisions like stopping effective chemo or unnecessary
surgery. Precisely.
The radiation causes this sterile inflammatory reaction

(18:41):
that just looks bad on the scan.So how do we tell the
difference, especially early on?This is where the RANO criteria
come in. Response assessment in neuro
oncology. They give us strict rules to try
and navigate. This OK?
What did the RANO criteria say? Well the key thing is timing.
Per Rano, within the 1st 12 weeks after finishing radiation,
you generally cannot definitively call true

(19:01):
progression based on MRI alone, unless unless there's new
enhancement clearly outside the high dose radiation field that
suggests the tumor really has spread beyond the treated zone.
OK, new enhancement outside the field counts.
What else? Or if you have pathologic
confirmation from a new biopsy. Obviously that's invasive, but

(19:22):
it gives a definite answer. So without those two things, you
have to be cautious in the 1st 12 weeks.
Why is making that distinction so critical for managing the
patient? Because as you said, you want to
avoid unnecessary interventions stopping chemo that might
actually be working during riskyRE operations.
If it's just pseudo progression,it often stabilizes or improves
on its own over time. Patients and careful follow up

(19:44):
are key in that early window. Maybe advanced imaging like Mr.
Perfusion or spectroscopy can help sometimes, but the standard
T1 post con is ambiguous early on.
OK, that ran a rule is vital then let's shift now to some
clinical pearls, those high yield takeaways for practice and
boards. Right, let's distill the key
messages. These are the things to really
lock in. Pearl one for GBM volumes.

(20:06):
Remember the modern standard single volume approach GTV is
cabbie plus T1 enhancement. Add a 1.5 centimeter CTV margin,
treat that whole volume to 60 Grays, and crucially,
intentionally exclude the surrounding T2 edema from that
high dose. OK GV MT1 plus cavity GTV 1.5cm
TTV 60G Exclude edema Perl 2. Perl 2 for anaplastic weoma

(20:30):
volumes. It's different.
GTV is the entire T2 FLR abnormality plus cavity at a 1
to 1.5 centimeter CTV margin. Treat that volume to 59.4 grey.
Remember, T2 FLAR is the tumor here.
Right Anaplastic T2 FLARGTV 11.5centimeter CTV 59.4 Guy Pearl 3
is about trimming. Pearl 3 Anatomical trimming trim

(20:52):
the CTV at bone forks and tantorium and natural barriers.
But the big exception never trimthe CTV at the corpus callosum,
but the tumor is nearby or the superhighway.
Don't trim the corpus callosum. OK Pearl 4 is OA Rs.
Pro 4 Key Max dose constraints to no cold brain stem less than
60 Gray. Chiasm less than 56 Gray, optic
nerves less than 55 Gray. Absolutely critical for safety.

(21:12):
Brain stem 60, Chiasm 56, Nerves55 and Pearl 5, Pearl 5.
Pseudo pro. Remember you can't reliably
distinguish it from true progression on standard MRI in
the 1st 12 weeks post RT per rano.
You need pathologic proof or newenhancement clearly outside the
radiation field to call true progression early.
Be patient. Excellent pearls.

(21:33):
Ready for a quick board blitz? Let's test these concepts with a
couple of cases. Think through the.
Y. OK, case one, you're planning
treatment for a grade 3 anaplastic astrocytoma, right?
Temporal lobe. It's mostly a big T2 FLR
abnormality, no significant enhancement.
How do you define the GTV? Is it AT1 post contrast
enhancing part only B the entireT2 FLR hyperintense signal?

(21:56):
C Surgical cavity only OR DT2 FLR signal plus A1 centimeter
margin. OK, thinking about anaplastic
astrocytoma non enhancing. The T2 FLR is a tumor, so the
GTV has to be that entire signal.
The answer must be B. Explain why the others are
wrong. Well, A is wrong because there's
no significant enhancement to target, C is wrong because just
the cavity misses the infiltrated tumor in the T2 FYR

(22:19):
signal, and D is tricky. But adding a margin makes it a
CTV, not a GTV. So the GTV itself is just the TT
of L full IR signal answers B. Perfect rationale.
Case two, you're reviewing a GB M plan.
CTV was made with a 1.5 centimeter expansion from GTV
standard practice, but to spare the optic nerve, the planner

(22:39):
trimmed the PTV by 2mm away fromthe nerve.
Is this correct? A Yes, sparing the optic nerve
is highest priority. B Yes, but 5mm trim is better.
C No, the CTV should have been trimmed, not the PTV or D No,
neither CTV nor PTV should ever be trimmed.
This is that critical PTV rule. You never trim the PTV to spare
an OAR. The PTV is purely geometric for

(23:00):
setup uncertainty, trimming at risks, underdosing the target if
our choice has to be made to spare an OAR at the expense of
coverage. That decision involved modifying
the CTV, the biological target volume.
If you're carefully and consciously so, trimming the PTV
is fundamentally incorrect. The also has to be C.
Excellent. That really nails down that PTV
principle. OK, let's summarize today's
workshop. We covered a lot of ground on

(23:21):
high grade glioma planning. We really did.
We started with the setup, the importance of CT SIM and
especially MRI fusion including that crucial pre op scan.
Then we contrasted the classic RTOG treat, the edema approach
for GBM with the modern Estro standard treat the recurrence,
which intentionally excludes theT2 edema from the high dose
volume. Big shift there.

(23:43):
And we detailed how to contour anaplastic astrocytomas,
focusing on the T2 FL air signal.
Is the GTV recognizing their different biology?
We mastered the trimming rules for the CTV trim at Bone Fox
Tantorium. But not at the corpus callosum.
Remember the superhighway? Got it.
And we reviewed those essential OAR constraints, brainstem,
optic structures, lens cochlea numbers, you just have to know.

(24:05):
And finally, the Rhino criteria for navigating that tricky
pseudo progression versus true progression dilemma in the early
post treatment face. So that wraps up our deep dive
into high grade gliomus for thisseries.
Hope this practical workshop wasreally helpful.
Yeah, we covered a lot. Planning these cases is complex
but so important. Our next series of episodes will
switch gears completely, moving to the other end of the

(24:26):
spectrum, low grade Gliomus. We'll kick off with Episode 6,
LDG Foundations, Risk stratification, and the Timing
debate. When do you treat these slower
growing tumors? Always a fascinating question,
and as a final thought for you today, consider where this is
all heading with better imaging,genomics, AI.

(24:46):
How will glioma planning change next?
Will we define targets based on,say, specific mutations or
metabolic activity rather than just T1 and T2 signal?
Something to think about. Definitely food for thought.
Remember, you could find [e-mail].
That's radonksmartlearn.com and subscribe to Radonk Smart Review
for our next session. Thanks for joining us.

(25:08):
Thanks for tuning in.