Leptomeningeal Carcinomatosis

[Neuronix]

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Case discussion time. Serious replies only please.

Imagine a hypothetical young woman, 35 years old with known breast cancer on systemic therapy, well controlled and limited systemic disease, comes in to ED with headaches that become controlled on dexamethasone. MRIs of brain and total spine show subtle faint streaks and dots consistent with leptomeningeal carcinomatosis in brain and cauda equina. Lumbar puncture is performed confirming cancer cells in CSF on cytology. You are the rad onc consulted for recommendations.

So you speak to the medical oncologist and they have nothing that they think will be active in the CNS. The patient is triple positive and has already been through all the hormonal and Her2 agents OR it's triple negative and they have no good ideas... Pick either scenario, and would that change anything? Next generation sequencing is already there from the blood and doesn't give you any additional options.

What would you do?
Does your center perform or consider intrathecal chemotherapy?
What do you think of this article: Clinical trial of proton craniospinal irradiation for leptomeningeal metastases - PubMed ? Would you consider CSI with photons or protons, whole brain radiation alone, or throw up your hands and tell the desperate young lady with the young children there's nothing else we can do?

 

[thecarbonionangle]

I think proton CSI is very reasonable in this situation especially if there is no systemic CNS options. I would do it for a case like this (young, good PS) as a last ditch effort. The trial you link has a quarter of patients without progression at 1 yr.

 

[StIGMA]

Alternative discussion- When very limited disease in the brain such as 2-3 lesions with long disease latency, I’ve given SRS for leptominengeal disease for similar cases as initial brain therapy. Would not SRS for this presentation. Happy to hear what others are doing.

 

[Upgrayedd]

INTRATHECAL HERCEPTIN. Have a 2 yr survivor for ER-/PR-/Her2+

 

[RickyScott]

INTRATHECAL HERCEPTIN. Have a 2 yr survivor for ER-/PR-/Her2+

I didn’t even know that was possible

 

[elementaryschooleconomics]

I didn’t even know that was possible

Agreed, that's some cowboy ish right there. @Neuronix, could you float that idea by MedOnc?

(I also agree with @thecarbonionangle about proton CSI)

 

[Palex80]

I had a similar case during the past couple of years.

Premenopausal patient with Her2 positive disease. Had Taxanes, Herceptin, Perjeta for extracerebral metastases and a clinical complete response. Then developed meningeal spread while on Herceptin and Perjeta. Disease limited to the brain, no spinal lesions. CSF done and proven. Patient was in an very good performance status, apart from headaches and some vertigo. She was able to work. Did not want chemotherapy.

She agreed to aggressive treatment, so I went for 30/2.5 for the entire spinal axis and boosted with WBRT up to 40/2.5.
VMAT-photons, 3 isocenters, feathering.

She tolerated treatment quite well. Follow up scan 3 months later showed a very good response, symptoms were gone.

Med onc put her on Kadcyla.

It didn't last long however. 3 months after that scan she was progressing again while on Kadcyla at multiple sites in the cerebellum and med onc was asking for options.


I was very reluctant at first, I must admit.
So, 6 months after 40/2.5, I gave her 30/3 to the entire cerebellum. I set constraints for the brain stem (Dmax <25 Gy) and medulla oblongata (Dmax <15 Gy).

She responded quite well to that course too, albeit she tolerated it worse than the first one with a lot of fatigue and some nausea.
After that, med onc started giving newer Her2-directed treatment (Tucatinib) which seems to have controlled the disease quite good, with the exception of small lesions that kept popping up every now and then.

In the past year I have irradiated 5 progressive lesions in the cerebrum with SRS (I'm not touching the cerebellum again). 🙂




I recently treated another young, however postmenopausal lady, who had ER+ PR+ Her2 - disease and presented with vertigo 3 years after BCS, RT and chemo for her stage II breast cancer and was on an aromatase inhibitor. Scans came back negative (PET-CT & MRI), CSF was however positive. Med onc put her on Ademaciclib (which apparently crosses that BB-barier well) and fulvestrant and asked me what I wanted to do.
I gave her a CSI with 40/2.5, didn't boost anything. Since no target on imaging, we only had the cell count in CSF to tell us if it's working. At the end of the treatment course the cell count was already dropping. 3 months out of treatment now and CSF is clear. She is free of symptoms. I do not know if it was the ademaciclib + fulvestrant or the CSI (or both), but she may experience a long lasting remission. Apparently, her metastatic tumor load was rather low.

 

[Palex80]

I think proton CSI is very reasonable in this situation especially if there is no systemic CNS options. I would do it for a case like this (young, good PS) as a last ditch effort. The trial you link has a quarter of patients without progression at 1 yr.

Actually I do not agree. I think proton CSI should rather be considered if the patient is scheduled to undergo chemotherapy and you want to spare bone marrow. Apart from sparing bone marrow, I cannot think of any clinically relevant benefits of protons in this indication.

 

[Neuronix]

Actually I do not agree. I think proton CSI should rather be considered if the patient is scheduled to undergo chemotherapy and you want to spare bone marrow. Apart from sparing bone marrow, I cannot think of any clinically relevant benefits of protons in this indication.

They all continue their systemic agents. The idea is for systemic therapy to control systemic disease while RT controls CNS in these cases. I personally believe in protons to reduce CSI toxicity (acute and long-term marrow) but maybe that's overstated.

Agree with intrathecal herceptin for her2+. Getting intrathecal at my institution is very challenging. I am posting an example patient, but I am the CNS attending so I have a case like this with varying histologies and receptor statuses more or less monthly. So I agree with IT herceptin for her2+ breast, but let's assume they aren't her2+ breast for sake of further discussion.

 

[Palex80]

They all continue their systemic agents. The idea is for systemic therapy to control systemic disease while RT controls CNS in these cases. I personally believe in protons to reduce CSI toxicity (acute and long-term marrow) but maybe that's overstated.

Agree with intrathecal herceptin for her2+. Getting intrathecal at my institution is very challenging.

Gotcha! Sure, makes sense. If the patient needs to continue systemic treatment which is bone marrow depleting, then CSI with protons may allow both treatments to happen.

We do not offer CSI in patients with active extracerebral disease. We only treat focally in those patients. FSRT/SRS if we can see a target that's causing specific symptoms in the brain. And focal RT to the spine for lesions that are producing symptoms. We will do WBRT if symptomatic, for instance with headaches.

 

[Upgrayedd]

Agreed, that's some cowboy ish right there. @Neuronix, could you float that idea by MedOnc?

(I also agree with @thecarbonionangle about proton CSI)

Intrathecal trastuzumab in the management of HER2+ breast leptomeningeal disease: a single institution experience - PubMed

IT trastuzumab was well tolerated with prolongation of OS over historical controls. IT trastuzumab should be considered for management of HER2+ leptomeningeal disease patients.

pubmed.ncbi.nlm.nih.gov

Giddyup

 

[thecarbonionangle]

Actually I do not agree. I think proton CSI should rather be considered if the patient is scheduled to undergo chemotherapy and you want to spare bone marrow. Apart from sparing bone marrow, I cannot think of any clinically relevant benefits of protons in this indication.

In a person who has had cardiotoxic chemo and herceptin lowering heart dose does not have “any clinical benefit”? News to me!!!

 

[radiation]

2 new FDA approved HER-2 drugs with pretty good CNS penetration

Intracranial Efficacy and Survival With Tucatinib Plus Trastuzumab and Capecitabine for Previously Treated HER2-Positive Breast Cancer With Brain Metastases in the HER2CLIMB Trial - PubMed

In patients with HER2-positive breast cancer with BMs, the addition of tucatinib to trastuzumab and capecitabine doubled ORR-IC, reduced risk of intracranial progression or death by two thirds, and reduced risk of death by nearly half. To our knowledge, this is the first regimen to demonstrate...

pubmed.ncbi.nlm.nih.gov

DEFINE_ME

Also always wondered why intrathecal herceptin neccessary when other antibodies like avastin cross the BBB

 

[deleted969641]

35 year old. You throw the kitchen sink at them. Or as we say "на всех парах"
End of discussion, seriously. Damn the torpedoes. I'll see the lawyers in court. Don't care, they won't win. And if they do, I'll still sleep soundly at night regardless whether it's in a Motel 6 and my kids are in public school because I refuse to work for Evicore 🤣.

 

[TheWallnerus]

I had a similar case during the past couple of years.

Premenopausal patient with Her2 positive disease. Had Taxanes, Herceptin, Perjeta for extracerebral metastases and a clinical complete response. Then developed meningeal spread while on Herceptin and Perjeta. Disease limited to the brain, no spinal lesions. CSF done and proven. Patient was in an very good performance status, apart from headaches and some vertigo. She was able to work. Did not want chemotherapy.

She agreed to aggressive treatment, so I went for 30/2.5 for the entire spinal axis and boosted with WBRT up to 40/2.5.
VMAT-photons, 3 isocenters, feathering.
View attachment 339140
She tolerated treatment quite well. Follow up scan 3 months later showed a very good response, symptoms were gone.

Med onc put her on Kadcyla.

It didn't last long however. 3 months after that scan she was progressing again while on Kadcyla at multiple sites in the cerebellum and med onc was asking for options.


I was very reluctant at first, I must admit.
So, 6 months after 40/2.5, I gave her 30/3 to the entire cerebellum. I set constraints for the brain stem (Dmax <25 Gy) and medulla oblongata (Dmax <15 Gy).

View attachment 339142
She responded quite well to that course too, albeit she tolerated it worse than the first one with a lot of fatigue and some nausea.
After that, med onc started giving newer Her2-directed treatment (Tucatinib) which seems to have controlled the disease quite good, with the exception of small lesions that kept popping up every now and then.

In the past year I have irradiated 5 progressive lesions in the cerebrum with SRS (I'm not touching the cerebellum again). 🙂




I recently treated another young, however postmenopausal lady, who had ER+ PR+ Her2 - disease and presented with vertigo 3 years after BCS, RT and chemo for her stage II breast cancer and was on an aromatase inhibitor. Scans came back negative (PET-CT & MRI), CSF was however positive. Med onc put her on Ademaciclib (which apparently crosses that BB-barier well) and fulvestrant and asked me what I wanted to do.
I gave her a CSI with 40/2.5, didn't boost anything. Since no target on imaging, we only had the cell count in CSF to tell us if it's working. At the end of the treatment course the cell count was already dropping. 3 months out of treatment now and CSF is clear. She is free of symptoms. I do not know if it was the ademaciclib + fulvestrant or the CSI (or both), but she may experience a long lasting remission. Apparently, her metastatic tumor load was rather low.

Why are your low isodose lines floating way outside the body. Do you guys tell the TPS the immobilizations are body? Or am I simply seeing it wrong.

 

[RadOncDoc21]

35 year old. You throw the kitchen sink at them. Or as we say "на всех парах"
End of discussion, seriously. Damn the torpedoes. I'll see the lawyers in court. Don't care, they won't win. And if they do, I'll still sleep soundly at night regardless whether it's in a Motel 6 and my kids are in public school because I refuse to work for Evicore 🤣.

It’s sad how I once used to feel this way but now I practice everything “by the book” since I truly started separating my job from my personal life. Let’s just say I’ve been jaded and believe that at the end of the day, nobody cares about the doctor and everyone is looking to sue, especially when “sh—t hits the fan.”

With that said, anyone who I feel the slightest could benefit receiving treatment somewhere else like at an “academic center” or even to a place that may have a treatment advantage (protons, MRI Linac, etc), I would send without any hesitation. Let’s just say I stopped fighting those wars.

 

[Neuronix]

35 year old. You throw the kitchen sink at them. Or as we say "на всех парах"
End of discussion, seriously. Damn the torpedoes. I'll see the lawyers in court. Don't care, they won't win. And if they do, I'll still sleep soundly at night regardless whether it's in a Motel 6 and my kids are in public school because I refuse to work for Evicore 🤣.

I don't know why you would get sued for throwing the kitchen sink at a patient like this? Patients are very grateful to be aggressive in a desperate situation, and I'm very clear that no treatment means death in short order so yes there are risks but we have to fight. It helps that other docs like the med oncs and surgeons are also saying the same thing and are all on board with the plan.

Even when we lose the fight or cause side effects, people are grateful you did everything you could. That's been my experience at least.

 

[communitydoc13]

Do we have any data to indicate CSI better than WBRT in this situation? I have always viewed CSI in an adult as the most exceptional of interventions.

 

[elementaryschooleconomics]

I don't know why you would get sued for throwing the kitchen sink at a patient like this? Patients are very grateful to be aggressive in a desperate situation, and I'm very clear that no treatment means death in short order so yes there are risks but we have to fight. It helps that other docs like the med oncs and surgeons are also saying the same thing and are all on board with the plan.

Even when we lose the fight or cause side effects, people are grateful you did everything you could. That's been my experience at least.

I mean yeah, this is America (for most of us, looking side-eyed at you, @Palex80) and it's something I'm always thinking about in the back of my mind.

That being said, I have seen the most wild things tried on patients in these situations, and I have never thought twice about it, and have never heard of lawsuits resulting from heroic efforts in desperate situations.

...and I love that intrathecal Herceptin case series, saving that to my personal drive.

 

[Neuronix]

Do we have any data to indicate CSI better than WBRT in this situation? I have always viewed CSI in an adult as the most exceptional of interventions.

No. I'm not clear how you'd randomize it. I can't imagine patients would agree to the randomization. I'm not clear that I'd even agree that there's equipoise there, and I'm very pro clinical trial.

Also selection criteria would make it tough to get accruals and power such a trial unless cooperative group (good luck--though I bet those in control of the cooperative groups are thinking about it) or maybe very high volume cancer center. Possibly better in another country where patients don't get to choose their treatment like mine do.

 

[RickyScott]

How quickly could you sim and plan this pt for proton csi?

 

[Neuronix]

How quickly could you sim and plan this pt for proton csi?

1 week if insurance cooperates. If patient acutely ill and needs treatment urgently they are not a candidate for CSI. I guess you could consider photon CSI but I've been giving those patients whole brain. Definitely will confound any retrospective series that comes out.

 

[communitydoc13]

No. I'm not clear how you'd randomize it.

Yeah, not expecting randomized data. Even patterns of failure data or retrospective series. To me, a positive CSF is equivalent to micrometastatic systemic disease. I'm usually not chasing it beyond treating whole brain (knowing that even low volume leptomeningeal disease causes cranial nerve symptoms, etc). If there is candy-coating of the entire spine, treatment probably futile (although if you have examples of pts doing well, please let me know).

Love the intrathecal Her-2 idea. Never heard of it. Around here, TDM-1 and 5FU seem to be systemic options employed.

 

[RickyScott]

1 week if insurance cooperates. If patient acutely ill and needs treatment urgently they are not a candidate for CSI. I guess you could consider photon CSI but I've been giving those patients whole brain. Definitely will confound any retrospective series that comes out.

Could you start with photon whole brain and then add protons to the spine?

 

[RickyScott]

Could you start with photon whole brain and then add protons to the spine?

May make arguing with insurance easier because could claim potential overlap

 

[emt409]

Case discussion time. Serious replies only please.

Imagine a hypothetical young woman, 35 years old with known breast cancer on systemic therapy, well controlled and limited systemic disease, comes in to ED with headaches that become controlled on dexamethasone. MRIs of brain and total spine show subtle faint streaks and dots consistent with leptomeningeal carcinomatosis in brain and cauda equina. Lumbar puncture is performed confirming cancer cells in CSF on cytology. You are the rad onc consulted for recommendations.

So you speak to the medical oncologist and they have nothing that they think will be active in the CNS. The patient is triple positive and has already been through all the hormonal and Her2 agents OR it's triple negative and they have no good ideas... Pick either scenario, and would that change anything? Next generation sequencing is already there from the blood and doesn't give you any additional options.

What would you do?
Does your center perform or consider intrathecal chemotherapy?
What do you think of this article: Clinical trial of proton craniospinal irradiation for leptomeningeal metastases - PubMed ? Would you consider CSI with photons or protons, whole brain radiation alone, or throw up your hands and tell the desperate young lady with the young children there's nothing else we can do?

Have had several patients in this exact situation. CSI. Proton if available. No HC sparing. I offer modafinil for fatigue/brain fog. Cheaper, better tolerated, and I think more effective than memantine.

Intrathecal chemo is available, but is falling out of favor as it’s hard to find people who respond to it, that didn’t respond to standard. Especially here, since her2+ agents have decent CSF concentration.

If there are large gross mets, we sometimes SIB

There are data that indicate her2+ breast cancer may be more radioresisant than other subtypes, so this patient is probably at risk for less durable control.

 

[RickyScott]

Have had several patients in this exact situation. CSI. Proton if available. No HC sparing. I offer modafinil for fatigue/brain fog. Cheaper, better tolerated, and I think more effective than memantine.

Intrathecal chemo is available, but is falling out of favor as it’s hard to find people who respond to it, that didn’t respond to standard. Especially here, since her2+ agents have decent CSF concentration.

If there are large gross mets, we sometimes SIB

There are data that indicate her2+ breast cancer may be more radioresisant than other subtypes, so this patient is probably at risk for less durable control.

I have had very limited success with provigil in cancer pts and hard to get insurance coverage even though it is generic. Was a big fan of it in college.

 

[deleted969641]

Have had several patients in this exact situation. CSI. Proton if available. No HC sparing. I offer modafinil for fatigue/brain fog. Cheaper, better tolerated, and I think more effective than memantine.

Intrathecal chemo is available, but is falling out of favor as it’s hard to find people who respond to it, that didn’t respond to standard. Especially here, since her2+ agents have decent CSF concentration.

If there are large gross mets, we sometimes SIB

There are data that indicate her2+ breast cancer may be more radioresisant than other subtypes, so this patient is probably at risk for less durable control.

I have narcolepsy. I have a prescription for Nuvigil, which I take as infrequently as possible. I absolutely hate taking these medications. It is completely absurd to prescribe this for cancer patients with "brain fog." I can't believe I just read this.

 

[RickyScott]

I have narcolepsy. I have a prescription for Nuvigil, which I take as infrequently as possible. I absolutely hate taking these medications. It is completely absurd to prescribe this for cancer patients with "brain fog." I can't believe I just read this.

provigil and Ritalin are not very helpful for cancer related fatigue, (quite a bit of literature) but have certainly seen pts on both. (Recall a young computer programmer in a high pressure job) Steroids and exercise probably best for fatigue.

 

[TheWallnerus]

Steroids and exercise probably best for fatigue.

at least there's randomized data for both

 

[Upgrayedd]

Do we have any data to indicate CSI better than WBRT in this situation? I have always viewed CSI in an adult as the most exceptional of interventions.of the leptomeningeal info

Have had several patients in this exact situation. CSI. Proton if available. No HC sparing. I offer modafinil for fatigue/brain fog. Cheaper, better tolerated, and I think more effective than memantine.

Intrathecal chemo is available, but is falling out of favor as it’s hard to find people who respond to it, that didn’t respond to standard. Especially here, since her2+ agents have decent CSF concentration.

If there are large gross mets, we sometimes SIB

There are data that indicate her2+ breast cancer may be more radioresisant than other subtypes, so this patient is probably at risk for less durable control.

My entire point is for Her2+ dz there is a subset of pts with a profound response to IT HER2 tx. Lit proves this; as does my anecdotal experience. I have had dozens die within days/wks on IT therapy for other histologies or non-targeted tx; but this is different.

 

[deleted969641]

at least there's randomized data for both

There is also randomized data for Nuvigil. Randomized data that shows it doesn't work.

Phase II double-blind placebo-controlled randomized study of armodafinil for brain radiation-induced fatigue (nih.gov)

"Interestingly, the participants who had significantly less fatigue at baseline were found to have worse fatigue measures in the armodafinil group versus the placebo group."

These drugs have significant side effects, are costly, and are inappropriate to prescribe cancer patients. You just radiated their whole brain. You gave them a hail mary for a few more months at the end of their life. There is no free lunch. This is something the wacky med onc I used to work with who regularly put cancer patients on testosterone to "improve their energy" would do.

 

[TheWallnerus]

There is also randomized data for Nuvigil. Randomized data that shows it doesn't work.

Phase II double-blind placebo-controlled randomized study of armodafinil for brain radiation-induced fatigue (nih.gov)

"Interestingly, the participants who had significantly less fatigue at baseline were found to have worse fatigue measures in the armodafinil group versus the placebo group."

These drugs have significant side effects, are costly, and are inappropriate to prescribe cancer patients. You just radiated their whole brain. You gave them a hail mary for a few more months at the end of their life. There is no free lunch. This is something the wacky med onc I used to work with who regularly put cancer patients on testosterone to "improve their energy" would do.

Actually the testosterone for guys may be a good idea 😉 May be some data for that

 

[deleted969641]

Actually the testosterone for guys may be a good idea 😉 May be some data for that

The data he threw around was a study with about 14 patients on it. I was not impressed. Is anyone here regularly giving patients testosterone?

 

[medgator]

The data he threw around was a study with about 14 patients on it. I was not impressed. Is anyone here regularly giving patients testosterone?

Only my Prostate pts....
















I kidd, i kidd*




* - whacky urologist in town will give testosterone to PCA patients down the road if they complain enough though

 

[TheWallnerus]

I’m gonna get pilloried but if you dig deep there is data... not saying I believe it or not... that some forms of breast and prostate may be associated with falling levels of E2 or T in older age. I would not have a problem with giving a guy physiologic T levels via T with several years of PSA control or undetectability after definitive therapy. Keep an open mind.

The ASCO Post

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Follow the Johns Hopkins Medicine newsroom for the latest updates in medicine, scientific discovery, and next generation medical education, expert sources, and media contact information.

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Estrogen's Dual Nature? Studies Highlight Effects on Breast Cancer

The story of estrogen's role in breast cancer is starting to look like Dr. Jekyll and Mr. Hyde. A spate of recent studies demonstrates that the hormone—lon

academic.oup.com

 

[medgator]

I’m gonna get pilloried but if you dig deep there is data... not saying I believe it or not... that some forms of breast and prostate may be associated with falling levels of E2 or T in older age. I would not have a problem with giving a guy physiologic T levels via T with several years of PSA control or undetectability after definitive therapy. Keep an open mind.

The ASCO Post

ascopost.com

Newsroom

Follow the Johns Hopkins Medicine newsroom for the latest updates in medicine, scientific discovery, and next generation medical education, expert sources, and media contact information.

www.hopkinsmedicine.org

ClinicalTrials.gov

clinicaltrials.gov

Estrogen's Dual Nature? Studies Highlight Effects on Breast Cancer

The story of estrogen's role in breast cancer is starting to look like Dr. Jekyll and Mr. Hyde. A spate of recent studies demonstrates that the hormone—lon

academic.oup.com

If someone were 5+years out and it was impacting qol significantly, maybe. Not sure I'd be a fan in high risk disease, and it is not something I'd push unless the patient was really being affected by it

 

[thecarbonionangle]

Yes if you talk to urologists who specialize in fertility and hormones for hypogonadism they will tell you that low testosterone gives people prostate cancer in some cases so the picture is more complicated. The data in T replacement in well selected prostate cancer patients is not bad.

 

[evilbooyaa]

CSI in a LMD patient is a big big hail mary, but not unreasonable in the appropriate consented patient. Young, limited extra CNS disease, etc. etc.

Unfortunately, these are ****ty situations and despite efforts you are not likely to durably help vast majority of these people.

 

[Palex80]

In a person who has had cardiotoxic chemo and herceptin lowering heart dose does not have “any clinical benefit”? News to me!!!

Of course, lowering cardiac dose in these patients is important, however do bear in mind the OS-estimate of patients with leptomeningeal spread.
You need to select correctly. The majority of these patients will never live long enough to experience any benefit from heart sparing with protons.

 

[Palex80]

Why are your low isodose lines floating way outside the body. Do you guys tell the TPS the immobilizations are body? Or am I simply seeing it wrong.

Yes, they are all contoured.

 

[TheWallnerus]

Yes, they are all contoured.

Huh. Well as you know when they aren't contoured it means significantly different isodose representations, and dose implications (and optimization implications, and MU implications), intra-body. And looks the the immobilization devices are behaving tissue-equivalent (which they can't be?). That's... wild stuff!

 

[RickyScott]

If someone were 5+years out and it was impacting qol significantly, maybe. Not sure I'd be a fan in high risk disease, and it is not something I'd push unless the patient was really being affected by it

Of course, lowering cardiac dose in these patients is important, however do bear in mind the OS-estimate of patients with leptomeningeal spread.$
You need to select correctly. The majority of these patients will never live long enough to experience any benefit from heart sparing with protons.

I think the main benefit of protons would be less lymphoneia.

 

[Palex80]

Huh. Well as you know when they aren't contoured it means significantly different isodose representations, and dose implications (and optimization implications, and MU implications), intra-body. And looks the the immobilization devices are behaving tissue-equivalent (which they can't be?). That's... wild stuff!

Understood. But they are not "air" now, are they? They do absorb some dose, wouldn't you agree?
We contour them and they are assigned a density value by the physics. That specific patient had a blue bag vacuum cushion.

 

[TheWallnerus]

Understood. But they are not "air" now, are they? They do absorb some dose, wouldn't you agree?

Not >1% absorption. Roughly seems air-equivalent here by HUs, but seems to be absorbing >>1%. IDK man. I'm not a physicist!

 

[StIGMA]

Not >1% absorption. Roughly seems air-equivalent here by HUs, but seems to be absorbing >>1%. IDK man. I'm not a physicist!

whatever it is, it's more accurate dose representation than proton CSI isodoses (major uncertainties in RBE effects as a primary concern)