Where are you getting that heterogeneity is part of the definition (not YOUR definition)? What about a solitary brain met in middle of parenchyma (what critical structureS? Just one - normal Brian?) or a bone met (just bone and soft tissue)? Fixation - Who’s definition? “Incredible” conformality? What does that mean?
Not devil’s advocate. I’m not arguing against a real opponent / platform. People just saying whatever they think.
Sounds very reasonable. I will forward this analysis to a bold grifter I know. Maybe get some greasy wheels moving!In this calc, we will completely eliminate time corrections from consideration. This may actually be reasonable for very slowly dividing, very large Tpot tumors.
There is, kind of, radiobiological clinical precedent of very small fraction prostate radiotherapy: that’s brachytherapy. If we assume that I-125 has a half life of 60 days, and thus has a useful life of ~90 days, we can break the 145 Gy Rx dose into 90 days of TID doses of 0.5Gy per “fraction,” which comes out to 135 Gy. That is to say, I can enforce a radiobiological similarity between brachy 145 Gy using I-125 and 90 continuous days of 0.5 Gy per fraction, three-times-a-day RT (like CHART in lung) w/ 8h interfraction intervals, and the clinical outcomes should be very similar.
I already pre-established I’m ignoring time.
If that’s the case, we can just say 270 fractions of 0.5 Gy. Again, completely ignoring time, we can break this into 0.5 Gy per day/fraction over 54 weeks.
Therefore, we could maybe run a rational trial of 270 fractions of 0.5 Gy/day over 54 weeks vs e.g. 81 Gy/45 fractions over 9 weeks. Interestingly, the alpha/beta that would make these two regimens similar is: 1.45.
And as we all know, a prostate alpha/beta of 1.45 is quite a very reasonable and nice result that we actually just “accidentally” derived by trying to adjust very long fractionation to be normative w/ brachy and 81/45. Unfortunately, we would predict late tissue effects to be 20 to 25% higher w/ 135 Gy over 54 weeks. The reimbursement per patient w/ 270 fractions would be beyond the dreams of avarice.
Sounds pretty arbitrary. Maybe just let us in on what the super top secret manual says at your New England academic satellite?
it works!
What I'm looking for is clarity. A lot of this stuff doesn't make sense.
All partial breast (and whole breast) in the UK is 26/5, not 30/5. They explained it’s a logical conclusion of IMPORT and FAST forward. So that’s what I now do when doing partial breast (I think @radmonckey does same?). However knowing that 30/5 is reasonable, I don’t mind a 115% hot spot on 26/5 (it’s still <30 Gy max!). I don’t seek it out, but I wouldn’t eschew it.
To pull back from my reductio ad absurdum, some payors call for a “high degree of accuracy and precision” w/ SBRT. The best analogy from science/engineering for precision in rad onc is how well (and homogenous actually) the prescribed 100% dose shape matches the target shape. Ie, IMRT with 20 fields or VMAT is going to be more highly precise than an AP PA w/MLCs. (Also and btw the larger the PTV to CTV/GTV ratio, the less the Rx dose will match the shape of the intended target. The conformity index never was intended for a PTV, so the bigger the PTV the worse the conformity index if you’re playing by the book. CI is a measure of precision too.) The best analogy for accuracy is IGRT. No payor has ever mentioned hetero/homogeneity of the dose for defining SBRT. Ever.
It’s logical but the toxicity is so good from 30/5 so I do sib 26/5 and 30/5 to smaller volume
Sounds like SBRT
lusty SBRT evenI still have trouble coming to grips with the logical conclusion that 27/5 is way more toxic than 26/5.
Perhaps, but by extrapolation, the question then is, is 26/5 truly isotoxic with 40/15, despite being barely less dose than 27/5, and numerically inferior to 40/15 in a trial not powered to detect toxicity differences?I have trouble with it too. That said, it's kind of like the Pascal's wager...
39/13 vs 40/15 was associated w/ significantly more "shoulder problems" (brach plexopathy? stiffness? what? IIRC not defined?) in one of the START trials and also I think non-significant trends in breast fibrosis. Not quite "barely less dose" with 39/13 vs 40/15 compared with 27/5 vs 26/5, but maybe everything we think we know about alpha/beta easily sliding along the mathematical scale the higher and higher we push the fractional dose we don't know. (Because when you say 26/5 is "barely less dose than 27/5" you can only rationally say that using linear quadratic.) Which just allows me to use my favorite phrase in breast: "The therapeutic window appears to be very narrow."
yes, but that was in the context of also keeping time of treatment equivalent right? 50/25 vs 39/13, both over 5 weeks. But yes, the 41.6 vs 39 in 5 weeks difference is also weird.
pubmed.ncbi.nlm.nih.gov
So what about the celiac plexus "SBRT" paper. No GTV... What about post op "SRT" for brain met? No GTV.
I think ~5-6
When I do prostate SBRT, or even IMRT for that matter, I always label it "Prostate CTV." And this prostate-as-CTV rubric is widespread throughout the literature. The ratio of normal cells to cancer cells inside the prostate must be >100 to 1 in most cases, and we irradiate the whole prostate "electively" (as seeing a tumor and irradiating said tumor-only are both problematic in prostate).
I would imagine only if you delineate a DIL to sib. Most of the dosimetrists/physicians I've spoken to dislike a central hotspot due to potential urethral tox.
I think "Hot In The Target" used to play at Peepland in Times Square in the 70s.
So prostate "SBRT" of 5 times 7.5 Gy:
It is what someone is willing to pay you.
Everyone adopting 5 fraction whole breast should reference Table A5 of the supplementary material for a blinded analysis of photographic breast appearance at 2 and 5 years between the 3 arms. 3-4% of women in either hypofractionated arm had a physician assessed marked change in breast appearance at 2 and 5 years. For 40/15 it was 0.5% (2 patients) at 2 years and 0.3% (1 patient) at 5 years. The tools they use to claim no statistically meaningful difference are wrong. It is unlikely that if we had a million patients these differences would not persist.I have trouble with it too. That said, it's kind of like the Pascal's wager...
Yes, thank you. This trial was designed backwards from the desired endpoints. It's an imrt trial as it was inversely planned.
I don’t do 5 fx whole breast. Either they are a candidate for 5 fx APBI or do 16 fx whole breast.
Have you submitted it as sbrt ?
Some kind of real time imaging/immobilization as well. I really don't see the issue with it at all
I can't seem to find the hard references/tweets right now but at ACRO 2022 it looked like... at least from afar... that 5 fraction breast is being called SBRT at many institutions. Whether billed as such who knows!!!
To me it’s due the volume being treated. Maybe arbitrary but I think at that size I don’t think of it is sbrt - it’s not focused or pinpoint in any way. But that’s subjective
