Tall Allen said...
CJ,
I haven't seen it -- have you? I suspect it's a clinical trial. MSKCC is doing a similar trial of LDR brachy with SBRT that also leaves me scratching my head.
Found the clinical trial here:
/clinicaltrials.gov/ct2/show/NCT01655836Tall Allen said...
There is no "may" about it - it does respond better to hypofractionation. Otherwise HDR brachy monotherapy which has been used since the 1990s, and SBRT, which has been used since 2003, would never have been curative.
Agreed, I just don't like to talk in such definitive terms myself since I'm not a doctor and I'm fairly new here to boot.
Tall Allen said...
No. These are apples and oranges. To make them comparable, ROs compute a value called "biologically effective dose (BED)" for cancer control as well as healthy tissue based on a factor called the alpha-beta ratio. Because of the very low alpha/beta ratio, somewhere around 1.5 for prostate cancer, there is a multiplier effect of hypofractionation - an SBRT schedule of 5 x 7.25 Gy (=36.25 Gy) is biologically equivalent in cancer killing power to a dose of about 97 Gy if it had been given in typical IMRT dose rates of 1.8 Gy. This is far beyond what can be safely given with IMRT.
At the same time, acute toxicity is based on a much higher alpha/beta ratio of about 10 for healthy tissues. This means that that SBRT schedule of 5 fx x 7.25 Gy has a third less acute toxicity than a typical IMRT treatment schedule of 44 fx x 1.8 Gy. Both the cure rates are higher, and the acute toxicity is lower with SBRT (or HDR brachy).
I think we were saying the same thing in lamen's terms - though you said it much more specifically and accurately then I did.
I was referring only to hyperfractionated RT initially - so your example proves my point - a hyperfractionated RT that would deliver 97Gy would deliver far higher SE's when compared to hypofractionated RT. I've read the studies on dosage levels between 70-82Gy for hyperfractionated RT that do have better cure rates and BC, but also worse SEs.
What I was not as aware of was the lower alpha/beta ratio for hypofractionated RT, resulting in lower acute toxicity. That's great info and thanks for passing this along with numeric examples.
I do understand that the
short term SE's for hypofractionated RT tend to be more acute - because of the higher fractional dosage, but the long term SEs are not as bad. Again, your numeric examples prove this out in more detail.
Tall Allen said...
All radiation therapies treat the entire prostate, so there is no need to localize where the tumors are. It is multifocal 80% of the time, so cancer foci are assumed to be everywhere. Some ROs have experimented with something called a Simultaneous Integrated Boost (SIB) to identified lesions within the prostate, with uncertain benefits and possibly higher toxicity. The argument against SIB is that since we know the cancer is spread throughout, perhaps microscopically, then the whole prostate should be given a curative dose consistent with low toxicity.
Got it, it was my understanding that if a tumor can actually be seen on MPMRI or CT, that the dosimetry and maps are sometimes altered to deliver, when possible, higher radiation dosage to the tumor area. I know in my case that my MPMRI results and my fusion biopsy results are being used in this manner for my HDR BT treatment - or at least that is what I was told. I'll have to follow up to be sure. In any case, thanks for sharing your knowledge - it helps us all to learn more about
Pca, which is a good thing at least for me.