What dose do you prescribe when treating the prostate in patients with low metastatic burden and what are your dose constraints?
I follow the STAMPEDE regimen - either 55 Gy in 20 daily fractions or 36 Gy in 6 weekly fractions. One of the imperatives of the STAMPEDE investigators was to not subject men with low volume metastatic prostate cancer to toxicity - and they found these doses of did not, while still significantly improving overall survival over ADT alone (+ docetaxel in 18%). While we know that higher doses (78 Gy/39, 70 Gy/28, 60 Gy/20) can be safe when treating for curative intent, we do not have evidence that using these doses will be any more effective than using the doses from STAMPEDE. Regarding dose constraints for 55 Gy/20, it would be reasonable to extrapolate from the CHHiP or PROFIT trials.
We've been debating this issue at our institution, too. STAMPEDE also used a 6 Gy x6 delivered one fraction per week regimen.
As these are patients likely to live a long time, even if not cured, then the goal in treating the prostate should be to achieve durable local control, while at the same time minimizing the risk of long term toxicity. In most cases, these men will have a large tumor burden in the prostate, so high radiation doses will be required. Thus far, we have been using the same regimens we would use for men with no evidence of metastatic disease, and for which we have well-defined time-tested normal tissue dose constraints, i.e., 78 Gy in 2 Gy fractions, 72 Gy in 2.4 Gy fractions or 60 Gy in 3 Gy fractions. We have not used SBRT doses in this setting.
Converting to U.S.-style 5 fraction prostate SBRT, would anyone give 7.25 Gy per fraction x 5 fractions for 36.25 Gy, or use a different dose? Treating once weekly, twice weekly, or thrice weekly?