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This parallel cohort analysis of two phase II trials compared PSA kinetics and biochemical failure after 2-fraction SABR and 2-fraction HDR brachytherapy. Of note, the 2-fraction HDR patients were part of a trial comparing 1-fraction and 2-fraction HDR. Enrolled patients had low to intermediate risk prostate cancer. In the SABR cohort, 30 patients received 26 Gy in two weekly fractions. A 3mm PTV margin was used and dose was prescribed such that 99% of the CTV received the prescribed dose. In the HDR cohort, 83 patients received 27 Gy in 2 fractions prescribed such that the V100 was >95%, V150<35%, and V200<12%. As you can see, dose heterogeneity was much greater in the HDR plans, which led the authors to hypothesize that HDR would be more effective. In fact, mean dose to the CTV was 26.7Gy with SABR and 40.5Gy with HDR. There was no difference between SABR and HDR with respect to PSA nadir (0.16 v 0.16), time to nadir (57.5 v 56.9mo), or the rate of PSA<0.4 at 4 years (56.7% v 63.9%). There was also no difference in the rate of biochemical failure with SABR v HDR (3.5% v 12.8% at 72 months). Overall there were no major differences in toxicity, however SABR tended toward slightly worse GI toxicity while HDR had slightly worse GU toxicity. | Correa, Radiother Oncol 2022