Nadir later.

We’ve seen that PSA kinetics can differ after SBRT compared to conventionally fractionated external beam radiation. But are the kinetics of biochemical failure different after SBRT compared to conventional EBRT? This retrospective study of > 2000 men treated with SBRT for low (46%) or intermediate risk (32% favorable, 22% unfavorable) prostate cancer tested different PSA thresholds for detecting BCR, but ultimately found that the Phoenix definition (nadir + 2) remained the most valid. Lower thresholds were more sensitive but less specific and had very high false positive rates (which was still ~30% using Phoenix). Median nPSA was higher among those who eventually developed BCR (0.63) than those who didn’t (0.15), and median time to nPSA was shorter (23 v 48 months). The authors found that two criteria (ratio of PSA@18 months to PSA@6 months ≥ 2.6 and nadir + 2 occurring ≥ 18 months after treatment) helped reduce the false positive rate of the Phoenix definition. | Ma, Radiother Oncol 2021