prostate-specific antigen kinetics after hypofractionated stereotactic body radiotherapy for localized prostate cancer

background: stereotactic body radiotherapy (sbrt) has emerged as an effective treatment for localized prostate cancer. however, prostate-specific antigen (psa) kinetics after sbrt has not been well characterized. the objective of the current study is to analyze the rate of psa decline and psa nadir following hypofractonated sbrt in localized prostate cancer. materials and methods: from 2008 to 2014, thirty-nine patients newly diagnosed, localized prostate (25.6% low risk, 66.7% intermediate risk and 7.7% high risk) cancer were treated with sbrt using cyberknife. total dose of 36.25 gy in 5 fractions of 7.25 gy were administered. no one received androgen deprivation therapy (adt). psa nadir and rate of change in psa (slope) were calculated and compared. results: with a median follow-up of 52 months (range, 13-71), the median rates of decline of psa were -0.372, -0.211 and -0.128 ng/ml/month, respectively, for durations of 1, 2 and 3 years after radiotherapy, respectively. the decline of psa was maximal in the first year and continuously decreased for durations of 2 and 3 year. the median psa nadir was 0.28 ng/ml after a median 33 months. there was one biochemical failure, occurring in a high risk patient. 5-year actuarial biochemical failure (bcf) free survival was 94.2%. conclusion: in this report of localized prostate cancer, continuous decrease of psa level for duration 1, 2 and 3 year following sbrt using cyberknife resulted in lower psa nadir. also, sbrt leaded to long-term favorable bcf-free survival.