Prostate‐specific antigen nadir within 12 months as an early surrogate marker of biochemical failure and distant metastasis after low‐dose‐rate brachytherapy or external beam radiotherapy for localized prostate cancer

Prostate‐specific antigen nadir (nPSA) after radiotherapy for localized prostate cancer has been investigated as a predictor. However, nPSA usually requires several years, limiting its clinical utility. We investigated the significance of nPSA within 12 months (nPSA12) after low‐dose‐rate prostate brachytherapy (LDR‐PB) or external beam radiotherapy (EBRT) on treatment outcomes. Between 2006 and 2014, 663 patients with prostate cancer were treated with LDR‐PB or EBRT at two institutions. Four hundred and seventy‐four men received LDR‐PB and 189 men received EBRT, without androgen deprivation therapy. The Kaplan–Meier method was used for biochemical failure (BF)‐free survival (BFFS) and distant metastasis (DM)‐free survival (DMFS) analyses, and multivariable Cox regression analysis was performed. The median follow‐up was 61.3 months. The median nPSA12 in the LDR‐PB and EBRT cohorts was 0.7 and 1.0 ng/mL, respectively. The 7‐year BFFS and DMFS rates in LDR‐PB patients with nPSA12 ≤ 0.7 ng/mL were 99.1% and 99.5%, respectively; when nPSA12 was >0.7 ng/mL, they were 90.2% and 94.8%, respectively. In EBRT patients with nPSA12 ≤ 1.0 ng/mL, BFFS and DMFS rates were 85.4% and 98.5%, respectively; when nPSA12 was >1.0 ng/mL, they were 67.1% and 87.2%, respectively. nPSA12 was an independent predictor of BF and DM in both cohorts (LDR‐PB, P = 0.004 and 0.020, respectively; EBRT, P = 0.005 and 0.041, respectively). The nPSA12 after LDR‐PB or EBRT is significantly associated with treatment outcomes of prostate cancer. Higher nPSA12 may identify patients at high risk of relapse who might benefit from salvage treatment.