['External beam radiation therapy (EBRT) for rectal cancer unavoidably delivers significant radiation dose to the prostate gland. The effect of this incidental exposure on subsequent prostate cancer diagnosis was investigated using the Surveillance, Epidemiology, and End Results (SEER) cancer registry.', 'Men diagnosed with localized or regional (L/R) rectal cancer from 1988-1997 and treated with EBRT and sphincter-sparing surgery (SSS) were identified. Men treated for L/R rectal cancer with SSS who did not receive EBRT, and men with L/R colon cancer who did not receive EBRT, were studied for comparison. Multiple Primary Standardized Incidence Ratios of observed to expected (O/E) cases of prostate cancer were calculated using SEER*Stat.', 'In all, 1574 men with L/R rectal cancer treated with EBRT and SSS were identified. The median age at diagnosis was 64 and median survival was 76 months. Twenty were subsequently diagnosed with prostate cancer, a number significantly less than expected compared with the general population of similar age and race. The ratio of O/E cases was 0.28 (95% confidence interval [CI], 0.17, 0.43). In contrast, 3114 men diagnosed with rectal cancer undergoing SSS who were not treated with EBRT and 24,578 men diagnosed with colon cancer who were not treated with EBRT were subsequently diagnosed with prostate cancer at rates similar to the general population (O/E of 0.94 and 1.09).'] Is external beam radiation treatment for rectal cancer associated with a decrease in subsequent prostate cancer diagnosis?