Radiation therapy for prostate cancer increases the risk of subsequent rectal cancer.

PURPOSE: To assess whether radiation therapy for prostate cancer (PCa) increases the risk of metachronous rectal cancer (RCa) and compare outcomes of RCa after radiation therapy and surgery.

PATIENTS AND METHODS: The Israel Cancer Registry was queried to identify patients with PCa and RCa diagnosed between 1982 and 2005. The age adjusted standardized incidence ratio (SIR) of RCa was defined as the ratio between the observed and expected (calculated) RCa cases and compared among the following: overall Israeli male population, patients with PCa treated with radiation therapy, patients with PCa treated surgically. The medical records of men diagnosed with RCa were reviewed and clinical characteristics retrieved.

RESULTS: Of 29,593 men diagnosed with PCa, 2163 were treated with radiation therapy, 6762 were treated surgically and 20,068 patients were treated with either primary androgen deprivation therapy or offered watchful waiting. Of the entire study cohort, 194 (0.65%) patients were diagnosed with subsequent RCa. Compared to the overall male population and stratified by treatment modality, the risk of developing RCa after radiation therapy was significantly increased (SIR = 1.81, 95% CI 1.2-2.5), whereas it was not increased in those managed by surgery (SIR = 1.22, 95% CI 0.85-1.65). RCa after radiation therapy was diagnosed at a more advanced stage, translating into inferior disease specific survival.

CONCLUSIONS: Compared to men diagnosed with PCa managed by surgery, we observed an increased risk of RCa in patients treated with radiation therapy. Further studies are needed to validate these findings and assess whether routine colonoscopic surveillance is warranted after pelvic radiation.