Increased risk of colon cancer after external radiation therapy for prostate cancer.

Radiotherapy can induce second cancers. Controversies still exist regarding the risk of second malignancies after irradiation for prostate cancer. We evaluated the risk of developing colon and rectum cancers after prostate cancer in irradiated and nonirradiated patients. Using data from the population-based Geneva cancer registry, we included in the study all men with prostate cancer diagnosed between 1980 and 1998 who survived at least 5 years after diagnosis. Of the 1,134 patients, 264 were treated with external radiotherapy. Patients were followed for occurrence of colorectal cancer up to 31 December, 2003. We calculated standardized incidence ratios (SIR) using incidence rates for the general population to obtain the expected cancer incidence. The cohort yielded to 3,798 person-years. At the end of follow-up 19 patients had developed a colorectal cancer. Among irradiated patients the SIR for colorectal cancer was 3.4 (95% confidence intervals [CI] 1.7-6.0). Compared to the general population, the risk was significantly higher for colon cancer (SIR = 4.0, 95% CI: 1.8-7.6), but not for rectal cancer (SIR = 2.0, 95% CI: 0.2-7.2). The risk of colon cancer was increased in the period of 5-9 years after diagnosis (SIR = 4.7, 95% CI: 2.0-9.2). The overall SIR of secondary cancer in patients treated with radiotherapy was 1.35 (p = 0.056). Nonirradiated patients did not have any increased risk of rectal or colon cancer. This study shows a significant increase of colon but not rectum cancer after radiotherapy for prostate cancer. The risk of second cancer after irradiation, although probably small, needs nevertheless to be carefully monitored.