Risk of Colorectal Cancer by Subsite in a Swedish Prostate Cancer Cohort.

BACKGROUND The relationship between sex hormone-related treatment for prostate cancer and the risk of colorectal cancer is controversial. METHODS A prostate cancer cohort was initiated from the Swedish Cancer Registry of patients diagnosed between 1961 and 2008. Patients diagnosed with prostate cancer between 1961 and 1980 were generally treated with estrogen. The cohort diagnosed between 1981 and 2008 was further divided into 3 subcohorts of orchiectomy, prostatectomy, and other treatment. Standardized incidence ratios (SIRs) for developing colorectal adenocarcinoma were estimated and 95% confidence intervals (CIs) were used to compare relative risk among these patients and the general male population. RESULTS Of 601,542 person-years of follow-up, 1,698 cases of colorectal adenocarcinoma were identified. Compared with the general male population, no association was detected in the cohort diagnosed between 1961 and 1980, whereas an increased risk of colorectal adenocarcinoma was observed among patients diagnosed with prostate cancer who received treatments other than estrogen. Following bilateral orchiectomy, the SIR was 1.30 (95% CI: 1.14-1.47); after prostatectomy, the SIR was 1.22 (95% CI: 1.04-1.43); among those who received treatment other than estrogen, the SIR was 1.37 (95% CI: 1.29-1.45). The increased risks were more apparent in cases of adenocarcinoma of the distal colon and rectum than in the proximal colon. CONCLUSIONS Patients with prostate cancer undergoing bilateral orchiectomy, prostatectomy, or other treatments, including antiandrogen therapy and radiation, may be at increased risk for colorectal adenocarcinoma.