Incidence of Right-Sided Colorectal Cancer After Breast Cancer: A Population-Based Study

OBJECTIVES: Estrogen levels, which are involved in the development of breast cancer, may also be responsible for a higher incidence of right-sided colorectal neoplasia in women. Our objective was to determine the incidence of right-sided colorectal cancer (CRC) after the diagnosis of breast cancer. METHODS: All cases of breast cancers diagnosed between 1956 and 2006 were identified from the Manitoba Cancer Registry (MCR) and followed up until the diagnosis of any invasive cancer, death, migration out of the province, or 31 December 2006. Standardized incidence ratios (SIRs) for all CRC and right-sided CRC (cecum, ascending colon, and hepatic flexure) were calculated to compare the observed CRC incidence with that expected in the general population. Stratified analysis was performed to determine the risk at different follow-up time intervals, age at breast cancer diagnosis, and for tamoxifen use. RESULTS: There were 23,377 cases of breast cancer diagnosed between 1956 and 2006 with a total follow-up of 221,364 patient-years. The SIR for all CRC was 0.96 (95% confidence interval (CI) 0.87-1.06) and for right-sided CRC it was 1.02 (95% CI 0.87-1.20). The SIRs remained close to unity at different time intervals, for different age groups, and in analysis restricted to more recent years (1985-2006). Tamoxifen use did not alter the risk of all CRC (SIR 1.22; 95% CI 0.92-1.62) or right-sided CRC (SIR 0.90; 95% CI 0.48-1.54). CONCLUSIONS: There is no increase in the overall risk for CRC or for right-sided CRC after the diagnosis of breast cancer. CRC screening strategy for breast cancer survivors should be similar to that for the general population.