Risk of colorectal cancer after breast cancer

Background History of breast cancer has been reported as a risk factor for colorectal cancer in women. In view of the ambiguous nature of existing evi dence and the growing interest in targeted colorectal cancer prevention, we sought to quantify this risk. Methods We used the Surveillance Epidemiology and End Results (SEER) databa se to estimate risk of colorectal cancer after breast-cancer diagnosis in w omen with first incident breast cancer between 1974 and 1995. Observed colo n and rectal cancer risk was compared with that expected in the general pop ulation. We stratified comparisons by age at breast-cancer diagnosis. stage of cancer, ethnic origin of patient, and follow-up time. Findings Overall, women with previous breast cancer were 5% less likely (95 % CI 1-9) to develop colon and 13% less likely (6-19) to develop rectal can cer than women in the general population. Stratified analyses suggested tha t the risk reductions observed for colon and rectal cancer were most pronou nced for women with breast cancer diagnosed after age 65 years, in white wo men, women with local stage breast cancer, and women diagnosed in the later study years (1990-94). Interpretations Breast cancer does not increase subsequent colorectal cance l risk, and reduced risk was seen for certain subgroups of women. Because n o biologically plausible endogenous protective factor has been identified. we suggest that reduced risk could stem from an accumulation of exposures t hat increase breast-cancer frequency but protect against colorectal cancer.