Increased risk of colorectal cancer in patients diagnosed with breast cancer in women

Background: Epidemiological studies have shown a potential association between sex hormones and colorectal cancer. The risk of colorectal cancer in breast cancer patients who may have been exposed to increased levels of endogenous sex hormones and/or exogenous sex hormones (e.g. anti-hormonal therapy) has not been thoroughly evaluated. Methods: Using the National Swedish Cancer Register we established a population-based prospective cohort of breast cancer patients in women diagnosed in Sweden between 1961 and 2010. Subsequent colorectal cancers were identified from the same register. Standardized incidence ratios (SIRs) and 95% confidence intervals (95% CIs) were used to estimate the risk of colorectal cancer after a diagnosis of breast cancer. The association between breast cancer therapy and risk of colorectal cancer was evaluated in a subcohort of breast cancer patients treated in Stockholm between 1977 and 2007. Hazard ratios (HRs) and 95% CIs were estimated using Cox regression models. Results: In a cohort of 179,733 breast cancer patients in Sweden, 2571 incident cases of colorectal cancer (1008 adenocarcinomas in the proximal colon, 590 in the distal colon and 808 in the rectum) were identified during an average follow-up of 9.68 years. An increased risk of colorectal adenocarcinoma was observed in the breast cancer cohort compared with that in the general population (SIR = 1.59, 95% CI: 1.53, 1.65). Adenocarcinoma in the proximal colon showed a non-significantly higher SIR (1.72, 95% CI: 1.61, 1.82) compared with the distal colon (1.46, 95% CI: 1.34, 1.58). In the subcohort of 20,171 breast cancers with available treatment data, 299 cases with colorectal cancers were identified. No treatment-dependent risk of colorectal cancer was observed among the breast cancer patients. Conclusion: An increased risk of colorectal adenocarcinoma - especially in the proximal colon - was observed in the breast cancer cohort. Breast cancer treatment did not alter this risk. (C) 2016 Elsevier Ltd. All rights reserved.