Bariatric Surgery and Breast Cancer Incidence: a Population-Based, Matched Cohort Study

Purpose Obesity is associated with increased breast cancer risk in women. Bariatric surgery induces substantial weight loss. However, the effects of such weight loss on subsequent breast cancer risk in women with obesity are poorly understood. To examine breast cancer incidence and related outcomes in women with obesity undergoing bariatric surgery. Materials and Methods This was a population-based matched cohort study of breast surgery outcomes utilizing linked clinical databases in Ontario, Canada. Women with obesity who underwent bariatric surgery were 1:1 matched using a propensity score to non-surgical controls for age and breast cancer screening history. The main outcomes were incidence of breast cancer after lag periods of 1, 2, and 5 years. Additional outcomes included tumor hormone receptor status, cancer stage, and treatments undertaken. Time-varying Cox proportional hazard models accounting for screening during follow-up were used to model cancer incidence. Results A total of 12,724 women per group were included, average age 45.09. After a 1-year lag, breast cancer incidence occurred in 1.09% and 0.79% of the control and surgery groups, respectively (adjusted hazard ratio, 0.81 [95%CI 0.69-0.95]; p = 0.01). This association was maintained after lag periods of 2 and 5 years. Women in the surgical cohort diagnosed with breast cancer were more likely to have low-grade tumors and less likely to have high-grade tumors (overall p < 0.01). No association was found for tumor hormone receptor status, although the surgical group was more likely to have her2neu-negative tumors (p = 0.01). Conclusion Bariatric surgery was associated with a lower incidence of breast cancer and lower tumor grade in women with obesity. Further evaluation of outcomes, including mortality, is required.