Body Mass Index and Risks of Recurrence and Mortality by Breast Cancer Subtype

Abstract

Background A key modifiable risk factor that may contribute to breast cancer prognosis is body mass index (BMI). Triple negative (TN) and HER2-overexpressing (H2E) breast cancers are particularly aggressive molecular subtypes where lifestyle-focused interventions may be particularly impactful. Methods We conducted a population-based case–case study of 4,557 women aged 20–69 years diagnosed with invasive breast cancer from 2004-2015. Case groups were defined by ER/PR/HER2 status—TN (ER-/PR-/HER2-, n=1,559), H2E (ER-/HER2+, n=615), luminal A (ER+/HER2-, n=2,048), luminal B (ER+/HER2+, n=335). Multivariable-adjusted Cox proportional hazards models were used to evaluate the impact of BMI at diagnosis on recurrence and mortality across each subtype, overall and stratified by menopausal status. Results Median follow-up was 7-years, with 736 (16%) observed recurrences and 733 (16%) deaths. Among women with TN-tumors, overweight (BMI=25-30kg/m2) women had lower risks of recurrence (HR=0.70, 95%CI=0.52-0.95) and mortality (HR=0.66, 95%CI=0.50-0.88) compared to women with a BMI<25kg/m2. Women with H2E-tumors who were obese (BMI>30kg/m2) had a 2.12-fold (95%CI=1.12-4.04) higher risk of recurrence and 2-fold (95%CI=1.19-3.641) higher risk of mortality than women with BMI<25kg/m2. BMI was not associated with risks of recurrence or mortality among patients with luminal A or luminal B tumors. Conclusions Previous studies indicate that obese breast cancer survivors have worse outcomes. However, our results suggest that obesity is associated with increased risks of recurrence and mortality only among younger women with H2E disease. The mechanism underlying these relationships is unclear, but if confirmed, these associations suggest that weight-loss interventions may be particularly beneficial for premenopausal H2E breast cancer patients.