Body mass index, tumor characteristics, and prognosis following diagnosis of early-stage breast cancer in a mammographically screened population.

Purpose: Many studies suggest increased body mass index (BMI) is associated with worse breast cancer outcomes, but few account for variability in screening, access to treatment, and tumor differences. We examined the association between BMI and risk of breast cancer recurrence, breast cancer-specific mortality, and all-cause mortality, and evaluated whether tumor characteristics differ by BMI among a mammographically screened population with access to treatment.

Methods: Using a retrospective cohort study design, we followed 485 women aged ≥40 years diagnosed with stage I/II breast cancer within 24 months of a screening mammogram occurring between 1988 and 1993 for 10-year outcomes. BMI before diagnosis was categorized as normal (<25 kg/m(2)), overweight (25-29.9 kg/m(2)), and obese (≥30 kg/m(2)). Tumor marker expression was assessed via immunohistochemistry using tissue collected before adjuvant treatment. Medical records were abstracted to identify treatment, recurrence, and mortality. We used Cox proportional hazards to separately model the hazard ratios (HR) of our three outcomes by BMI while adjusting for age, stage, and tamoxifen use.

Results: Relative to normal-weight women, obese women experienced increased risk of recurrence (HR 2.43; 95 % CI 1.34-4.41) and breast cancer death (HR 2.41; 95 % CI 1.00-5.81) within 10 years of diagnosis. There was no association between BMI and all-cause mortality. Obese women had significantly faster growing tumors, as measured by Ki-67.

Conclusions: Our findings add to the growing evidence that obesity may contribute to poorer breast cancer outcomes, and also suggest that increased tumor proliferation among obese women is a pathway that explains part of their excess risk of adverse outcomes.