The effect of body mass index on breast cancer stage and breast cancer specific survival.

Purpose: Underweight women and those with obesity, defined as having a body mass index (BMI) ≥ 30 kg/m, diagnosed with breast cancer (BC) are known to have worse prognosis. Whether BMI impacts BC stage at diagnosis and BC specific survival (BCSS) is not understood. We aim to better understand the relationship between BMI with stage at BC diagnosis and BCSS.

Methods: Women age ≥ 15 years old diagnosed with BC between 2014 and 2019 were identified from the California Cancer Registry. BMI at diagnosis was classified as underweight (< 18.5 kg/m), normal weight (18.5-24.9 kg/m), overweight (25-29.9 kg/m), obesity class 1-2 (30-39.9 kg/m), and obesity class 3 (≥ 40 kg/m). BC late stage of diagnosis was defined as American Joint Committee on Cancer stage 3 and 4. Multivariate logistic regression was performed to compare sociodemographic and clinical factors associated with late stage. Multivariable cox proportional hazards regression models assessed association of BMI and BCSS.

Results: Of 159,248 patients: 2.2% were underweight, 34.6% normal weight, 30.5% overweight, 26.7% obesity class 1-2, and 6.0% obesity class 3. Compared to normal weight, patients who were underweight [Hazard Ratio (HR) 1.54, 95% Confidence Interval (CI) 1.51-1.57], obesity class 1-2 [HR 1.06, 1.05-1.07], and obesity class 3 [HR 1.14, 1.12-1.16] were more likely to be diagnosed with late-stage BC. In models stratified by age, patients ≥ 40 years who were underweight had worse BCSS, while patients ≥ 51 years with obesity class 1-2 had better BCSS.

Conclusion: Patients with obesity class 1-2 were more likely to be diagnosed with a later stage, but had improved BCSS, supporting an "obesity paradox" in BC and suggesting that other measures are needed to better assess body composition, adipose distribution, and metabolic health of patients. Patients who were underweight had worse survival, suggesting this high-risk group may benefit from being assessed and treated for possible sarcopenia and malnourishment.