Is Body Mass Index a Prognostic Factor in Metastatic HER2-Positive Breast Cancer? A Real-World Multicenter Study

Background and Objectives: The prognostic significance of body mass index (BMI) in metastatic HER2-positive breast cancer (BC) remains unclear, with previous studies yielding conflicting results. This multicenter real-world study aimed to investigate the prognostic role of BMI in this patient population. Materials and Methods: A total of 169 female patients with metastatic HER2-positive BC who received trastuzumab-based treatment between 2010 and 2024 were included. Patients were categorized by BMI (<30 kg/m2 vs. ≥30 kg/m2). The primary endpoints were overall survival (OS) and progression-free survival (PFS). Kaplan–Meier and Cox regression analyses were performed overall and in subgroups stratified by hormone receptor (HR) status. Results: In the overall cohort, a BMI ≥ 30 was not significantly associated with OS or PFS. However, in the HR-positive/HER2-positive subgroup, BMI ≥ 30 kg/m2 was linked to significantly shorter OS (p = 0.024) and PFS (p = 0.047) by Kaplan–Meier analysis. However, these associations did not remain statistically significant in multivariate analyses. No significant BMI-related differences were observed in the HR-negative subgroup. Other independent negative prognostic factors included recurrent disease, the presence of brain metastases, and a high Ki-67 index. Conclusions: BMI was not identified as an independent prognostic factor in the overall population. However, among HR-positive/HER2-positive patients, obesity was associated with poorer survival in univariate analysis, but this was not confirmed in multivariate analysis. These findings underscore the need for prospective studies to clarify the prognostic role of adiposity, considering biological subtypes.