Skeletal muscle index as a prognostic biomarker for survival in HER2-positive and triple-negative breast cancer: a retrospective cohort study.

[OBJECTIVES] Sarcopenia, characterised by reduced skeletal muscle mass and strength, is increasingly recognised as a prognostic marker in oncology. Its role in breast cancer and its variation across molecular subtypes remains unclear. This study examined the prognostic value of sarcopenia, defined by Skeletal Muscle Index (SMI), and its association with overall survival (OS) among breast cancer subtypes.

[METHODS] This retrospective study included 523 women diagnosed with breast cancer at Ege University between January 2014 and January 2015. Clinical, pathological and anthropometric data were collected. Sarcopenia was evaluated using bioelectrical impedance analysis (BIA) with the TANITA SC-330 device to estimate appendicular muscle mass, and defined as SMI <5.5 kg/m². Survival outcomes were analysed using Kaplan-Meier curves and Cox regression models adjusted for age, tumour stage, molecular subtype and body mass index. Subgroup analyses were performed for luminal A, luminal B, HER2-positive and triple-negative breast cancer (TNBC) subtypes.

[RESULTS] The mean age was 57.6±13.0 years, and 9.7% were sarcopenic. Sarcopenia was significantly associated with reduced OS (HR: 1.789, p=0.025). While no significant effect was seen in luminal A or B subtypes, sarcopenia predicted markedly worse survival in HER2-positive patients (HR: 11.247, p=0.001) and TNBC patients. Recurrence rates were similar between sarcopenic and non-sarcopenic individuals.

[CONCLUSION] Sarcopenia assessed by BIA is an independent predictor of poor survival in breast cancer, particularly in HER2-positive and TNBC subtypes. Incorporating muscle assessment into routine evaluation may help identify high-risk patients and guide supportive care.