Pretreatment Body Composition and Its Association with Pathologic Complete Response and Survival in Breast Cancer Patients Undergoing Neoadjuvant Chemotherapy: A Retrospective Cohort Study

Background and Objectives: We investigated how pretreatment body-composition parameters relate to pathologic complete response (pCR), disease-free survival (DFS), and overall survival (OS) in women with non-metastatic breast cancer who received neoadjuvant chemotherapy (NAC). Materials and Methods: We retrospectively analyzed 278 women treated with NAC at a single institution between January 2010 and December 2022. On computed tomography (CT) or PET-CT, visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT), skeletal muscle mass (SMM), and skeletal muscle density (SMD) were measured at the third lumbar vertebra (L3) with a semi-automated technique. Receiver operating characteristic (ROC) analysis was used to define optimal SMM cut-offs, and survival was examined by Kaplan–Meier estimation and Cox proportional-hazards regression. Results: The median follow-up was about 62 months. BMI was correlated with VAT (r = 0.449) and SAT (r = 0.482) but not with SMM (r = 0.075; p = 0.211). SMM was higher in women who attained pCR (107.4 ± 14.4 vs. 101.1 ± 16.1; p = 0.003). Derived by ROC analysis against distant metastasis—the event defining DFS—the SMM cut-off was 105 cm2 (AUC = 0.725); values below it predicted poorer DFS on both univariable (HR = 4.80; p < 0.001) and multivariable testing (adjusted HR = 3.83; 95% CI 1.78–8.26; p = 0.001). The recurrence/metastasis rate was far higher among sarcopenic obese women (n = 34) than non-sarcopenic obese women (n = 56) (41.2% vs. 10.7%; log-rank p = 0.0004). With adjustment for molecular subtype, greater SMM remained an independent correlate of pCR (adjusted OR 1.26 per 10-cm2 increment; 95% CI 1.04–1.53; p = 0.020). SMM was not significantly related to OS (univariable HR 1.75; p = 0.081). Sensitivity and specificity of the 105-cm2 cut-off were 92.3% and 48.2%, respectively. Conclusions: In this retrospective cohort, greater pretreatment skeletal muscle area was independently linked to pathologic complete response and lower area to worse disease-free survival, while no significant link emerged with overall survival. Since BMI captures muscle mass poorly, body composition obtained from CT/PET-CT may add complementary prognostic information; nevertheless, because the design was observational and the cut-offs internally derived, these hypothesis-generating results need prospective external validation before clinical adoption.