A Comparative Study of Breast Tumors Before and After Neoadjuvant Therapy

Background: Pathological response after neoadjuvant therapy in breast cancer varies substantially across immunohistochemical surrogate subtypes. This study aimed to compare pretreatment biopsy features with post-treatment surgical pathology findings and to explore baseline predictors of favorable pathological response. Methods: We retrospectively analyzed female patients with invasive breast carcinoma treated with neoadjuvant therapy followed by surgery at a single institution between January 2020 and December 2024. Pretreatment histological features, stromal tumor-infiltrating lymphocytes, tumor cellularity, necrosis, in situ component, lymphovascular/perineural invasion, and immunohistochemical surrogate subtype were compared with post-treatment findings. Residual disease was assessed using the Residual Cancer Burden system, and vital status was analyzed as an exploratory outcome. Results: The analytical dataset included 146 breast carcinoma records. RCB-0, RCB-I, RCB-II, and RCB-III were observed in 20, 14, 64, and 46 of 144 evaluable records, respectively, and pCR was identified in 18/142 evaluable records. Tumor cellularity decreased significantly after therapy, whereas stromal TILs and tumor necrosis did not change significantly. Favorable pathological response and pCR were more frequent in HER2-positive and triple-negative tumors than in luminal tumors. In univariable Firth regression, higher baseline stromal TILs and HER2-positive/triple-negative subtype were associated with increased odds of RCB 0/I and pCR, and both associations persisted in multivariable models adjusted for age and grade. Nineteen patients (13.0%) deceased; crude mortality increased across RCB classes (0%, 14.3%, 7.8% and 26.1% for RCB-0 to RCB-III; p = 0.008), with no deaths among patients achieving pCR. Conclusions: In conclusion, favorable pathological response after neoadjuvant therapy was uncommon in this cohort and was observed mainly in HER2-positive and triple-negative tumors. Baseline stromal TILs and immunohistochemical surrogate subtype showed exploratory associations with response, supporting their further evaluation in larger, treatment-stratified cohorts with formal survival follow-up.