Prognostic Significance of Selected Tumor Stroma Parameters in Patients with HER2-Positive Breast Cancer Treated with Adjuvant Trastuzumab

Background: While the prognostic and predictive value of tumor cell–derived features such as grade, estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2) status, and Ki67 index is well established in breast cancer, less is known about the prognostic role of tumor stroma. This study aimed to evaluate stromal parameters in HER2-positive breast cancer patients treated with adjuvant trastuzumab. Material and methods: The study included 224 patients (T ≥ 1, N ≥ 0, M0) who underwent radical treatment followed by adjuvant chemotherapy, hormone therapy (if ER/PR-positive), and trastuzumab. The following histological and immunohistochemical parameters were analyzed: stroma type, tumor-infiltrating lymphocytes (TILs), eosinophils, neutrophils, central area of fibrosis, necrosis, and programmed cell death protein ligand 1 (PD-L1) expression in tumor and stromal cells. Results: Low TILs percentage (≤50%) was associated with lower tumor grade (G2) (p = 0.013) and ER/PR positivity (p = 0.001). Tumors lacking PD-L1 expression had a lower percentage of TILs (p < 0.001), less frequently exhibited tumor-associated neutrophilia (p = 0.019), and more often presented with desmoplastic stroma (p < 0.001). The following parameters were associated with prognosis: TILs percentage, stroma type, and PD-L1 expression. High TILs percentage (>50%) was an independent positive prognostic factor. Conclusions: In patients with HER2-positive breast cancer treated with adjuvant trastuzumab, the percentage of TILs, stroma type, and PD-L1 expression are prognostically relevant. Specifically, a TILs percentage >50% independently predicts favorable outcomes. Routine evaluation of stromal features may provide additional prognostic information and support treatment planning.