ESR1 and breast cancer: This study aimed to evaluate stromal parameters in HER2-positive breast cancer patients treated with adjuvant trastuzumab.<h4>Material and methods</h4>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.