Localized HER2-positive breast cancer represents an aggressive subtype with a high risk of recurrence. Despite advances in adjuvant therapy, a significant proportion of patients do not achieve a complete pathological response after neoadjuvant treatment, which increases the risk of disease progression. Case Report: This report discusses the case of a 40-year-old premenopausal woman diagnosed with localized, multifocal HER2-positive breast cancer. The patient received neoadjuvant chemotherapy with anthracyclines, taxanes, and dual anti-HER2 blockade (trastuzumab and pertuzumab). After neoadjuvant therapy, residual disease was detected, leading to adjuvant treatment with T-DM1. Following eight cycles, the patient developed grade 2 pneumonitis, prompting the discontinuation of T-DM1 and the initiation of corticosteroid therapy. After recovery, adjuvant trastuzumab was administered. Given th...