ERBB2 and breast cancer: This study investigates the effectiveness and safety of a T-DM1 biosimilar in routine oncology practice across adjuvant and metastatic HER2-positive breast cancer populations.<h4>Methods</h4>In this prospective, single-institute study conducted at a tertiary oncology center in India, 116 patients with HER2-positive breast cancer (51 adjuvant and 65 metastatic) received T-DM1 biosimilar (Ujvira) between June 2021 and October 2023.