This study involves two non-randomized groups of patients: the observation group and the comparison group. The comparison group will retrospectively include data on 29 patients with oligometastatic HER2-positive breast cancer who received treatment with trastuzumab-emtansine and had a history of SRT.

The prospective part of this study aims to include 29 patients with oligometastatic HER2-positive breast cancer. These patients will undergo SRT followed by the administration of trastuzumab emtansine 24 hours after the SRT.

The combined effect of stereotactic radiation therapy on a metastatic lesion followed by anti-HER2 therapy in the 2nd line - trastuzumab emtansine, remains unexplored. This study plans to evaluate the effectiveness of combining systemic therapy and local control methods (SRT) in patients with oligometastatic HER2-positive breast cancer. It will be the first time the efficacy and toxic profile of this new combined treatment method in this patient population will be studied. This basket trial evaluates trastuzumab emtansine for oligo-metastatic breast cancer with the aim of inducing deep responses, long-lasting disease remissions, and potentially cure.