• (1) Women ≥18 years of age :(2) pathological diagnosis of HER2-positive breast cancer, defined as positive immunohistochemistry detection (+++) or FISH(Fluorescent In Situ Hybridization); (3) Imaging evidence of brain metastases; (4) Progression of brain metastases after prior treatment with pyrotinib, defined as progression of new or pre-existing brain metastases during treatment with pyrotinib; (5)ECOG score ≤3 points; (6) The efficacy of T-DXd should be evaluated at least once after treatment; (7) Bone marrow and organ functions are basically normal; (8) Complete medical records