A Phase II Study of T-DXd Plus SRT in HER2-positive Breast Cancer Brain Metastases

Inclusion Criteria: * Pathologically confirmed HER2 positive advanced breast cancer * Age\>18 years. * Brain metastases confirmed by enhanced brain MRI. Metastases number less than 15. * KPS≥70 or KPS ≥60 with neurologic symptoms caused by BM * Life expectancy of more than 6 months * Prior therapy of oral dexamethasone not exceeding 16mg/d * Time interval from prior therapy was more than 2 weeks, and evaluation of adverse events is no more than grade 1. * Screening laboratory values must meet the following criteria( and should be obtained within 28 days prior to registration): 1. Absolute Neutrophil Count (ANC) ≥ 1.5 x 109/L, Platelets≥ 90 x 109/L, Hemoglobin ≥ 90 g/L 2. Aspartate aminotransferase/alanine aminotransferase (AST/ALT) ≤2.5 x ULN without liver metastasis,≤ 5 x ULN with liver metastases 3. Serum BUN and creatinine ≤ 1.5 x Upper Limit of Normal (ULN) 4. LVEF ≥ 50% 5. QTcF \< 480 ms 6. INR≤1.5×ULN，APTT≤1.5×ULN * Signed the informed consent form prior to patient entry Exclusion Criteria: * Leptomeningeal or hemorrhagic metastases * Uncontrolled epilepsy * Severe or uncontrolled disease: severe cardiovascular disease, end-stage renal disease, severe hepatic disease, history of immunodeficiency, including HIV positive, active HBV/HCV or other acquired congenital immunodeficiency disease, or organ transplantation history, active infection, etc. * History of allergy to treatment regimens * Pregnancy or lactation period， women of child-bearing age who are unwilling to accept contraceptive measures. * Inability to complete enhanced MRI * Not suitable for inclusion for specific reasons judged by sponsor