A prospective, single-arm study to explore the efficacy and safety of SHR-A1811 combined with radiotherapy in patients with HER2-positive or HER2-low brain metastases

Inclusion Criteria:

1. Pathologically confirmed HER2 positive (IHC 3+ or IHC 2+/FISH+)or HER2-low(IHC1+ or 2+/FISH-) advanced breast cancer
2. Age>18 years.
3. newly diagnosed Brain metastases confirmed by enhanced brain MRI. Metastases number less than 6.
4. No more than four lines of previous advanced therapy
5. Life expectancy of more than 3 months
6. There were no known or suspected leptomeningeal brain metastases
7. The interval from previous therapy was more than 2 weeks, and acute toxicity from previous therapy had to resolve to grade 1 or less
8. The use of mannitol, bevacizumab, or hormone therapy was allowed before enrollment, but the dose was stable for at least a week without the need for an increment
9. Screening laboratory values must meet the following criteria( and should be obtained within 28 days prior to registration):

Absolute Neutrophil Count (ANC) ≥ 1.5 x 109/L, Platelets≥ 90 x 109/L, Hemoglobin ≥ 90 g/L Aspartate aminotransferase/alanine aminotransferase (AST/ALT) ≤2.5 x ULN without liver metastasis,≤ 5 x ULN with liver metastases Serum BUN and creatinine ≤ 1.5 x Upper Limit of Normal (ULN) LVEF ≥ 50% QTcF < 480 ms INR≤1.5×ULN，APTT≤1.5×ULN 10.Signed the informed consent form prior to patient entry

Exclusion Criteria:

1. Leptomeningeal or hemorrhagic metastases
2. previously used T-dxd
3. Uncontrolled epilepsy
4. 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.
5. History of allergy to treatment regimens
6. Pregnancy or lactation period， women of child-bearing age who are unwilling to accept contraceptive measures.
7. Inability to complete enhanced MRI
8. Not suitable for inclusion for specific reasons judged by sponsor -