Approximately 5-15% breast cancer patients develop brain metastases. Current local treatments (surgery/radiotherapy) are associated with significant complications, and systemic treatments have limited efficacy. Although new-generation ADC drugs, such as trastuzumab deruxtecan, have demonstrated breakthrough efficacy in patients with brain metastases, the mechanisms of action are not fully elucidated, moreover the current treatment regimens still have limitations for patients with HER2-low expressing, safer and more effective systemic treatment options are urgently needed to improve patient survival. BCBM-006 is an open-label, prospective, single-arm, single-center Phase II clinical study to evaluate the relationship between cerebrospinal fluid drug concentration and efficacy of trastuzumab deruxtecan in central nervous system metastatic breast cancer, and explore the incidence of leptomeningeal metastases diagnosed via lumbar puncture in patients with brain parenchymal metastases, facilitate early intervention to improve prognosis.