Molecular Analysis of Cerebrospinal Fluid Tumor-Derived DNA to Aid in the Diagnosis and Targeted Treatment of Breast Cancer Brain Metastasis

A woman in her 40s with a history of ER/PR+, HER2-negative breast cancer presented with a seizure three years after mastectomy. Magnetic resonance imaging (MRI) revealed a right caudate head mass, which was concerning for either high-grade glioma or metastatic disease, but biopsy was deemed too high risk. Cerebrospinal fluid (CSF) tumor-derived DNA (tDNA) analysis by next-generation sequencing (NGS) was ordered, revealing a gain-of-function variant in <i>PIK3CA</i>, <i>ERBB2</i> copy number gain, and high aneuploidy, findings consistent with breast cancer brain metastasis. Based on these results, the patient was treated with stereotactic radiosurgery (SRS) followed by trastuzumab deruxtecan, a HER2-targeted therapy. This case highlights the diagnostic and therapeutic value of CSF tDNA analysis in central nervous system (CNS) lesions when biopsy is not feasible. The report also illustrates how clonal evolution, such as acquired <i>ERBB2</i> amplification, can occur in metastatic disease and influence treatment decisions.