Efficacy of trastuzumab deruxtecan in treating HER2-low breast cancer leptomeningeal metastasis: a case report.

We report the efficacy of trastuzumab deruxtecan (T-DXd) in treating human epidermal growth factor receptor 2 (HER2) low, type ID leptomeningeal breast cancer (LMD) (with positive cerebrospinal fluid [CSF] cytology and hydrocephalus as the only abnormal imaging finding) and the diagnostic and monitoring utilization of a novel microfluidic platform called CNSide™. Breast cancer LMD is associated with poor prognosis, and effective treatments are lacking. Our case highlights two crucial aspects related to the treatment and monitoring of breast cancer LMD. First, T-DXd was chosen based on immunocytochemistry (IHC) data from CSF malignant cells and follow-up revealed effectiveness of T-DXd in treating HER2-low LMD. While the efficacy of T-DXd has been established in treating metastatic HER2-low breast cancer, our case represents, to our knowledge, the first demonstration of T-DXd's effectiveness in HER2-low breast cancer LMD. Second, since this is type 1D LMD with absence of unequivocal measurable radiological disease in both the central nervous system (CNS) and extra-CNS, we employed a novel microfluidic CSF assay to monitor disease response. This novel assay outperformed standard CSF cytology in our case. There is an urgent need to develop CSF tumor cell assessment tool that surpasses the capabilities of conventional CSF cytology.