Clinical Utility of a Circulating Tumor Cell–Based Cerebrospinal Fluid Assay in the Diagnosis and Molecular Analysis of Leptomeningeal Disease in Patients With Advanced Non–Small Cell Lung Cancer

PURPOSE: Leptomeningeal disease (LMD) is associated with significant morbidity and mortality for metastatic non-small cell lung cancer (NSCLC). We describe our clinical experience in evaluating the use of cerebrospinal fluid (CSF)-derived circulating tumor cells (CTCs) for the diagnosis of LMD and the detection of genomic alterations in CSF cell-free DNA (cfDNA). METHODS: Patients with NSCLC who had CSF collection as part of routine clinical care for suspected LMD were included in the study. CSF was evaluated for CTCs and cfDNA using a commercial assay (CNSide; Biocept, San Diego, CA), and molecular profiling was performed. Molecular testing results from sequencing of tumor tissue and plasma circulating tumor DNA were collected. cMET and human epidermal growth factor receptor 2 (HER2) expression analysis was performed using fluorescence in situ hybridization (FISH). RESULTS: mutation. Thirteen of the 22 patients (59%) were diagnosed with LMD using the CSF CTC assay. Five of these 13 patients (38%) had negative CSF cytology for LMD, and two patients (15%) had normal magnetic resonance imaging brain imaging. Seven of the 13 patients (54%) had sufficient CTCs to perform molecular profiling. The concordance with tissue next-generation sequencing was 100%, and the driver mutation was identified in all seven patients with the CSF cfDNA assay. cMET expression and HER2 expression via FISH were noted in 11 patients (50%) and four patients (18%) respectively. CONCLUSION: We detected higher sensitivity to diagnose LMD using CSF CTC-based assay; 38% of LMD cases identified using this assay were missed by standard CSF cytology. CSF molecular testing using CSF cfDNA demonstrated high concordance with tissue-based molecular testing.