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 Twenty-two patients were included (77% female; median age 60 years). Sixty-four percent had sensitizing EGFR mutations, and 32% had an atypical EGFR 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.