BIOM-46. REAL-WORLD EXPERIENCE WITH CIRCULATING TUMOR DNA IN CEREBROSPINAL FLUID IN PATIENTS WITH CENTRAL NERVOUS SYSTEM TUMORS

Abstract Next generation sequencing (NGS) of circulating tumor DNA (ctDNA) from cerebrospinal fluid (CSF) offers a minimally invasive approach to identify somatic alterations for individuals with central nervous system (CNS) cancers. Here, we review our ‘real world’ experience using a clinically validated, FDA-authorized platform, MSK-IMPACTTM, to comprehensively profile ctDNA from CSF samples with and without clinically documented CNS involvement by cancer (1007 samples, 711 patients). The patient cohort included patients with over 90 distinct tumor types of primary CNS and metastatic origin, with lung cancer (n = 188), breast cancer (n = 150), and glioma (n = 148) representing the most common broad categories. We identified genetic alterations (ctDNA-positive) in 489/922 (53.0%) CSF samples from patients with clinically documented CNS cancer. Over half of these ctDNA-positive samples (248, 50.7%) possessed a biomarker that was predictive of response to an FDA-approved drug (OncoKB level 1). Furthermore, the distribution of clinically actionable somatic alterations was consistent with tumor-type specific alterations across the AACR GENIE cohort. Of patients without clinical documentation of CNS cancer, none of these 85 CSF samples were ctDNA-positive (specificity = 100%). Successive CSF sequencing over time identified clonal evolution and the emergence of resistance mechanisms to therapies. ctDNA positivity corresponded with shortened overall survival after CSF collection (HR: 3.23, 95% CI: 2.58-4.05, P < 0.001). NGS of CSF cfDNA can detect clinically actionable somatic alterations in a large subset of CNS cancer patients and allows tracking of tumor evolution. Detection of somatic alterations in CSF may be a useful prognostic biomarker for survival in cancer patients.