57. CIRCULATING TUMOR CELLS (CTC) IN CEREBROSPINAL FLUID (CSF) AS A PREDICTOR OF SURVIVAL IN CNS METASTASES

BACKGROUND CSF-CTC testing using the CellSearch® platform is a validated diagnostic tool for leptomeningeal metastases (LM) from solid tumors. CSF-CTCs can also be detected in patients with brain metastases (BM), but their significance is unclear. Our objective was to evaluate the utility of CSF-CTC measurement in predicting outcomes in CNS metastases. METHODS We conducted a retrospective single-institution review of patients who underwent CSF-CTC testing from 2016–2019. Information on neuroaxis imaging, CSF results, systemic cancer status, tumor molecular profile and survival was collected. LM was diagnosed by unequivocal MRI findings and/or positive or suspicious CSF cytology. Survival analyses were performed using Cox proportional hazards modeling, and CSF-CTC splits associated with survival were identified through recursive partitioning analysis. RESULTS A total of 407 patients (38% lung primary, 34% breast, 28% other tumor types) were included; of these, 144 had LM and 233 had BM diagnosed before or around the time of CSF analysis (97 had both). We identified a subgroup of newly diagnosed CNS metastases, comprising 144 patients with LM, BM, or both diagnosed within 30 days of CSF sampling: 70 patients with LM, 43 with BM, and 31 with both. For 101 patients with newly diagnosed LM, mean and median CSF-CTCs were 127.3 and 200, respectively, compared to 44.6 and 0 in the overall cohort; 73/101 had positive (66) or suspicious (7) cytology. CSF-CTCs predicted survival in patients with newly diagnosed LM, with optimal cutoff identified at 61 CSF-CTCs, above which the risk of death doubled (HR=2.09, 95% CI: 1.13–3.87, p=0.02). For this group, positive/suspicious cytology was also associated with higher risk of death, but this result was not statistically significant (HR=1.79, 95% CI: 0.95–3.35, p=0.07). CONCLUSION In newly diagnosed LM, quantification of CSF-CTCs predicts survival. CSF-CTC measurement can be used as a prognostic tool in patients with CNS metastases.