BSLD-07 Similar Survival in Leptomeningeal Metastases With Negative CSF Cytology/Positive Cnside Circulating Tumor Cells vs. Positive CSF Cytology

Molecularly targeted therapies are emerging as effective treatments for subtypes of leptomeningeal metastases (LM) with formal clinical trials on the horizon, yet diagnosis of LM remains a challenge because up to 50% of cases have persistently negative CSF cytology. The CNSide assay confers improved sensitivity by using a proprietary antibody cocktail to capture circulating tumor cells in CSF. We report our center’s experience with the CNSide assay performed on 85 patients between 2020 and 2023. 9/85 patients tested were later determined by investigator not to have LM, and all of these patients tested negative by CNSide. Of the remaining 76 patients with suspected LM, 44 had positive CSF cytology (58%). 43/44 (98%) of those with positive CSF cytology had positive CNSide testing, with one negative CNSide test resulting in a patient with cancer of unknown primary. CNSide testing was positive in an additional 22/32 (69%) LM patients with negative CSF cytology. Overall survival for LM patients with cytology negative/CNSide positive was similar to LM patients who were cytology positive (mOS 23.7 weeks vs 30.6 weeks, HR=1.016, p=0.95). There was a trend toward decreased survival for LM patients with CNSide positive compared to patients who were CNSide negative (mOS 27.0 weeks vs. 90.2 weeks, HR=2.078, p=0.08). Our results indicate that patients with LM who have negative CSF cytology but positive circulating tumor cells in CSF have a similar survival outcome as those with positive CSF cytology.