Abstract PURPOSE : The purpose of this study was to identify circulating tumor cells (CTC) and evaluate tumor heterogeneity in cerebrospinal fluid (CSF) of advanced gastric cancer (GC) patients with leptomeningeal seeding. METHOD : We used the DEPArray system to identify and align single enabled CTCs in CSF based on multi-parallel fluorescence analysis. CSF were stained with an antibody cocktail recognizing cytokeratin, EpCAM, and HER2. CTC subpopulations of CSF were quantified as both absolute cell count and relative frequency. Based on the stained morphology of each single CSFTCs, five groups (Group A~E) were categorized. RESULTS : CSF samples were obtained from a HER2 positive GC patient with leptomeningeal seeding. Each single CTC was collected according to the characteristics of morphology and the difference of marker expression. Group A CTC revealed HER2 positive, CK positive and EpCAM positive cells. The ratio of group A cells to total cells was 56.7%. Although the tumor of patient was HER2 positive cancer, HER2 negative CTCs in CSF (Group D + E) accounted for 21.8% of the total cells. Among CTCs we detected in CSF, there was no CTC revealing HER2 positive, CK negative and EpCAM negative. CONCLUSIONS : DEPArray analysis might be a novel tool to capture CTC in CSF samples and evaluate the intra-patient's tumor heterogeneity. These findings suggested that DEPArray analysis could be used as a useful tool for the precision medicine. Characterization of HER2 + CSFTCs on gastric cancer patientHER2CytokeratinEpCamCell#%/total cellGroupPositivePositivePositive13556.7APositivePositiveNegative20.8BPositiveNegativePositive4920.6CPositiveNegativeNegative00.0NegativePositivePositive52.1DNegativePositiveNegative00.0NegativeNegativePositive4719.7E Citation Format: Jang Ho Cho, Sun Young Kim, Kyung Kim, Jusun Kim, Su Jin Lee, Jeeyun Lee, Se Hoon Park, Joon Oh Park, Young Suk Park, Ho Yeong Lim, Won Ki Kang, Seungtae Kim. The identification of circulating tumor cells in the cerebrospinal fluid of gastric cancer patients with brain metastasis and the evaluation of intra-patient tumor heterogeneity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1173.