Biom-51. Detection of Pd-L1 in Cerebrospinal Fluid of Leptomeningeal Metastasis From Solid Tumors Based on Thinprep Cytology

BACKGROUND This study assessed the feasibility of PD-L1 detection via ThinPrep cytology and explored its application in cerebrospinal fluid (CSF) from leptomeningeal metastasis (LM) patients. METHODS Six human tumor cell lines (lung, breast and gastric) with validated high/low PD-L1 expression were prepared using ThinPrep cytology and cell-block sections. PD-L1 immunocytochemistry/immunohistochemistry were performed using four antibodies. CSF samples from LM patients were ThinPrep-processed and PD-L1 immunocytochemistry stained. PD-L1 expression was assessed via tumor proportion score (TPS). Inter-assay concordance was evaluated using Cohen’s κ statistics. RESULTS High PD-L1 expression cell lines (NCI-H358, MDA-MB-231, SGC-7901) showed strong staining (TPS: 94-100%) with both 22C3 (Dako) and SP263 (Ventana) assays, while low-expressing lines (A549, MCF7, AGS) had TPS <1%. Concordance was excellent between ThinPrep slides and cell-block sections in these two antibodies. The other two antibodies (28-8 and SP142 Abcam) produced false-positive staining in the low-PD-L1-expressing cell line. 122 CSF samples from 61 LM patients (53 lung, 6 breast, and 2 gastric cancers) were detected by 22C3 and SP263. PD-L1 positivity rates were 46% (28/61) with 22C3 and 51% (31/61) with SP263 (κ=0.83). 64 slides had ≥100 tumor cells, PD-L1 positivity rates of which were 50% (16/32) with 22C3 and 56% (18/32) with SP263 (κ=0.88). The remaining 58 slides had 20-100 tumor cells, PD-L1 positivity rates of which were 41% (12/29) with 22C3 and 45% (13/29) with SP263 (κ=0.79). All breast/gastric cancer cases were negative. Lung adenocarcinoma cases showed 53% (28/53) for 22C3 and 58% (31/53) for SP263 positivity (κ=0.81). Among 28 patients with paired extracranial samples, PD-L1 expression concordance rates between CSF and extracranial samples were 57% (16/28) with 22C3 and 61% (17/28) with SP263. For 24 patients receiving intrathecal immunotherapy, the correlation between PD-L1 expression and prognosis remained under evaluation in ongoing follow-up. CONCLUSION PD-L1 immunocytochemistry with 22C3 (Dako) and SP263 (Ventana) in CSF samples from LM patients using ThinPrep cytology demonstrated feasibility and potential clinical significance.