Comparative evaluation of PD-L1 expression in cytology imprints, circulating tumour cells and tumour tissue in non-small cell lung cancer patients

Alternative sources of tumour information need to be explored in patients with non-small cell lung cancer (NSCLC). Here, we compared programmed cell death ligand 1 (PD-L1) expression on cytology imprints and circulating tumour cells (CTCs) with PD-L1 tumour proportion score (TPS) from immunohistochemistry staining of tumour tissue from patients with NSCLC. We evaluated PD-L1 expression using a PD-L1 antibody (28-8) in representative cytology imprints, and tissue samples from the same tumour. We report good agreement rates on PD-L1 positivity (TPS ≥ 1%) and high PD-L1 expression (TPS ≥ 50%). Considering high PD-L1 expression, cytology imprints showed a PPV of 64% and a NPV of 85%. CTCs were detected in 40% of the patients and 80% of them were PD-L1+ . Seven patients with PD-L1 expression of < 1% in tissue samples or cytology imprints had PD-L1+ CTCs. The addition of PD-L1 expression in CTCs to cytology imprints markedly improved the prediction capacity for PD-L1 positivity. A combined analysis of cytological imprints and CTCs provides information on the tumoural PD-L1 status in NSCLC patients, which might be used when no tumor tissue is available.