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.