Programmed death‐ligand 1 immunoexpression in matched biopsy and liquid‐based cytology samples of advanced stage non‐small cell lung carcinomas

Objective Programmed death‐ligand 1 (PD‐L1) immunohistochemistry (IHC) is essential in patients of advanced non‐small‐cell lung cancer to determine eligibility for immunotherapy. PD‐L1 IHC assays have been clinically validated only on formalin‐fixed paraffin‐embedded tissue; however, lung cancer is frequently diagnosed on cytology. PD‐L1 immunocytochemistry (ICC) has shown high concordance of immunoexpression between cytology samples and paired small biopsies. Feasibility of liquid‐based cytology (LBC) smears for PD‐L1 ICC has not been analysed previously. Methods PD‐L1 ICC and IHC (clone SP263) were performed on paired LBC smears and small biopsies, respectively, in patients with advanced non‐small‐cell lung cancer. Cases with fewer than 100 viable tumour cells on LBC smear/biopsy were excluded from analysis. PD‐L1 was interpreted positive when 25% or more tumour cells showed membranous and/or cytoplasmic protein expression of any intensity greater than background staining. Results A total of 26 patients, harbouring adenocarcinomas (50%) and squamous cell carcinomas (50%), had available bronchial brushings/washings processed as LBC smears and concurrently obtained endobronchial biopsies. PD‐L1 IHC was interpreted positive in 46% (12/26) biopsies. PD‐L1 ICC was interpreted positive in 35% (9/26) LBC smears, all of which were IHC‐positive. No IHC‐negative case was positive on cytology. The overall concordance between LBC smears and small biopsies was 88.4%. Conclusion PD‐L1 ICC can be performed on LBC processed smears, with certain challenges in interpretation inherent to LBC smears and their processing methods. Nevertheless, they represent a potential resource for ICC, especially when alternate histology material is not available. Future studies are required to validate the predictive value of PD‐L1 ICC on LBC smears. Analysis of PD‐L1 expression in liquid based cytology samples and matched biopsies in advanced stage of NSCLC.