The Clinicopathological and Molecular Associations of PD-L1 Expression in Non-small Cell Lung Cancer: Analysis of a Series of 10,005 Cases Tested with the 22C3 Assay

PD-L1 expression testing is mandatory prior to pembrolizumab prescription in non-small cell lung cancer. Our service offers PD-L1 testing using the PD-L1 IHC 22C3 pharmDx assay, in parallel with EGFR, ALK, ROS1 and (in some cases) KRAS testing. We correlate PD-L1 expression in 10,005 tumours with patient age and sex, with tumour histological subtypes, with the sampling modality and type of tissue, and with the presence of other molecular alterations. PD-L1 expression testing was performed using the aforementioned assay; tumour proportion scores (TPS) of 1 and 50% were taken as cut-offs for low and high positivity, respectively. EGFR testing was performed using the cobas® EGFR Mutation Test v2. ALK testing was performed using the VENTANA ALK (D5F3) CDx Assay. KRAS testing was performed using pyrosequencing. TPS <1% was seen in 44.4% of tumours, 1–49% in 25.0% and ≥ 50% in 30.6%. We identified no significant relationship with age. Female patients were slightly more likely to express PD-L1. Poorly-differentiated tumour histology and ALK translocation were significantly associated with PD-L1 expression. Rare EGFR mutations tended to be associated with PD-L1 expression. Pleural and nodal metastases were more likely to express PD-L1 than primary tumours, but biopsy and cytological specimens did not show different PD-L1 expression rates. Our data show that the means of acquiring a tumour sample (biopsy versus cytology) does not have a significant impact on PD-L1 expression. However, we found that certain metastatic sites were associated with significantly higher expression rates, which has substantial implications for selection of tissue for testing.