PD-L1 expression on routine samples of non-small cell lung cancer: results and critical issues from a 1-year experience of a centralised laboratory.

Aims: Our laboratory is a centralised centre receiving routine non-small cell lung cancer (NSCLC) samples for programmed death ligand-1 (PD-L1) immunohistochemical (IHC) evaluation. Since literature data are not concordant here we review our clinical records to assess the rate of PD-L1 positive and negative NSCLC cases in real-world practice.

Methods: PD-L1 expression was evaluated by a validated 22C3 IHC laboratory developed test on 211 prospectively collected routine NSCLC samples, received from 10 outside institutions. PD-L1 expression was assessed by the tumour proportion score (TPS) and reported by using a three cut-point system: TPS<1, TPS 1%-49% and TPS≥50%.

Results: Overall, 193 out of 211 samples (91.5%) meet the criteria for adequacy (more than 100 viable neoplastic cells). In 62.7% (121/193) of samples TPS was <1%; 17.6% of samples (34/193) expressed PD-L1 with a TPS of 1%-49% and 19.7% (38/193) with a TPS of >50%. There was no significant difference in PD-L1 expression rates between different histotypes and site of sampling. Instead, a statistically significant difference was associated to the type of samples: in fact, cytological samples were more frequently negative for PD-L1 expression (TPS<1%) and less often displayed PD-L1 expression at high levels (TPS>50%) than surgical resections and biopsies.

Conclusions: We present a referral laboratory experience on IHC PD-L1 expression of prospectively collected routine NSCLC samples. Data from the real-world practice can better clarify the percentage of PD-L1 positive and negative cases, to establish benchmarks for good practice standards.