The usefulness of various cytologic specimen preparations for PD-L1 immunostaining in non-small cell lung carcinoma

Evaluation of programmed cell death ligand-1 (PD-L1) on formalin-fixed paraffin-embedded (FFPE) histologic specimens is required for immunotherapy with pembrolizumab in non-small cell lung carcinoma (NSCLC). A significant percent of patients may be diagnosed on cytology samples alone; however, FFPE tissue may not always be available. Here, we evaluated the feasibility of PD-L1 staining on a variety of cytologic preparations including conventional cell blocks (CB), cell-transfer cell blocks (CTCB), and direct smears.

We identified 30 NSCLC cytology cases that had PD-L1 status evaluated on a concurrent core needle biopsy. Papanicolaou-stained smears (PS) were reviewed and a moderately cellular smear was selected per case to prepare a CTCB. CTCB, CB, and PS (when available) were stained with PD-L1 22C3 and tumor proportion scores (TPS) were compared across all preparations with the concurrent core biopsies.

Concordance of PD-L1 positivity in CB and PS versus core biopsy was high, 80% (12 of 15) and 94.4% (17 of 18), respectively. CTCB versus core biopsy concordance was lower in comparison, 62% (13 of 21) for PD-L1 positivity. Overall, TPS was lower in CTCB compared with CB and PS. Among the 3 preparations, CTCB and CB sections were easier to interpret as PS displayed various artifactual staining patterns.

In summary, our study demonstrates that CB and PS preparations are suitable surrogates for histologic FFPE tissue for determining PD-L1 status in NSCLC patients. CTCBs, on the other hand, show high discordance and are not optimal specimens. Pre-analytic factors in unconventional cytology preparations may need optimization and negative results should be interpreted with caution.