PD-L1 expression by immunohistochemistry in salivary duct carcinoma

Immune checkpoint inhibitors play an increasing role in oncologic care. PD-L1 expression is associated with survival and predicts response to PD-1 or PD-L1 inhibitors in a variety of tumors. Our aim is to evaluate the frequency and prognostic significance of PD-L1 expression in salivary duct carcinoma.

We retrospectively evaluated the expression of PD-L1 by two different antibodies (PD-L1 28-8 and PD-L1 22C3) in salivary duct carcinomas. PD-L1 expression in at least 1% of tumor cells was considered immunoreactive. Kaplan-Meier analysis was performed to determine the impact of PD-L1 expression on survival; differences between survival curves were assessed by the chi-square test, and pairwise comparisons of factors were assessed with the log-rank test.

A total of 113 patients' specimens were evaluated. Seventy-six (76%) of the patients were male. Mean age at time of presentation was 61.2 (SD = 12.4) years. PD-L1 expression was found in 26% of the samples. Median follow-up time was 36.6 months (range = 1.4-249 months). Overall survival at 3, 5 and 10 years were 52.6%, 37.9% and 25.6%, respectively. There was no statistical difference in survival between patients with PD-L1-immunoreactive tumors and those without, regardless of which antibody was used (chi² result for all plots: p = 0.53; log rank test for pairwise comparison: p > 0.256).

In our analysis, PD-L1 expression occurred in a small proportion of salivary duct carcinomas, usually at low levels, and did not correlate with survival. Its predictive value and utility in selecting patients with salivary duct carcinoma who might benefit from PD-1/PD-L1 inhibitors warrants further investigation.