Concordance of PD-L1 Positivity Between Fine-Needle Aspiration Biopsy and Histologic Specimens in Metastatic Triple Negative Breast Cancer

Summary: Introduction: Immune checkpoint inhibitors targeting programmed death ligand (PD-L1) are a primary therapy for metastatic triple negative breast cancer (TNBC). With most presenting at advanced stages, fine-needle aspiration (FNA) of metastases is a useful diagnostic approach. However, assessment of immunohistochemical PD-L1 expression in this context is not well studied Materials and Methods: Consecutive patients with metastatic TNBC and adequate-cellularity FNA cell block and correlative core biopsy/excision specimen from 2016-2023 were included. Three blinded board-certified pathologists independently scored 223C PD-L1 expression by categorical combined positive score (CPS >10 = positive). Results: Twenty patients with 40 paired samples were available. A unanimous consensus was present in 27 cases with 8 disagreements (7 cytology) on adequacy alone, 5 disagreements (3 cytology) on positivity alone, and no combined disagreements. Table 1 provides a categorized account of concordance by majority rating for all cases and by specimen type, along with diagnostic accuracy of cytology compared to histology. Conclusions: Evaluation of PD-L1 expression in cytology specimens shows moderate interrater concordance and high positive predictive value. These results support the use of FNA specimens for initial assessment, especially when alternative specimens are inadequate, although limited sensitivity should prompt repeat evaluation for cases scored as negative. Sensitivity likely suffers due to limited sampling and tumor heterogeneity. Concordance is likely limited by the lack of preserved architecture, borderline cellularity, and staining heterogeneity in FNA cell blocks. Future research may evaluate digital image assistance for improved concordance.