Value of Routine Immunohistochemistry in Improving Diagnostic Accuracy of Fine Needle Aspiration of Salivary Gland Lesions

Summary: Introduction: Fine Needle Aspiration (FNA) plays an important role in triaging and management of salivary lesions requiring accurate and effective cytologic interpretation. Due to the complexity and variety of neoplasms often ancillary testing such as immunohistochemistry (IHC) and molecular studies are required. Most institutions are not equipped to perform these extensive tests. The goal of this study was to determine the utility of common IHC markers in improving diagnostic accuracy of FNA of salivary gland lesions and to develop a practical IHC panel to aid in daily practice. Materials and Methods: The pathology database was reviewed for salivary gland FNA cases with subsequent resection specimens from November 2020 to December 2023, upon IRB approval. All cases were reviewed blindly by two cytopathologists with interpretation documented after review of smears only, followed by cell block/core biopsy (CB) and IHC slides using the Milan System for Reporting Salivary Gland Cytopathology (MSRSGC). Concordance rates between final surgical diagnosis and cytology (pre-CB, post-CB and post-IHC) interpretations were determined. Concordance and discordance were defined as shown in Table 1. Results: 90 cases, of which 69 with CB and 21 without CB were reviewed. 20 of the 69 cases with CB had IHC (Figure 1). Of these 20 cases, concordance was noted in 17 cases by smear alone and 2 required CB for concordance. One case remained discordant despite IHC (Figure 2A). In 5 of 7 MSRSGC VI cases determined on Cytology smears alone, IHC aided in rendering a specific diagnostic entity. 2 of 4 MSRSGC IVB cases on cytology smears alone, remained indeterminate despite IHC (Figure 2B). 13 of 21 (62%) cases without CB or IHC were concordant. One case of squamous cell carcinoma remained SUMP after IHC due to lack of features of invasion. Conclusions: The highest concordance rate was noted in cases which had both IHC and CB. Highest discordance rate was noted in cases without CB and IHC. A slight increase in concordance rate was noted upon performing IHC. In MSRSGC VI cases determined on cytology smears alone, IHC aided in rendering a specific diagnosis. In MSRSGC IVB cases determined on cytology smears alone, IHC was less helpful in rendering a specific diagnosis. Sampling error led to discordance despite IHC.