The puzzle of diagnosing salivary gland tumors with immunohistochemistry

Salivary gland tumors are among the most diagnostically challenging tumors due to their overlapping histomorphology, morphological diversity, and heterogeneity. This difficulty is particularly amplified in small biopsies due to marked crushing artifacts and sampling errors, affecting diagnosis due to heterogeneity of tumors, limited tissue availability for immunohistochemistry (IHC) and molecular analyses, and inability to evaluate the interface of the tumor with adjacent native tissues.[1-3] Although IHC has a limited role in diagnosis, it helps identify the lineage of tumors. Kumar et al.[4] have discussed a holistic approach to diagnose salivary gland neoplasms in cytology and small biopsies through the application of IHC, with a focus on current management guidelines. These are critical problems faced by pathologists across institutes. Broadly, luminal/ductal cells are positive for low-molecular-weight cytokeratin, cytokeratin 7, carcinoembryonic antigen, and epithelial membrane antigen, while the myoepithelial markers are calponin, smooth muscle actin, p63, p40, S100, and SRY-transcription factor 10 (SOX10). S100 and SOX10 are the least specific markers of myoepithelial lineage. Acinic cell carcinoma and secretary carcinoma are SOX10 positive. Secretary carcinoma, polymorphous adenocarcinoma, and canalicular adenoma are S100 positive. Clear cell carcinoma is positive for p40 and p63. All these are monophasic luminal origin tumors. A recent review article published in Cancer Research, Statistics and Treatment Journal by Kumar et al. does not highlight that a large proportion of salivary gland tumors have rearrangements of tumor-specific genes with varying partners, which can be evaluated by fluorescent in situ hybridization, polymerase chain reaction, and next-generation sequencing.[4] Some of these molecular markers can be detected by IHC as a surrogate for those mutations. Pleomorphic adenoma gene 1 (PLAG1) and high-mobility group A2 (HMGA2) IHC markers are used for pleomorphic adenoma and are also expressed in carcinoma ex pleomorphic, beta-catenin for basal cell adenoma, and in very few cases of basal cell adenocarcinoma, v-myb avian myeloblastosis viral oncogene homolog (MYB) for adenoid cystic carcinoma, pan-tropomyosin receptor kinase (Pan-Trk) for secretory carcinoma, nuclear receptor subfamily 4 group A member 3 (NR4A3) for acinic cell carcinoma and rat sarcoma virus (RAS) Q61R for epithelial-myoepithelial carcinoma. MYB IHC is sensitive, but shows suboptimal specificity and expression in numerous mimickers such as polymorphous adenocarcinoma, myoepithelioma and myoepithelial carcinoma, sailoblastoma, and even in other nonsalivary gland tumors such as squamous cell carcinoma. NR4A3 IHC has high sensitivity and specificity.[1-3] Strong nuclear expression for pan-TRK IHC can be considered a surrogate marker for the detection of neurotropic receptor tyrosine kinase 3 (NTRK3) fusions in secretary carcinoma. These patients are eligible candidates for tropomyosin receptor kinase (Trk) inhibitors, for example, larotrectinib, selitrectinib, and entrectinib.[3] However, weak cytoplasmic expression of pan-TRK IHC needs further validation by other cytogenetic or molecular analyses if the patient is considered a candidate for targeted therapy.[3] It is equally essential to emphasize that every salivary gland tumor type has alternate genetic findings; hence, a negative molecular test should not be considered as evidence to entirely rule out any diagnosis. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.