This issue focuses on select salivary gland neoplasms. The ever-expanding list of salivary gland neoplasms primarily but not exclusively reflects recent/new discoveries in the molecular biology of this group of neoplasms. Tumors previously unknown and/or “lumped” within the spectrum of other tumors have been defined and recognized as unique types. Still, the more common types of salivary gland neoplasms can present diagnostic difficulties owing to their relative rarity and to the fact that such cases are not common and infrequently encountered in day-to-day surgical pathology.
The first case focuses on a common diagnostic problem in salivary gland pathology albeit with a slight clinical twist. Dr Deborah Chute from the Cleveland Clinic discusses the diagnosis and differential diagnosis of mucoepidermoid carcinoma by fine-needle aspiration biopsy arising in a periparotid lymph node. In addition to discussing the key cytologic features associated with mucoepidermoid carcinoma, Dr Chute provides an overview on mucoepidermoid carcinomas developing at ectopic sites, which can add confusion to the diagnosis as the clinical differential diagnosis usually does not initially include a salivary gland tumor. Aside from the cytomorphologic findings, awareness among pathologists that salivary gland tumors can occur at unusual sites is a key to arrive at the correct interpretation.
In the next case, Dr Justin Bishop from Johns Hopkins University School of Medicine discusses a case of mammary analog secretory carcinoma (MASC) representing a recently defined low-grade carcinoma associated with ETV6 translocation. Prior to its identification as a unique salivary gland neoplasm with analogous features to secretory carcinoma of the breast, MASC was included within the spectrum of acinic cell adenocarcinoma as a “zymogen-poor” subtype. Mammary analog secretory carcinoma and acinic cell adenocarcinomas can be remarkably similar in their histologic features, and while the presence of ETV6 translocation would differentiate them, many laboratories do not perform such analyses. However, as discussed in this case, the immunohistochemical findings do allow for their distinction in lieu of performing molecular analysis.
Although in part defined by the presence of multiple growth patterns as well as bland cytomorphology, polymorphous low-grade adenocarcinoma (PLGA) is not the only salivary gland neoplasm with such histologic findings. Drs Vaidehi Avadhani and Nebras Zeizafoun from the Mount Sinai Health System discuss a case of PLGA detailing the light microscopic and immunohistochemical findings that allow for diagnosis and differentiating it from other histologically similar neoplasms, including pleomorphic adenoma and adenoid cystic carcinoma. Furthermore, previous classifications of salivary gland neoplasms “lumped” the cribriform adenocarcinoma of minor salivary glands within the spectrum of PLGAs, but as detailed in this article, the presence of ARID1A-PRKD1 and DDX3X-PRKD1 fusions in cribriform adenocarcinoma of minor salivary glands and rarity of such fusions in PLGA support distinct classification but suggest a possible shared pathogenesis between these tumor types.
Clear cells can be seen across a wide spectrum of salivary gland lesions including benign and malignant tumors. In the next case, Dr Andrew Turk from Columbia University reviews the pathologic features of the low-grade epithelial-myoepithelial carcinoma with a discussion on the differentiating features from other salivary gland neoplasms with clear cells. His discussion also includes the utility of molecular chromosomal rearrangements, specifically the presence of EWSR1-ATF1 fusion in clear cell carcinoma.
The last 2 articles in this issue are reviews of specific topics in salivary gland neoplasia. In the first of these reviews, Dr Joaquín García from Mayo Clinic provides an overview of the fusion transcripts that characterize malignancies of salivary gland origin. Although discussed in several of the previous case reviews, this review details the unique molecular signatures that have been identified to date in salivary gland neoplasms.
Finally, Frederik Petersson from the National University Health System of Singapore provides an exhaustive overview of high-grade transformation of differentiated salivary gland neoplasms heretofore considered “dedifferentiation,” a term more aptly used for soft tissue neoplasms. His review essentially covers all salivary gland neoplasms that may be associated with high-grade transformation discussing the process of malignant transformation of salivary gland neoplasms.
We hope that you find the cases and reviews interesting, informative, and useful in your daily practice.