Progress in salivary gland pathology: new entities and selected molecular features

The Role of Molecular Testing in the Differential Diagnosis of Salivary Gland Carcinomas

Salivary gland neoplasms are a morphologically heterogenous group of lesions that are often diagnostically challenging. In recent years, considerable progress in salivary gland taxonomy has been reached by the discovery of tumor type-specific fusion oncogenes generated by chromosome translocations. This review describes the clinicopathologic features of a selected group of salivary gland carcinomas with a focus on their distinctive genomic characteristics. Mammary analog secretory carcinoma is a recently described entity characterized by a t(12;15)(p13;q25) translocation resulting in an ETV6-NTRK3 fusion. Hyalinizing clear cell carcinoma is a low-grade tumor with infrequent nodal and distant metastasis, recently shown to harbor an EWSR1-ATF1 gene fusion. The CRTC1-MAML2 fusion gene resulting from a t(11;19)(q21;p13) translocation, is now known to be a feature of both low-grade and high-grade mucoepidermoid carcinomas associated with improved survival. A t(6;9)(q22-23;p23-34) translocation resulting in a MYB-NFIB gene fusion has been identified in the majority of adenoid cystic carcinomas. Polymorphous (low-grade) adenocarcinoma and cribriform adenocarcinoma of (minor) salivary gland origin are related entities with partly differing clinicopathologic and genomic profiles; they are the subject of an ongoing taxonomic debate. Polymorphous (low-grade) adenocarcinomas are characterized by hot spot point E710D mutations in the PRKD1 gene, whereas cribriform adenocarcinoma of (minor) salivary glands origin are characterized by translocations involving the PRKD1-3 genes. Salivary duct carcinoma (SDC) is a high-grade adenocarcinoma with morphologic and molecular features akin to invasive ductal carcinoma of the breast, including HER2 gene amplification, mutations of TP53, PIK3CA, and HRAS and loss or mutation of PTEN. Notably, a recurrent NCOA4-RET fusion has also been found in SDC. A subset of SDC with apocrine morphology is associated with overexpression of androgen receptors. As these genetic aberrations are recurrent they serve as powerful diagnostic tools in salivary gland tumor diagnosis, and therefore also in refinement of salivary gland cancer classification. Moreover, they are promising as prognostic biomarkers and targets of therapy.

Newly described salivary gland tumors

This review concentrates on three salivary gland tumors that have been accepted in the recent literature as new neoplastic entities: mammary analog secretory carcinoma (MASC), sclerosing polycystic adenoma (SPA) and cribriform adenocarcinoma of tongue and other minor salivary glands (CAMSGs). MASC is a distinctive low-grade malignant salivary cancer that harbors a characteristic chromosomal translocation, t(12;15) (p13;q25) resulting in an ETV6-NTRK3 fusion. SPA is a rare lesion often mistaken histologically for low-grade salivary carcinoma. Previously thought to be a reactive fibroinflammatory process, but recent evidence of clonality, recurrences in up 30%, and dysplastic foci suggest it may be truly neoplastic. CAMSG is a distinct tumor entity that differs from polymorphous low-grade adenocarcinoma (PLGA) by location (ie, most often arising on the tongue), by prominent nuclear clearing, alterations of the PRKD gene family and clinical behavior with frequent metastases at the time of presentation of the primary tumor. Early metastatic disease seen in most cases of CAMSG associated with indolent behavior makes it a unique neoplasm among all low-grade salivary gland tumors. Salivary glands may give rise to a wide spectrum of different tumors. They are often diagnostically challenging as morphological features often overlap between different entities. Although conventional morphology in combination with immunohistochemical findings still provide the most important clues for diagnosis, recent advances in molecular pathology offer new diagnostic tools in investigating the differential diagnosis, as well as providing potentially valuable prognostic indicators. In the last two decades, several new salivary gland tumor entities have been described, namely MASC, SPA and CAMSGs.

Mammary analogue secretory carcinoma of salivary gland origin: an update and expanded morphologic and immunohistochemical spectrum of recently described entity

Mammary analogue secretory carcinoma of salivary gland origin (MASC) is a recently described tumor with ETV6 translocation. Akin to secretory breast cancer, MASC expresses S-100 protein, mammaglobin, vimentin, and harbors a t(12;15) (p13;q25) translocation which leads to ETV6-NTRK3 fusion product. Histologically, MASC displays a lobulated growth pattern and is often composed of microcystic, tubular, and solid structures with abundant eosinophilic homogeneous or bubbly secretions. Colloid-like secretory material stains positive for periodic acid-Schiff (PAS) with and without diastase and for Alcian blue. The cells of MASC are devoid of PAS-positive secretory zymogen granules. These features help to exclude the most important differential diagnostic considerations, namely acinic cell carcinoma, low-grade cribriform cystadenocarcinoma, cystadenocarcinoma (not otherwise specified), and low-grade mucoepidermoid carcinoma. To date the presence of the ETV6-NTRK3 fusion gene has not been demonstrated in any other salivary gland tumor than MASC. It is likely that MASC is more common than currently recognized and with further studies, the clinical need for molecular studies of the ETV6-NTRK3 fusion may diminish. However, molecular testing is recommended at this time to arrive at the diagnosis of MASC.