Mammary Analogue Secretory Carcinoma of the Thyroid Mimicking Locally Advanced Papillary Thyroid Carcinoma: A Rare Case Report.

Mammary analogue secretory carcinoma (MASC) harboring ETV6 gene rearrangements was first described in the salivary gland with a relatively favorable prognosis and a possible molecular therapeutic target with pan-Trk inhibitors. Recently, primary MASC of the thyroid gland has been reported. We report a case of a 4.0 cm MASC arising from the left thyroid of a 58-year-old female with extrathyroidal extension. Initially, it was diagnosed by fine needle aspiration as suspicious for papillary thyroid carcinoma (PTC) and subsequently called a poorly differentiated carcinoma on resection. A final diagnosis of primary MASC of the thyroid was confirmed after an expanded immunohistochemical panel and identification of an ETV6 gene rearrangement by fluorescence in situ hybridization. Morphologically, the tumor was composed of solid, microcystic and focally papillary growth with dense fibrotic stroma and necrosis. Overlapping cytological features with PTC were identified, including foci of enlarged cells with irregular nuclear membranes/grooves. However, most of the cells contained prominent nucleoli with intraluminal and intracytoplasmic eosinophilic secretions. Immunohistochemically, the tumor cells were strongly positive for pancytokeratin, cytokeratin 7, PAX8, mammaglobin, and GCDFP-15, with rare staining for GATA3 and S100 and negative for TTF-1 and thyroglobulin. We report a rare case of a primary thyroid MASC, initially misdiagnosed as PTC. Pathologists should be aware of this entity and, given the similarities to PTC, have a high index of suspicion, prompting the addition of immunohistochemical and molecular studies. Furthermore, an accurate diagnosis is important because of the possible prognostic and treatment implications.