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

Abstract

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.

Secretory carcinoma of the major salivary gland: Provincial population‐based analysis of clinical behavior and outcomes

“…SC is also known to occur in the lip, buccal mucosa, soft palate, and thyroid. 1,5,32,35,[38][39][40][41][42][43][44] Similar to previous studies, 1,2,6,37 cases of SC show a slight male preponderance and occur at an earlier age than reported populations of acinic cell carcinoma or other mimics of SC. 38 Although the majority of cases of SC follow an indolent course and present with a slowly enlarging painless mass, pain was a feature in a quarter of SC cases and one case demonstrated aggressive growth and facial palsy.…”

“…An exception may be SC of thyroid origin which, while rare, seems more likely to display aggressive behavior. All eight cases described so far have presented with extrathyroid extension or lymph node involvement and four of the eight cases have subsequently recurred …”

“…All of the cases of SC in this study occurred in the parotid gland, which may relate to the search strategy in which only tumors of the major salivary glands were reviewed. SC is also known to occur in the lip, buccal mucosa, soft palate, and thyroid …”

“…All eight cases described so far have presented with extrathyroid extension or lymph node involvement and four of the eight cases have subsequently recurred. 35,[41][42][43][44]…”

Secretory Carcinoma: A Silent Mass Increasing in the Parotid Gland

“…Its incidence is <0.5% in parotid malignant neoplasms [5] and usually presents as an asymptomatic mass in the parotid gland with slight male predilection (1.5:1) and the age span for SC is wide (5-77 years) [4]. It has also been reported in the nasal cavity, vulva, lacrimal gland [6], eyelid [7], skin [8], thyroid [9], and lung [10]. Most patients with SC present with a slowly growing painless tumor.…”