Mucoacinar Carcinoma of the Parotid Gland

Mucoepidermoid carcinoma (MEC) is the most common salivary gland malignancy, displaying recurrent MAML2 gene rearrangement. Multiple histomorphologic presentations and subtypes have been described, including clear, sclerosing, ciliated, oncocytic, Warthin-like, and lacking detectable squamoid/squamous differentiation [1‐3]. Mucoacinar carcinoma refers to a recently described rare MEC subtype displaying serous acinar differentiation, with only 12 cases reported to date [4, 5]. Distinguishing it from acinic cell carcinoma can be challenging, particularly with the emergence of the phenotypically comparable squamoglandular subtype [6].

This is a case of a 66-year-old female patient with a slowly enlarging parotid gland mass. Imaging revealed a solid-cystic mass, measuring < 2.0 cm in greatest dimension. Fine needle aspiration was attempted but the cytology specimens were comprised of cyst content only. The lesion was subsequently and completely excised, and histologic examination revealed a predominantly cystic salivary proliferation with scattered squamoglandular and intermediate cell components, surrounded focally by tumor-associated lymphoid proliferation. Variable serous acinar differentiation (the granules additionally highlighted by PAS-D special stain) and rare luminal intracytoplasmic mucin (highlighted by Mucicarmine special stain) were identified (Fig. 1). Importantly, the components were intimately admixed in a single nodule and did not display geographically separate and morphologically distinct cellular proliferations, arguing against a collision of two tumors [7]. No infiltrative growth, perineural or angiolymphatic invasion, marked cytologic atypia, increased mitoses or tumor necrosis were identified, corresponding to a low-grade categorization per the Brandwein, MSK and modified MSK MEC grading systems [8].

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By immunohistochemistry, the lesional cells showed positive expression for CK7 (diffuse), CK5/6 (diffuse and accentuated basally/peripherally), SOX10 (variable, focally strong in the acinar component), S100 (variable, overall patchy), and DOG1 (luminal staining in the acinar component). p40 highlighted basal/abluminal cells in the serous acinar units and the simple cystic component but showed more “parabasal” staining in the complex squamoglandular areas sparing the luminal/ductal cells. Figure 2. A custom solid tumor gene fusion next-generation sequencing panel (ArcherDX FusionPlex) identified a sole CRTC1::MAML2 fusion, without NR4A3 gene rearrangement. No extended transcriptomic analysis was performed. The molecular results argue against a collision tumor, and the collective evidence is consistent with a MEC subtype. No further management was indicated, and recent post-operative follow-up data is not available.

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This case highlights that serous acinar differentiation is not restricted to acinic cell carcinomas, and it further supports the relation of mucoacinar carcinoma to the expanding phenotypic spectrum of MEC. Given the overlapping histomorphology and immunoprofile, molecular testing is mandatory to distinguish it from squamoglandular acinic cell carcinoma. The prognostic implications of this subtype are not known, but it should be histologically graded and clinically managed as a MEC.