SALIVARY MICROSECRETORY ADENOCARCINOMA - CASE REPORT OF A RECENTLY DESCRIBED ENTITY.

A 35-year-old female patient presented with a 1.5-cm left hard palate swelling. The incisional biopsy was inconclusive, and imaging suggested a nonaggressive lesion with no evidence of bone invasion. The lesion was excised and sent for histopathologic examination. Histologic examination showed a well-defined and unencapsulated multilobular lesion within the connective tissue exhibiting morphological diversity with areas of ductal differentiation, prominent cytoplasmic clearing, and areas with an eosinophilic and epithelioid appearance. In places, cords and trabeculae of tumor cells with a prominent microcystic appearance were seen with surrounding fibromyxoid stroma. Focally, the tumor cells showed a spindled and somewhat infiltrative appearance, but there was no evidence of single-cell filing or perineural or lymphovascular invasion. Immunohistochemistry showed diffuse staining for CK7, Cam5.2, and S100 with p63 staining in a significant proportion of the tumor cells. There was no evidence of a biphasic population, and other myoepithelial markers (i.e., calponin and p40) were also negative. Ki67 staining showed a low proliferation index of approximately 5%. Targeted RNA sequencing showed the presence of MEF2C-SS18 gene fusion, leading to a definitive diagnosis of salivary microsecretory adenocarcinoma (MA). Salivary MA is a recently described entity characterized by the MEF2C-SS18 gene fusion. The differential diagnosis includes an adenocarcinoma not otherwise specified, and the immunohistochemical profile is similar to a polymorphous adenocarcinoma; however, both of these entities lack the characteristic molecular signature. Only 5 cases of MA have been reported to date, with our case adding to the existing literature on this emerging entity. [ABSTRACT FROM AUTHOR]
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