Cutaneous "Microcystic" Microsecretory Adenocarcinoma With Marked Adnexal Hyperplasia: A Simulant of Microcystic Adnexal Carcinoma.

Microsecretory adenocarcinoma (MSA) was first described in 2019 as a low-grade salivary gland neoplasm of intraoral origin with distinct histopathologic features and a characteristic  MEF2C::SS18  fusion. Recently, skin was also identified as a primary site for MSA in a report by Bishop et al. Due to its rarity and resemblance to other adnexal tumors, MSA is a challenging diagnosis. Herein, we present a case of cutaneous MSA that was unique for the presence of a significant microcystic component and marked adnexal hyperplasia, which mimicked myxoid microcystic adnexal carcinoma (MAC). A 58-year-old presented with a 1 year history of an enlarging eyelid nodule. Histopathologic analysis revealed dermal tumor composed of small tubules containing inspissated bluish mucinous material. Accompanying marked adnexal hyperplasia and many microcysts were also present. Tumor cells expressed S100 protein, which is distinct from MAC, while p63 immunohistochemistry showed marked loss of myoepithelial labeling, as is common in primary adenocarcinomas. Next generation gene sequencing detected the characteristic MSA fusion protein  MEF2c::SS18 . We report a novel presentation of MSA that simulated MAC because of the presence of associated microcystic change. The presence of S100 immunopositivity and the identification of  MEF2C::SS18  fusion confirmed the diagnosis of cutaneous MSA.