Adenocarcinoma Arising in Sporadic Apocrine Papillary Cystadenoma of the Scalp.

Hidrocystomas and cystadenomas are benign tumors, with malignant transformation being exceptionally rare, most commonly reported in the periocular region. We present a rare case of adenocarcinoma in situ arising within an apocrine papillary cystadenoma of the scalp in a 53-year-old man, without an associated nevus sebaceus. Microscopically, the tumor appeared as a predominantly cystic lesion and was located in the deep dermis. The neoplasm was composed of cuboidal to cylindrical cells, displaying decapitation secretion and forming glands, micropapillae, and occasional papillae with fibrovascular cores projecting into the cystic lumen. In addition, a considerable extraluminal proliferation of small, closely packed glands with a visible peripheral layer of myoepithelial cells was evident. Whereas most tumor cells were monomorphic, a focal area exhibited significant cellular atypia and pleomorphism. There were numerous necrotic cells, both isolated and clustered, and frequent mitotic figures, including atypical ones. RNA sequencing was performed using the Archer FusionPlex panel and revealed an HMGA2 fusion transcript of unknown significance. The lesion was surgically removed, and the patient had no evidence of disease 4 months after the diagnosis.