<scp><i>CRTC1</i></scp>::<scp><i>MAML2</i></scp>‐Positive Hidradenoma With Sebaceous Differentiation

Hidradenoma can exhibit several cell types, including clear cells, polyhedral eosinophilic cells, squamoid cells, mucinous cells, oxyphilic (oncocytic) cells, and transitional (intermediate) cells. However, sebocytes have not yet been described in hidradenoma. Here, we present a case of CRTC1::MAML2-positive hidradenoma with sebaceous differentiation. In addition, this tumor had two cell-type layers that were morphologically similar to apocrine glandular cells and myoepithelial cells throughout the lesion. A 66-year-old male presented with a 20 mm nodule on his upper lip with a 3-year history. Histopathological examination revealed a multinodular tumor adherent to the overlying epidermis. No surrounding salivary glands were seen. The tumor consisted of eosinophilic ductoglandular cells surrounded by basaloid cells at the outer periphery. Numerous sebocytes were scattered throughout the tumor. Severe nuclear atypia or mitotic figures were not observed. Immunohistochemical examination showed no expression of α-smooth muscle actin, calponin, and S100 protein in the tumor cells. Adipophilin highlighted scattered sebocytic tumor cells. Immunoexpression of MLH1, PMS2, MSH2, and MSH6 was preserved. MAML2 break-apart fluorescence in situ hybridization revealed frequent split signals. Sanger sequencing revealed a CRTC1(e1)::MAML2(e2) fusion. Based on this report, hidradenoma should be included in the differential diagnosis of cutaneous adnexal tumors with sebaceous differentiation other than sebaceous adnexal tumors. In addition, sebocytes should be added to the list of tumor cell types in hidradenoma.