Melanotrichoblastic Carcinosarcoma: A Histopathological Case Report of a Previously Undescribed Nosological Unit

<b>Background and Clinical Significance</b>: Trichoblastomas and their variants are rare and underrecognized tumors, with their differential diagnosis being predominantly the much more common basal cell carcinoma. Variants of trichoblastoma, such as melanotrichoblastoma, and malignant counterparts, such as trichoblastic carcinoma and trichoblastic carcinosarcoma, are also rare and probably further underrecognized. <b>Case Presentation</b>: Herein, we present the morphological findings of a tumor located on the right arm of an 86-year-old female patient. The tumor presented with a mixed morphology comprising malignant epithelial nests and retiform structures with focal keratinization and comedo-type necrosis, admixed with dendritic melanocytes, and it had a strikingly bizarre and hypercellular stroma. Immunohistochemistry was positive for BerEp4 in the epithelial nest, HMB-45 was positive only in dendritic melanocytes admixed in some of the nests, and CK20 showed a focal dot-like reaction in intermixed Merkel cells. The stroma was negative for epithelial and melanocytic markers; however, it also exhibited hypercellularity and a bizarre appearance, in addition to a high Ki-67 proliferative index, which further defined it as malignant. Based on the morphology and immunohistochemical profile, the tumor was defined as melanotrichoblastic carcinosarcoma-a previously undescribed nosological unit with unique morphology. <b>Conclusions</b>: Melanotrichoblastomas, as well as trichoblastic carcinosarcomas, are exceedingly rare and underrecognized tumors due to their mimicry of other, more common malignancies. The currently presented case, to the best of the authors' knowledge, is the first reported one of melanotrichoblastic carcinosarcoma.