Morphologic and Immunohistochemical Characteristics of Anorectal Melanoma

Anorectal melanoma is a rare aggressive disease. Due to its rarity and considerable histologic and immunohistochemical variabilities, misdiagnosis as lymphoma, carcinoma, sarcoma, and/or gastrointestinal stromal tumor is not uncommon, particularly in amelanotic cases. We reviewed histologic features and immunohistochemical stains of 19 anorectal melanoma cases. Histopathologic features were evaluated including junctional activity, melanin pigment, and morphologic features. Immunohistochemical stains were performed using Sox10, S100 protein, HMB-45, melan-A, CD56, and cytokeratins. Epithelioid histopathologic morphology was observed in 63.2% of the cases followed by 47.4% of the cases with spindle-cell, 26.3% with lymphoma-like, and 26.3% with pleomorphic morphologies. Junctional melanocytic activity was seen in almost half of the cases. Melanin pigment was absent (amelanotic) in nearly 40% of the cases. Immunohistochemically, diffuse positive expression of Sox10, S100 protein, melan-A, and HMB-45 was seen in 100%, 40%, 53.3%, and 38.5% of the cases, respectively. Cytokeratins were negative and CD56 was positive in 2 cases. These findings indicate that anorectal melanomas often show one or combined histolopathologic features without presence of melanin pigment and absence of junctional melanocytic activity. Anorectal melanoma should be kept in mind in the differential diagnosis of malignant neoplasms of anorectal region with epithelioid, spindle-cell, lymphoma-like, and pleomorphic morphologies. Sox10 immunohistochemistry stain can be used as a first-line screening tool to avoid extensive or unnecessary workups and/or potential misdiagnosis.