Melanotrichoblastoma: Two Cases with Expanded Histopathologic and Immunophenotypic Features

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ABSTRACT: Melanotrichoblastoma is an exceptionally rare benign follicular neoplasm recognized by the fifth edition of the WHO classification as a heavily pigmented variant of trichoblastoma. It is characterized by a prominent proliferation of basaloid follicular germinative cells intimately colonized by active dendritic melanocytes, alongside significant stromal and intratumoral melanin deposition. Owing to its dense pigmentation and basaloid morphology, it is frequently mistaken for pigmented basal cell carcinoma, melanoma, or other pigmented adnexal tumors. We report 2 cases occurring in women aged 43 and 79 years involving the scalp and cheek, respectively. Both lesions were well-demarcated dermal neoplasms composed of basaloid nests and cords lacking epidermal connection. Architectural patterns included solid, reticulated, cribriform, and cystic areas with focal follicular keratin cyst formation. Numerous dendritic melanocytes colonized the epithelial nests, successfully verified via red chromogen SOX10 and Melan-A staining. Both tumors showed diffuse AE1/AE3 and BER-EP4 positivity, wildtype p53 expression, and highly variable CK7/CD10 staining. These cases expand the limited literature and reinforce that diagnosis depends primarily on recognition of the characteristic epithelial-melanocytic architecture, rather than a rigid reliance on single ancillary immunohistochemical markers. Awareness of this rare variant is essential to avoid diagnostic pitfalls and patient overtreatment.

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