Abstract
Melanoma is known as “the great mimicker”, looking like almost any other tumor and able to metastasize anywhere in the body. All this can lead to misdiagnosis, especially when a patient is already diagnosed with metastatic melanoma. We report a case of an 80-year-old male with a 0.4 cm black anterior scalp papule over 3 years. The patient admitted to scratching it and clinically was suspicious for melanoma. His past medical history was significant for melanoma of the calf with a Breslow thickness of 1.4 mm, 1 mitotic figure/mm2 and negative sentinel lymph node. Histologic sections of the shave biopsy showed multiple tumor nodules with regular borders within the dermis. The tumor was composed of polygonal cells with moderate amphophilic cytoplasm and abrupt central keratinization with ghost cells, melanin deposits, and pigmented dendritic melanocytes. There is significantly increased mitotic activity. Frank invasion, necrosis or significant cytological atypia are not identified. Immunohistochemical stains revealed a strong and diffuse p63 positivity. Beta-catenin demonstrated cytoplasmic positivity in tumor cells with focal nuclear positivity. SOX10 and S100 stains highlighted scattered dendritic melanocytes. A re-excision was performed with no residual tumor. The histologic and immunohistochemical findings support the diagnosis of melanocytic matricoma. It is a rare cutaneous adnexal tumor thought to recapitulate the bulb of the anagen hair follicle with numerous intratumoral pigmented dendritic melanocytes. Most cases have occurred in elderly men. The tumor contains a mixed population of basaloid cells, shadow cells, and dendritic melanocytes. Due to the clinical diversity, histopathological overlap, and potential interpretation as melanoma, it is crucial to recognize melanocytic matricoma. Dermatopathologists and pathologists should be aware of this rare entity and use immunohistochemical stains to achieve the correct diagnosis.
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