Abstract

A 49-year-old female presented with a 1.3 x 0.7 cm itchy, brown macule on the right axilla. Histopathological examination of a shave biopsy showed a background banal intradermal melanocytic nevus with an area composed of distinct, well-defined rounded nests infiltrating the mid reticular dermis. Higher magnification of the nests revealed epithelioid cells with ample gray cytoplasm and small oval to round nuclei, surrounded by numerous melanophages, without significant atypia or mitoses. The presence of epithelioid cells near the base of the lesion exhibited a lack of maturation with dermal descent. However, the exact depth evaluation was limited as the lesion was transected at the base. A Mart-1/Ki-67 showed a low proliferative rate (<1%) within the Mart-1 positive melanocytes, and HMB-45 was negative in most dermal components. Combining histopathological and immunohistochemical findings, a diagnosis of inverted type-A melanocytic nevus was made with a recommendation for complete excision of the lesion. The maturation of melanocytes is the diagnostic histopathological feature of benignancy; that is, the melanocytes transform from epithelioid (type A) to lymphocyte-like (type B) to spindled or fibroblast-like (type C) with increasing dermal depth. The lack of maturation is a diagnostic challenge in histological differentiation of inverted type-A nevus from melanoma, which occurs in about 5% of the cases. Remarkably, the inverted type-A nevus also has overlapping clinical and histopathological features with a deep penetrating nevus, combined nevus, and clonal nevus supported by most recent studies showing an activating mutation of the beta-catenin pathway.

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