Pigmented extramammary Paget disease of the thigh mimicking a melanocytic tumor: report of a case and review of the literature.

IMPORTANCE: Extramammary Paget disease (EMPD) is an uncommon tumor that presents in apocrine-rich skin as an irregular, pruritic plaque. Histopathologically, EMPD consists of an intraepidermal proliferation of atypical epithelioid cells. Rarely, the tumor cells contain intracytoplasmic melanin pigment, and the lesion clinically and histopathologically can mimic a melanocytic proliferation. OBSERVATIONS: A 51-year-old female with a history of breast carcinoma presented with a pigmented patch on her right thigh of 6 months duration. The clinical impression was an atypical melanocytic nevus. Histopathologic examination revealed an intraepidermal proliferation of epithelioid cells along the dermal-epidermal junction with pagetoid migration. The tumor cells exhibited increased cytoplasm containing conspicuous melanin pigment and enlarged oval-irregular nuclei. Immunohistochemical studies showed the tumor cells to be strongly and diffusely positive for cytokeratin 8/18, cytokeratin 7 and p63; focally and weakly positive for epithelial membrane antigen (EMA), but negative for cytokeratin 5/6, Cam5.2, carcinoembryonic antigen (CEA), human melanoma black 45 (HMB-45), tyrosinase and Sox-10, supporting the diagnosis of pigmented EMPD. The lesion was subsequently excised, and the patient is free of disease after 24 months.
CONCLUSION: We present this unusual case of pigmented EMPD arising on the thigh to draw attention to the entity and to underscore the potentially misleading clinical, histopathologic and immunophenotypic features that mimic other cutaneous intraepidermal lesions.