LAUSR

A 37-year-old man of skin type VI consulted our dermatology department for a nodule on the posterior surface of the right thigh, above the popliteal fossa, which had appeared with no obvious reason one year ago. The lesion was occasionally tender. Medical history was remarkable for sickle-cell anaemia. Physical examination showed a 6-mm firm pigmented nodule with a keratotic surface (figure 1A). There was no regional (inguinal) lymphadenopathy. Dermatoscopic examination revealed a bluish colour. The lesion was excised under local anaesthesia with the clinical diagnostic hypotheses of melanoma, blue naevus, keratotic papilloma and squamous cell carcinoma. Histopathological examination showed a slightly raised dermal tumour overlaid by a hyperkeratotic, acanthotic and papillomatous epidermis, showing an aspect of pseudoepitheliomatous hyperplasia (figure 1B, C). The tumour was made of a dense proliferation of epithelioid melanocytes with vesicular nuclei and visible nucleoli. These cells contained abundant cytoplasmic melanin deposits (figure 1D). No junctional activity was noted, although the epidermis contained melanin deposits and pigmented dendritic cell processes. On immunohistochemistry, the large majority of dermal melanocytes expressed the gp100/HMB-45 antigen. A few cells expressed MIB-1/Ki-67 antigen and/or p16 protein. These findings were diagnostic of pigmented epithelioid melanocytoma. Pigmented epithelioid melanocytoma (PEM), also referred to as epithelioid blue naevus, was previously described as “animal-type melanoma” because of the clinical and histological similarities with equine melanoma [1]. It is a rare melanocytic neoplasm of unknown pathogenesis affecting all racial groups, with a predilection for young patients, including children [2]. Clinically, PEM manifests usually as a slow-growing heavily pigmented (dark blue) papule or nodule with a homogenous blue colour on dermatoscopic examination [3]. PEM is either a pure tumour (as in the patient presented here), or can be a component of a combined naevus (in association with an adjacent common acquired naevus). In 30% of cases, PEM contains small junctional nests. Histopathological examination shows a mixture of dermal epithelioid and spindled melanocytes with heavy pigmentation, indistinguishable from epithelioid blue nevi associated with Carney complex [4]. The tumour cells express the melanocytic antigens, including gp100/HMB-45. Epidermal (occasionally pseudoepitheliomatous) hyperplasia is often present, as in our patient. Sentinel lymph node biopsy may reveal deposits of melanocytes within lymph nodes, simulating metastasis, but the evolution of PEM is usually indolent with no fatal outcome [4, 5]. A