Abstract

Metastatic melanoma can have several radiographic and histologic mimics. We present a case of a 69-year-old man with history of melanoma of the left calf who presented with a self-palpated mass in the contralateral leg four years later. Ultrasound showed a 2.2 x 1.1 cm hypoechoic mass with internal flow within the gastrocnemius in the right upper calf and a PET/CT scan showed increased activity, raising suspicion for metastatic melanoma. A fine needle biopsy showed sparse atypical, medium-sized epithelioid cells with fine brown cytoplasmic granules in a background of lymphocytes, histiocytes, and occasional multinucleated giant cells, consistent with the possibility of melanoma with an associated reparative process. Surgical excision revealed a well-circumscribed proliferation of mononuclear cells admixed with osteoclast-like giant cells, foamy histiocytes and pigmented macrophages. Immunohistochemical studies showed that lesional cells were positive for CD163 and negative for SOX10, pan-melanoma cocktail, and pan-keratin cocktail. The pigment deposits were positive for iron, confirming they were hemosiderin and not melanin. A diagnosis of tenosynovial giant cell tumor was made and no further treatment was planned. This case highlights the importance of considering benign entities in the differential diagnoses of new lesions appearing in patients with a history of malignancy especially in the setting of melanoma cases.

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