Xanthogranulomas with inconspicuous foam cells and giant cells mimicking malignant melanoma: a clinical, histologic, and immunohistochemical study of three cases.

Histiocytic proliferations can mimic melanocytic tumors and vice versa. The authors describe the clinical, histologic, and immunohistochemical findings of three predominantly mononuclear xanthogranulomas that were misdiagnosed as malignant melanoma by experienced pathologists. All lesions occurred in male patients ranging in age from 14 to 75 years. The tumors presented as dermal nodules, two of which were surrounded by an epidermal collarette and were ulcerated focally. The tumors were composed of a mixed population of large epithelioid and plump spindle cells with pink or pale cytoplasm arranged in nests and short fascicles. Occasional mononuclear cells had cytoplasmic vacuolar changes, but none had well-developed foamy cytoplasm. Rare, multinucleated giant cells were present, but they were not of the Touton type. Mitotic figures were found in all lesions. Immunohistochemically, most tumor cells (80%-90%) were strongly positive for CD68 and a minority of cells (10%-15%), located typically at the periphery of the tumor, was positive for factor XIIIa. Two tumors contained rare cells positive for S-100 protein (5% of tumor cells or less). All tumors were completely negative for tyrosinase (T311), gp100 (HMB-45), and Melan-A (A103). Giant and foam cell-poor variants of juvenile xanthogranuloma have been reported previously, mainly in young children. Their occurrence in adolescents and adults is underrecognized. Knowledge of this variant is important to avoid misdiagnosing a benign tumor as malignant melanoma.