Histiocytoid melanoma presenting as lymphadenopathy and mimicking Rosai-Dorfman disease.

We report a case of a patient with large, unilateral inguinal lymphadenopathy. Excisional biopsy demonstrated expansion of the lymph node sinuses by a population of large histiocytoid cells with abundant granular eosinophilic cytoplasm and large, eccentric nuclei with prominent central nucleoli. The findings were morphologically suspicious for Rosai-Dorfman disease; however, emperipolesis was conspicuously absent. Immunohistochemical work-up demonstrated that the infiltrate consisted of metastatic melanoma with histiocytoid morphology and admixed reactive sinus histiocytes.