Extensive pigmented villonodular synovitis with markedly pigmented lymphadenopathy and its implication for differential diagnosis with malignant melanoma

A 51-year-old male presented with a 5 cm left knee mass. Fine needle aspiration revealed large epithelioid cells with prominent nucleoli and abundant cytoplasmic pigment, consistent with malignant melanoma. Left inguinal lymphadenopathy was present, which was suspicious for metastatic disease by ultrasound examination. A dark perianal skin lesion was also identified, therefore raising the possibility of a primary melanoma. The knee and perianal lesions were resected and inguinal sentinel node biopsy was performed. In the specimen from the knee, there were clusters and fascicles of spindle and epithelioid cells with prominent nucleoli. Many of the cells displayed abundant, granular, brown, cytoplasmic pigment. The lymph node showed clusters of similar cells located in the subcapsular sinus. Immunohistochemical study showed that the cells expressed CD68, but failed to express S-100, MART-1, and gp100. The cytoplasmic pigment was positive for iron staining. The final diagnosis was pigmented villonodular synovitis. This case illustrates that pigmented villonodular synovitis may present with lymphadenopathy, mimicking a malignant process, including melanoma. Immunohistochemical studies may be essential for establishing the correct diagnosis.