A Case of Metastatic Malignant Melanoma Simulating Granuloma Annulare

Malignant melanoma and particularly metastatic melanoma represent a diagnostic challenge due to the wide variety of histologic patterns, resemblance to benign entities, and extensive range of clinical presentations.  A high index of suspicion for melanoma is important for accurate diagnosis, especially when there is a previous history of malignancy.  Here, we present a patient with a history of melanoma and locally metastatic melanoma, who subsequently developed a nodule on his right forearm near the site of his previous melanoma excision.  Histologically, the melanoma appeared as granuloma annulare (GA) with benign cytologic features, but was identified as metastatic melanoma using SOX-10 immunohistochemical staining.  Other malignancies, including lymphomas, leukemias, sarcomas, and cutaneous metastases of internal malignancies, have mimicked GA and interstitial granulomatous processes.  Therefore, further immunohistochemical staining should be performed to assess for metastatic disease in the setting of a histological pattern that resembles a benign granulomatous process in a patient with a history of malignancy, including malignant melanoma.