Malignant Melanoma Metastatic to the Testis: A Report of Three Cases with Clinically Significant Manifestations.

Three cases of malignant melanoma metastatic to the testis, each of which was a challenge in pathologic interpretation, are reported. The patients were 43, 55, and 80 years of age, respectively. A history of malignant melanoma was not known to the initial examining pathologist in two of the cases. The testicular tumors were all unilateral. One took the form of multiple nodules, but the others were discrete solitary masses. On microscopic examination diffuse, nested, follicular, and fascicular patterns, usually in combination, were seen. The tumor cells had moderate to abundant eosinophilic cytoplasm in two cases, but scanty cytoplasm in a third. In one case there was also a component of cells with copious foamy cytoplasm. Rare areas of melanin pigment were identified in two tumors. Immunohistochemistry for S-100 and HMB-45 was positive in the two cases in which it was performed. In the third case electron microscopy revealed aberrant melanosomes within the tumor cells. The differential diagnosis in these and rare other cases of malignant melanoma involving the testis during life is broad and includes seminoma, malignant lymphoma, and Leydig cell tumor. When the diagnosis is entertained, a search for melanin pigment, immunohistochemical stains for S-100 protein and HMB 45, and further inquiry into the clinical history are often crucial. Int J Surg Pathol 8(1):49-57 2000