Answer: Metastatic malignant melanoma

Histology: The lymph node is completely replaced by an anaplastic appearing tumor with focal necrosis. The tumor shows varying histologies. In some areas it is composed of a predominantly large cell proliferation with abundant pink cytoplasm and multiple atypical nuclei. Some of the nuclei show prominent intra-nuclear cytoplasmic inclusions. In other areas the tumor appears to be more spindled in appearance, mimicking the growth pattern of a sarcoma. In other areas, the tumor is composed of smaller cells, growing in a sheet-like fashion. There is a high mitotic rate associated with this tumor and large areas of necrosis.

Discussion: This tumor raises the differential diagnosis of an anaplastic pink cell tumor with both epithelial and spindled growth pattern. The primary differential diagnosis rests between a metastatic anaplastic carcinoma with spindled metaplasia vs. a malignant melanoma. The diagnosis of lymphoma, such as a KI-1 lymphoma, would be unlikely, particularly with the prominent spindled cell pattern noted in this tumor. Malignant fibrous histiocytoma also is a diagnosis that should be considered as this tumor can on occasion metastasize to lymph nodes.

Immunohistochemical studies were particularly helpful in arriving at the proper diagnosis in this case. The neoplastic cells are positive for HMB45, S-100, and Melan A. The tumor cells are negative for cytokeratin. These findings support the diagnosis of metastatic malignant melanoma.