Answer: Metastatic melanoma

Histology: The lymph node is almost entirely replaced by a proliferation of spindle shaped cells with eosinophilic cytoplasm, hyperchromatic nuclei and occasional prominent nucleoli. The tumor shows some mitotic activity. Immunohistochemical stains show that the tumor cells express S100. HMB-45 and Melan-A were negative.

Discussion: A spindle cell lesion in the lymph node raises the differential diagnosis of metastatic sarcoma, Kaposi’s sarcoma and metastatic melanoma. Metastatic sarcomas to lymph nodes are rare; most sarcomas do not metastasize to lymph nodes, with the exception of synovial sarcoma. Kaposi’s sarcoma is characterized by a proliferation of spindle cells with slit-like vascular spaces, which is not apparent in this case. Metastatic melanoma may have varied histologic appearances, and many will lack melanin pigment. Spindle cell or desmoplastic melanoma should always be considered in the differential diagnosis in lymph nodes showing atypical spindle cells. Immunohistochemical staining will show that these cells express S-100 protein. Spindle cell melanoma is usually negative for the usual melanocytic markers, HMB-45 and Melan-A; therefore the lack of staining with these markers should not exclude the diagnosis.