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Large Cell Undifferentiated Malignant NeoplasmThe 'large cell undifferentiated malignant neoplasm' generally, but not always presenting as a lymph node based process, usually suggests the differential diagnosis of metastatic carcinoma, metastatic melanoma, or high grade lymphoma (e.g., anaplastic large cell lymphoma or diffuse large B-cell lymphoma), but other more uncommon entities such as epithelioid angiosarcoma, epithelioid malignant nerve sheath tumor, epithelioid sarcoma, and malignant angiomyolipoma might also be in the formal differential diagnosis, depending upon the histology and clinical setting. A basic panel of antibodies to CD45, cytokeratin, and S100 (or, perhaps, a melanoma-restricted antigen such as the HMB-45 defined gp100 protein) can be used in the initial analysis of these tumors, but it must be remembered that among large cell undifferentiated malignancies, there are many exceptions to the general rules. For example, cytokeratin, a marker of epithelial differentiation, can be expressed by a significant subset of metastatic melanomas as well as angiosarcomas, particularly those with epithelioid histologic features. While CD45 shows a very high specificity for hematolymphoid processes, a number of these malignancies (e.g., ALCL, granulocytic sarcoma, plasmacytoma) may be CD45 negative, and other markers are required to positively identify these tumors.
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