Which marker is NOT part of the IHC panel for primary soft tissue small round neoplasms (second question)?

• A. Leukocyte Common Antigen
• B. S100
• C. Neuron-Specific Enolase
• D. Epithelial Membrane Antigen

Answer: (B)

In evaluating primary soft tissue small round cell neoplasms, the immunohistochemical panel focuses on identifying lineage and differentiation to separate lymphoid, epithelial/synovial, neural, and muscle origins. Leukocyte Common Antigen (CD45) is included to exclude hematolymphoid tumors like lymphomas. Epithelial Membrane Antigen (EMA) helps detect epithelial or synovial differentiation, which is important when considering tumors such as synovial sarcoma. Neuron-Specific Enolase (NSE) can indicate neuronal or neuroendocrine features, providing supportive information in the differential. S100, while useful for neural crest–derived tumors and melanoma, is not part of the core panel for these soft tissue small round cell neoplasms because its expression is not specific enough to distinguish the common entities in this category. Therefore, S100 is the marker that is not typically included in this IHC panel.