Which marker is NOT included in the IHC panel for primary epithelioid soft tissue malignancy?

• A. Epithelial Membrane Antigen
• B. CD15
• C. Actin
• D. Vimentin

Answer: (D)

Immunohistochemistry panels for epithelioid soft tissue malignancies are built to reveal epithelial-like differentiation and specific lineage features, not just any mesenchymal marker. Epithelial Membrane Antigen indicates epithelial-type differentiation and is often positive in epithelioid tumors, helping distinguish them from non-epithelial sarcomas. CD15 can be expressed in epithelioid sarcoma and related epithelioid tumors, providing diagnostic value within the panel. Actin identifies myofibroblastic or smooth muscle differentiation that can be present in some epithelioid soft tissue tumors, adding further context to the tumor’s phenotype. Vimentin, while a common mesenchymal marker, is expressed in nearly all soft tissue tumors and offers little discriminative power for this specific group, so it is not included in the focused panel.