Which marker can help differentiate Hemangiosarcoma from Telangiectatic OSA when osteoid is not obvious?

• A. Ki-67
• B. Factor VIII-related antigen/von Willebrand factor (FVIII-RAg/vWF)
• C. CD31
• D. Desmin

Answer: (B)

When you can’t rely on osteoid to tell you a vascular tumor from an osteogenic one, you turn to lineage-specific markers. Hemangiosarcoma is of endothelial origin, so staining for endothelial markers helps confirm it, while telangiectatic osteosarcoma, being osteogenic, should not show endothelial differentiation.

Von Willebrand factor (FVIII-RAg) is a classic endothelial marker. Its presence supports a vascular tumor like hemangiosarcoma, even if osteoid isn’t obvious. Ki-67只是 a proliferation marker and doesn’t identify cell type; CD31 is another endothelial marker but vWF is particularly useful for establishing endothelial lineage in this differential. Desmin marks muscle, which isn’t relevant here.

So, using FVIII-related antigen/von Willebrand factor staining helps distinguish hemangiosarcoma from telangiectatic osteosarcoma when osteoid is not evident.