A 14-year-old boy presents with fever, weight loss, and an aggressive lytic diaphyseal femoral lesion with an 'onion-skin' periosteal reaction. Biopsy shows sheets of small round blue cells. Which immunohistochemical marker is characteristically POSITIVE in this tumor and helps confirm the diagnosis?
- A S-100 protein
- B CD99 (MIC2) ✓
- C Cytokeratin
- D Desmin
Correct answer: B. CD99 (MIC2)
Explanation
Ewing sarcoma characteristically shows strong membranous CD99 (MIC2) positivity, which is a useful diagnostic marker. The tumor cells also contain glycogen (PAS positive). S-100 is seen in neural tumors and melanoma. Cytokeratin is seen in carcinomas and adamantinoma. Desmin is seen in muscle tumors like rhabdomyosarcoma.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.