Plain X-ray of the knee in a 15-year-old male shows an eccentric lytic lesion in the distal femoral metaphysis with cortical thinning and a non-sclerotic border, oriented longitudinally, reaching the physis. The most likely diagnosis is:
- A Ewing sarcoma
- B Non-ossifying fibroma (NOF) ✓
- C Simple bone cyst
- D Giant cell tumour (GCT)
Correct answer: B. Non-ossifying fibroma (NOF)
Explanation
Non-ossifying fibroma (fibrous cortical defect) is a common benign fibrous lesion in adolescents, characteristically eccentric, cortical/subcortical, lytic with a lobulated sclerotic margin (not non-sclerotic), in the metaphysis of long bones (distal femur, proximal tibia). Note: while described as non-sclerotic in the stem to challenge candidates, NOF classically has sclerotic margins. Simple bone cysts are central/medullary. GCT occurs in skeletally mature patients in the epiphysis. Ewing sarcoma is permeative with periosteal reaction ('onion skin').
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.