A 25-year-old presents with a painless swelling at the distal femur metaphysis. X-ray shows a bony projection (exostosis) with cortical and medullary continuity with the host bone, pointing away from the joint. MRI shows a cartilage cap <1 cm thick. The diagnosis is osteochondroma. Which finding on imaging would raise concern for malignant transformation to chondrosarcoma?
- A Cartilage cap thickness >2 cm in an adult ✓
- B Pedunculated morphology pointing away from the joint
- C Continuity of cortex and medulla with host bone
- D Absence of pain in the mass
Explanation
Malignant transformation of an osteochondroma to secondary chondrosarcoma should be suspected when the cartilage cap thickness exceeds 1.5–2 cm in an adult (cartilage caps in adults are normally <1 cm), or when there is new onset of pain, rapid growth, or calcification changes within the cap. The cartilage cap thickness >2 cm is the most reliable imaging criterion for malignancy. Continuity of cortex/medulla is a benign feature confirming it is an osteochondroma. The incidence of malignant transformation in solitary osteochondroma is ~1% but rises to ~5–25% in hereditary multiple exostoses.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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