A 20-year-old man has a solitary osteochondroma (osteocartilaginous exostosis) of the distal femur that has been stable for years. On routine MRI, the cartilage cap thickness is measured at 3.2 cm. The significance of this finding is:
- A Normal variant — cartilage cap up to 5 cm is acceptable in adults
- B Concerning for malignant transformation to secondary chondrosarcoma — excision recommended ✓
- C Indicates rapid growth phase — observe with 3-monthly MRI
- D Requires prophylactic radiotherapy to prevent malignant transformation
Explanation
The cartilage cap of an osteochondroma is normally <1.5 cm in thickness in adults (and may be up to 2 cm in skeletally immature patients). A cap thickness >2 cm in adults is the accepted MRI threshold highly suspicious for malignant transformation to secondary chondrosarcoma (which occurs in 1% of solitary osteochondromas but 5–25% of hereditary multiple exostoses). A cap of 3.2 cm in a 20-year-old adult is clearly concerning, and wide excision is recommended. Irregular, lobulated margins on MRI and cap thickness >2 cm are the two most reliable imaging indicators of transformation. Radiotherapy is not indicated for musculoskeletal tumors of this type.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.