A 28-year-old woman presents with a painless, slowly enlarging mass on the posterior distal femur. X-ray shows a pedunculated bony outgrowth with a cartilage cap pointing away from the joint. The medullary cavity of the lesion is continuous with the host bone medullary cavity. The most likely diagnosis and its malignant transformation rate is:
- A Parosteal osteosarcoma; extremely low malignant transformation rate
- B Osteochondroma (exostosis); solitary lesions have <1% malignant transformation risk ✓
- C Periosteal chondroma; <5% risk of chondrosarcoma transformation
- D Enchondroma; 3–5% malignant transformation risk when in a long bone
Explanation
This description is classic for an osteochondroma (osteocartilaginous exostosis): pedunculated or sessile bony protuberance with cartilage cap, pointing away from the adjacent joint, with medullary continuity between the lesion and host bone. Solitary osteochondromas have a malignant transformation rate of <1% (secondary chondrosarcoma, heralded by growth after skeletal maturity or cartilage cap >2 cm). Multiple hereditary exostosis (MHE) has a 2–5% risk. Parosteal osteosarcoma does not have medullary continuity. Enchondromas are intramedullary and lack a cartilage cap at the surface.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.