A 25-year-old man presents with a slowly enlarging, painless posterior distal femoral mass. X-ray shows a dense lobulated sessile bony mass attached to the posterior cortex of the distal femur. Biopsy shows well-differentiated spindle cells within a bony trabecular scaffold with minimal atypia. What is the diagnosis and its prognostic significance?
- A Conventional osteosarcoma, poor prognosis
- B Parosteal osteosarcoma, good prognosis if widely excised ✓
- C Periosteal chondrosarcoma, intermediate prognosis
- D High-grade surface osteosarcoma, poor prognosis
Correct answer: B. Parosteal osteosarcoma, good prognosis if widely excised
Explanation
Parosteal osteosarcoma is a low-grade surface osteosarcoma arising on the posterior distal femur. It shows well-differentiated spindle cells with minimal atypia and bony trabeculae. Prognosis is excellent with wide local excision, unlike conventional osteosarcoma. High-grade surface osteosarcoma shows marked atypia. Periosteal chondrosarcoma is cartilaginous, not bony.
Reference: Campbell's Operative Orthopaedics, 14th ed.
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