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- Lower grade of OS
- Patients in 3rd-4th decade
- M<F – 1:1.5 (ie. opposite to central OS)
- Presents with painless block to knee flexion
Location
- Arises from cortex
- Most commonly in posterior aspect distal femur (> 75%)
- Also tibia & humerus
Radiology
- X-ray
- Dense mass adjacent to cortex demarcated from the adjacent soft tissues
- May resemble exostosis
- Lytic areas can occur
- Wraps around bone with intervening periosteum
- Underlying cortex may be thickened
- 25% invade periosteum
- Arises superficial to periosteum
- Often has more pronounced appearance than periosteal OS
- Often see lucent thin line separating it from the cortical bone – “String sign”
- CT Scan
- Differentiates from exostosis
- Parosteal OS
- Attached to cortex growing into soft tissue
- Normal cortex intact
- Exostosis
- Cortex of bone becomes cortex of exostosis
- Medullary canal confluent with exostosis
- Posterior femur rare
Pathology
- Low grade
- Irregularly arranged bone
- Background of spindle cells & fibrous tissue
- May have cartilage cap
- Can encircle bone
- Mass firmly adherent to bone & on cross section may exhibit bony, cartilaginous & fibrous areas
- Well defined lobulated mass with extensive bone & occasionally cartilage formation
- Contain bland, well-differentiated fibrosarcomatous stroma
Differential Diagnosis
- Osteochondroma
- Myositis Ossificans
- More mature in periphery
- “Like an Egg”
- Not attached to bone
- Classic OS
- Periosteal OS
Treatment
- Wide excision of mass
- 7cm Proximal & 5cm Distally
- As 25% involve medulla
- 80% cure with surgery alone
- Adjuvant chemotherapy not used unless there is intramedullary spread
- 25% of parosteal osteosarcomas are high-grade
- Histologically similar to central high-grade lesions
- Poor prognosis
