Parosteal Osteosarcoma

• 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

L>Location

• Arises from cortex
• Most commonly in posterior aspect distal femur (> 75%)
• Also tibia & humerus

R>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

Patholo>Pathology

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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

Differe>Differential Diagnosis

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Osteochondroma

Myositis Ossificans

• More mature in periphery
• “Like an Egg”
• Not attached to bone

Classic OS

Periosteal OS

Treatme>Treatment

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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