Parosteal osteosarcoma

Tumor biology and incidence

• Rare, low-grade lesion arising on surface of cortex
  • Classified as high-grade surface osteosarcoma if high-grade cytological changes observed
• This lesion may contain fibrous, osseous or cartilaginous elements
• Most common osteosarcoma variant, accounting for 5% of all osteosarcomas
• Lobulated, ossified mass on posterior distal femur is characteristic of 73% of parosteal osteosarcomas

Gender

M:F=2:3

Presentation

• Slow-growing lesion; usually presents as painless mass
• Restriction of joint motion less frequently the presenting complaint

Location

• Predilection for posterior aspect of distal femur (two thirds of cases)
• May also occur in proximal medial humerus or lateral proximal tibia

Radiographic appearance

Juxtacortically based lesion with densely sclerotic, often lobulated appearance

Size

Usually large and lobulated, with broad base

Tumor effect on bone

• Located on intact cortex as if a piece of plasticene were applied to bone
• Lesion may encircle entire bone.
• Sclerotic lesion, although areas of fibro-cartilaginous lucency may be observed on CT
• Controversy: Does invasion of tumor into medullary canal (observed on MRI or pathologic evaluation of the resected sample) indicate worse prognosis?
  

Differential diagnosis

• Periosteal osteosarcoma
• Periosteal chondrosarcoma
• Myositis ossificans
• Osteochondroma
  

Natural history

Approximately 30% or more of these lesions dedifferentiate into high-grade lesions and invade central canal

Prognosis

• Has best prognosis of osteosarcomas; lesions are usually low grade
• Complete resection results in cure of most low-grade parosteal osteosarcomas
• If high grade areas are present, prognosis approaches that of conventional high-grade osteosarcoma; patient requires chemotherapy and wide excision