Parosteal osteosarcoma

Last revised by Dr Calum Worsley on 10 Oct 2021

Parosteal osteosarcoma is a subtype of osteosarcoma and arises from the outer layer of the periosteum. 

It is the most common type of juxtacortical or surface osteosarcoma and accounts for ~5% of all osteosarcomas 1. It typically presents in early adulthood and middle age with a peak incidence in the third decade. It affects females slightly more than males 2,3

The diagnosis of parosteal osteosarcomas is based on a combination of typical histological and radiographic features.

Diagnostic criteria according to the WHO classification of bone tumors (2020 - blue book) 3:

  • imaging features of a bone tumor
  • parosteal location with the origin on the cortical surface of the bone
  • histology of a low-grade spindle cell tumor with woven bone formation

MDM2 amplification is a further desirable criterion.

Patients usually present with a painless, slowly enlarging mass 2.

They are composed of a dense osteoid component attached to the outer cortex over a narrow zone. Parosteal osteosarcoma originates from the outer fibrous layer of the periosteum. It exhibits extensive bone matrix and minimal fibroblastic cellular atypia, and as such is considered to be a low-grade tumor 4

  • usually located at the metaphysis (80-90%), most commonly at the posterior aspect of the distal femur (60%), followed by either end of the tibia, and then the proximal humerus
  • large lobulated exophytic, cauliflower-like mass with central dense ossification adjacent to the bone
  • string sign: thin radiolucent line separating the tumor from the cortex, seen in 30% of cases 5
  • tumor stalk: grows within the tumor in late stages and obliterates the radiolucent cleavage plane 6
  • cortical thickening without aggressive periosteal reaction is often seen
  • tumor extension into the medullary cavity is frequently seen

It exhibits predominantly low signal intensity on T1 weighted imaging, though T2 weighted imaging characteristics vary from heterogeneous to high intensity depending on the presence of hemorrhage or necrosis. High signal intensity on T2 is suggestive of a high-grade tumor. 7

As parosteal osteosarcomas tend to be low-grade lesions, they are usually treated with surgical resection, without chemotherapy or radiation. As they are frequently metaphyseal in location, large parosteal osteosarcomas or those with deep medullary invasion may require limb salvage, including joint replacement.

Parosteal osteosarcomas have an excellent prognosis (80-95% long-term survival) 1

On imaging consider

  • cortical desmoid: avulsive injury of the posterior femoral cortex
  • myositis ossificans: the ossification pattern of parosteal osteosarcoma shows progressively increased ossification from the periphery to the center; the ossification pattern of myositis ossificans is the opposite, with the densest ossification at the periphery and usually associated with soft tissue edema 5
  • sessile osteochondroma: parosteal osteosarcoma lacks a communication between the medullary canal of the bone and the cortical tumor 5 
  • juxtacortical chondrosarcoma
  • high-grade surface osteosarcoma
  • parosteal lipoma: radiolucent mass adjacent to the cortical surface on plain film and on cross-sectional imaging will usually show a definite fat component 8

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Cases and figures

  • Case 1
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  • Case 2
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  • Case 3
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  • Case 4: with string sign
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  • Case 5
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  • Case 6
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