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. It typically presents in early adulthood and middle age with a peak incidence in the third decade. It affects females slightly more than males 9.
Patients usually present with a painless, slowly enlarging mass 9.
They are 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.
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 periosteum. It exhibits extensive bone matrix and minimal fibroblastic cellular atypia, and as such is considered to be a low-grade tumour.
- large lobulated exophytic, 'cauliflower-like' mass with central dense ossification adjacent to the bone
- string sign: thin radiolucent line separating the tumour from the cortex, seen in 30% of cases
- tumour stalk: grows within tumour in late stages and obliterates the radiolucent cleavage plane
- +/- soft tissue mass
- cortical thickening without aggressive periosteal reaction is often seen
- tumour extension into the medullary cavity is frequently seen
It exhibits predominantly low signal intensity both on T1 and T2 weighted imaging. High signal intensity on T2 is suggestive of a high-grade tumour.
Treatment and prognosis
Parosteal osteosarcomas are usually low-grade lesions are usually treated with surgical resection and no neoadjuvant 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).
On imaging consider
- cortical desmoid: avulsive injury of the posterior femoral cortex
- myositis ossificans: the ossification pattern of parosteal OS is the radiographic inverse of that seen in myositis ossificans, with the densest ossification in the centre of the lesion and the least radio-opaque bone at the periphery (eggshell pattern); there is also often a history of trauma, progressive decrease in size or constant size and significant oedema in early stages on MRI.
- sessile osteochondroma: parosteal OS lacks corticomedullary continuity between the tumour and the underlying medullary canal.
- 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 2
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The differential diagnosis for bone tumours is dependent on the age of the patient, with a very different set of differentials for the paediatric patient.
- bone-forming tumours
- cartilage-forming tumours
- chondromyxoid fibroma
- fibrous bone lesions
- bone marrow tumours
- other bone tumours or tumour-like lesions
- aneurysmal bone cyst
- benign fibrous histiocytoma
- giant cell tumour of bone
- Gorham massive osteolysis
- haemophilic pseudotumour
- intradiploic epidermoid cyst
- intraosseous lipoma
- musculoskeletal angiosarcoma
- musculoskeletal haemangiopericytoma
- primary intraosseous haemangioma
- simple bone cyst
- impending fracture risk