Parosteal osteosarcoma

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

Epidemiology

It is the most common type of juxtacortical or surface osteosarcoma and accounts for ~4% of all osteosarcomas ^1-3. 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. 

Diagnosis

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

Diagnostic criteria

Diagnostic criteria according to the WHO classification of soft tissue and bone tumours (5^th edition) ³:

• imaging features of a bone tumour

• parosteal location with the origin on the cortical surface of the bone

• histology of a low-grade spindle cell tumour with woven bone formation

MDM2 amplification is a further desirable criterion.

Clinical presentation

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

Complications

Parosteal osteosarcomas might progress to a high-grade sarcoma including other osteosarcoma variants, undifferentiated spindle cell sarcoma or rhabdomyosarcoma ³.

Pathology

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 tumour ⁴.

Macroscopic appearance

Grossly, parosteal osteosarcomas are characterised by the following features ^2,3:

• exophytic lobulated osseous mass

• broad-based attachment to the underlying bone surface

• colour: white-tannish

• texture: gritty

• large tumours possibly surround the bone

• possible satellite lesions

• occasional incomplete cartilage cap

• possible focal necrosis, haemorrhage or fluid-filled cavities

• soft-fleshy areas might represent dedifferentiation

Microscopic appearance

Microscopically, parosteal osteosarcomas display the following histological features ^2,3:

• biphasic appearance with various patterns ²:

  • streamer pattern – well-formed bony trabeculae with intermeddled spindle cell fascicles (most common)

  • fibrous dysplasia-like – haphazard arrangement of trabeculae looking like Chinese calligraphy

  • desmoplastic fibroma-like – predominant background of spindle cells with abundant collagen

• usually hypocellular tumour with minimal nuclear atypia, low mitotic activity

• more cellularity and moderate atypia in about 20% of cases

• possible invasion of the skeletal muscle

• possible focal lack of bone formation

• possible scattered cartilaginous nodules or cartilaginous cap

Immunophenotype

Immunohistochemistry stains are usually reactive to MDM2 and CDK4 ^2,3 and might help in the differentiation to a variety of benign fibrous and fibro-osseous lesions such as reactive periostitis, juxtacortical myositis ossificans, fibrous dysplasia protuberans or desmoplastic fibroma ^2,3 as well in the diagnosis of dedifferentiated parosteal osteosarcoma ³.

Radiographic features

• 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 tumour from the cortex, seen in 30% of cases ⁵

• tumour stalk: grows within the tumour in late stages and obliterates the radiolucent cleavage plane ⁶

• cortical thickening without aggressive periosteal reaction is often seen

• tumour extension into the medullary cavity is frequently seen

MRI

It exhibits predominantly low signal intensity on both T1-weighted and T2-weighted imaging. High signal intensity on T2 is considered suggestive of a high-grade tumour ⁶, however, the T2 signal can vary depending on the presence of haemorrhage or necrosis and is therefore not always reliable in predicting higher-grade tumours or differentiation ⁷.

In addition, well-differentiated tumours may also have appearances indicating fatty marrow ³.

Radiology report

The radiological report should include a description of the following ^3,8:

• form, location and size

• tumour margins and transition zone

• circumferential extent

• cortical erosion, cortical breakthrough

• medullary components

• soft tissue component

• T2 signal: higher signal might suggest a higher grade ^6-8

• infiltration of neurovascular structures

Treatment and prognosis

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. 

History and etymology

Parosteal osteosarcomas were first characterised by the American surgeons Charles F Geschickter and Murray M Copeland in 1951 ^2,9.

Differential diagnosis

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 centre to the periphery (hardened heart); the ossification pattern of myositis ossificans is the opposite, with the densest ossification at the periphery (soft heart) and usually associated with soft tissue oedema ⁵

• sessile osteochondroma: parosteal osteosarcoma lacks communication between the medullary canal of the bone and the cortical tumour ⁵ 

• periosteal chondrosarcoma

• periosteal osteosarcoma

• high-grade surface osteosarcoma

• parosteal lipoma: radiolucent mass adjacent to the cortical surface on plain film and cross-sectional imaging will usually show a definite fat component ⁹