Osteosarcomas are malignant bone forming tumours and the second most common primary bone tumour after multiple myeloma. They account for ~20% of all primary bone tumours and occur in primary and secondary forms, each with different epidemiology and distribution. Although plain radiography can provide a lot of information, MRI is used for local staging by assessing intraosseous tumour extension (e.g. growth plate/epiphysis) and soft-tissue-involvement. Chest CT and bone scanning have a role in distant staging.
Osteosarcomas can be either primary or secondary, and these have differing demographics:
- primary osteosarcoma: typically occurs in young patients (10-20 years) with 75% occurring before the age of 20 because the growth centers of the bone are more active during puberty/adolescence 3; slight male predominance
- secondary osteosarcoma: occurs in the elderly; usually secondary to malignant degeneration of Paget disease, extensive bone infarcts, post-radiotherapy for other conditions, osteochondroma, and osteoblastoma
Patients usually present with bone pain, occasionally accompanied by a soft-tissue mass or swelling. At times, the first symptoms are related to pathologic fracture.
The distribution of primary and secondary osteosarcomas is also different.
- primary tumours typically occur in the metaphyseal regions of long bones, and have a striking predilection for the knee, with up to 60% occurring there
- secondary tumours on the other hand, have a much wider distribution largely mirroring the combined incidence of their underlying condition, and thus much have a higher incidence in flat bones, especially the pelvis (a favourite site of Paget disease)
Osteosarcomas can be divided into a number of subtypes according to degree of differentiation, location within the bone, and histological variants 3.
These subtypes vary in imaging findings, demographics and biological behaviour, and include:
- intramedullary: ~80%
- surface or juxtacortical: ~10-15%
- extraskeletal: ~5%
Macroscopically osteosarcomas are bulky tumours where a heterogeneous cut surface demonstrates areas of haemorrhage, fibrosis and cystic degeneration. Their extension within the medullary cavity is often much more extensive than the bulky part of the tumour would suggest. Areas of bone formation are characteristic of osteosarcomas, with the degree of bone formation varying widely.
Microscopically poorly formed trabecular bone is seen with (in the typical high grade conventional subtype) cellular pleomorphism and mitoses. Variable amounts of fibrocytic and chondroblastic appearing cells may also be encountered.
They typically occur at the metadiaphysis of tubular bones in the appendicular skeleton. Common sites include:
- femur: ~40% (especially distal femur)
- tibia: ~16% (especially proximal tibia)
- humerus: ~15%
Other less common sites include:
- innominate bone (i.e. os coxae)
- mandible (gnathic osteosarcoma)
Serum alkaline phosphatase (ALP) may be raised (particularly with advanced disease).
Conventional radiography continues to play an important role in diagnosis. Typical appearances of conventional high grade osteosarcoma include:
- medullary and cortical bone destruction
- wide zone of transition, permeative or moth-eaten appearance
- aggressive periosteal reaction
- soft-tissue mass
- tumour matrix ossification/calcification
- variable: reflects a combination of the amount of tumour bone production, calcified matrix, and osteoid
- ill-defined "fluffy" or "cloud-like" cf. to the rings and arcs of chondroid lesions
The role of CT is predominantly in assisting biopsy and staging, but adds little to plain radiography and MRI in direct assessment of the tumour. The exception to this rule is predominantly lytic lesions in which small amounts of mineralised material may be inapparent on both plain film and MRI 4.
MRI is proving essential in accurate local staging and assessment for limb sparing resection, particularly for evaluation of intraosseous tumour extension and soft-tissue involvement. Assessment of the growth plate is also essential as up to 75-88% of metaphyseal tumours do cross the growth plate into the epiphysis 4.
- soft tissue non-mineralised component: intermediate signal intensity
- mineralised/ossified components: low signal intensity
- peritumoural oedema: intermediate signal intensity
- scattered regions of haemorrhage will have variable signal (see ageing blood on MRI)
- enhancement: solid components enhance
- soft tissue non-mineralised component: high signal intensity
- mineralised/ossified components: low signal intensity
- peritumoural oedema: high signal intensity
Treatment and prognosis
Work-up includes local staging by MRI (for skip lesions) prior to biopsy and distant staging with bone scan and chest CT.
Cure, if achievable, requires aggressive surgical resection often with amputation followed by chemotherapy. If a limb-salvage procedure is feasible, a course of multidrug chemotherapy precedes surgery to downstage the tumour, followed by wide resection of the bone and insertion of an endoprosthesis. The outcome depend on different factors such as age, sex, site, size, and type but the most important predictor is the histologic degree of necrosis post induction chemotherapy; 90% histologic necrosis is associated with much better prognosis 6. Currently, the 5-year survival rate after adequate therapy is approximately 60-80% 4.
The most frequent complications of conventional osteosarcoma are pathologic fracture and the development of metastatic disease, particularly to bone, lung and regional lymph nodes.
General differential considerations include:
When the lesion is at the posteromedial distal femur consider:
When planning to biopsy a potential sarcoma, the treating surgeon should be consulted to plan the biopsy track as this will require excision to reduce the change of seeding. A poorly planned track that crosses compartments can result in a more extensive resection, potentially with poor outcomes for the patient.
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
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- 2. Zwaga T, Bovée JV, Kroon HM. Osteosarcoma of the femur with skip, lymph node, and lung metastases. Radiographics. 28 (1): 277-83. doi:10.1148/rg.281075015 - Pubmed citation
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