Osteosarcoma

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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.

Epidemiology

Osteosarcomas can be either primary or secondary, and these have differing demographics:

  • primary osteosarcoma: typically occurs in young patients (10-20 years) with 75% taking place before the age of 20 because the growth centres 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

Clinical presentation

Patients often 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)

Pathology

Osteosarcomas can be divided into a number of subtypes according to the degree of differentiation, location within the bone, and histological variants 3. 

These subtypes vary in imaging findings, demographics and biological behaviour, and include:

Macroscopic appearance

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.

Histology

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. 

Location

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:

Associations
Markers

Serum alkaline phosphatase (ALP) may be raised (particularly with advanced disease).

Radiographic features

Plain radiograph

Conventional radiography continues to play an important role in diagnosis. Typical appearances of conventional high-grade osteosarcoma include:

CT

The role of CT is predominantly utilised in assisting biopsy and staging. CT adds little to plain radiography and MRI in the 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

MRI is proving an essential tool to determine accurate local staging and assessment for limb-sparing resection, particularly for evaluation of intraosseous tumour extension and soft-tissue involvement. Evaluation of the growth plate is also essential as up to 75-88% of metaphyseal tumours do cross the growth plate into the epiphysis 4.

  • T1
    • 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 a variable signal (see ageing blood on MRI)
    • enhancement: solid components enhance
  • T2
    • 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 a pathologic fracture and the development of metastatic disease, particularly to bone, lung and regional lymph nodes.

Differential diagnosis

General differential considerations include:

When the lesion is at the posteromedial distal femur consider:

Practical points

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 chance of seeding. A poorly planned track that crosses compartments can result in a more extensive resection, potentially with poor outcomes for the patient. 

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