NeuroradiologyOsteogenic sarcoma
malignant tumour of mesenchymal cells characterized by a direct formation of osteoid or bone by the
tumour cells. It is overall one of the two most common primary
malignant bone tumours accounting for about 15% of all tumours arising in the bone and it is the most common in the paediatric population. It occurs most frequently in the first and second decades and more commonly in males. In the elderly patients osteosarcomas most frequently arise against a background of predisposing cancerogenic conditions such as bone irradiation (with 5- to 25-year latent period), or pre-existing bone pathology such as Paget's bone disease and osteochondroma. These are usually referred to as secondary osteosarcomas and account for no more than 6 to 10% of all osteosarcomas. Juvenile osteosarcomas arise most frequently in the medullary cavity of metaphyseal and of the long bones of the
extremities. However, any bone can be involved and after the age of 25 the incidence in flat bone and long bones is almost equal. Sites of neuroradiological interest such as the
spine, skull and facial bones are affected more commonly in the context of secondary osteosarcoma. In the
spine metastatic disease from osteogenic sarcoma arising elsewhere is more common. In the head and neck region
de novo osteosarcomas most frequently arise in the maxillary
sinus, mandible and calvarium.
Pathologically, osteosarcomas are calcified and firm lesions. Histologically they may be variously composed of fibroblastic cells or have abundant bone formation or chondroid differentiation (e.g. osteosarcoma in the jaw) or be highly vascular but all have tumour-produced osteoid marked by trapping of anaplastic tumour cells within the lacunae of the osteoid matrix.
On imaging, CT can show osteoblastic or osteolytic bony changes. On MR, as with other invasive bone tumours, on short TR sequences, the tumour appears of low signal compared with the high signal in the bone marrow whereas on long TR sequences, an intraosseous tumour can display low intensity, high intensity, or a combination of signal intensities. When the normal signal void of cortical bone is infiltrated by tumour, a mottled appearance will result. On long TR images, an extraosseous tumour is usually of a high signal which provides a demarcation from uninvolved muscle. In addition, after the administration of gadolinium, these tumours often show immediate enhancement, again marking them out from muscle, which only minimally enhances. Enhancing areas may reflect the more vascular and probably the more "aggressive" viable areas of the tumour. Associated necrotic or sclerotic areas will either slowly enhance or fail to enhance.
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