Osteosarcoma

General characteristics

Osteosarcoma is a malignant neoplasm characterized by production of osteoid by the tumor cells. It is the second most common primary malignant tumor of bone after plasmocytoma.

Epidemiology

It is a rare tumor, representing only 0.2% of all malignant tumors. There is a male predominance, with a male-to-female ratio of 2:l. Its peak incidence is in the second decade, and it is rare before 10 and after 30 years of age.

Morphology

Osteosarcoma can occur in any bone, but it has a strong predilection for the distal femur, proximal tibia, and proximal humerus. lbo thirds of osteosarcomas are localized to the knee or shoulder.

Osteosarcomas can be divided into two categories:

  • primary osteosarcomas, which arise de novo,
  • secondary osteosarcomas, which develop in abnormal bones.

Underlying bone abnormalities include:

  • Paget's disease,
  • complications of radiation therapy,
  • multiple enchondromas,
  • multiple osteochondromas,
  • chronic osteomyelitis,
  • fibrous dysplasia,
  • infarct.

Secondary osteosarcomas, rather than primary osteosarcomas, usually affect older people.

Telangiectatic osteosarcomas contain large, cystic, blood-filled spaces with fluid-fluid levels that may mimic an aneurysmal bone cyst, but these tumors are usually less well defined than aneurysmal bone cysts.

Imaging

MRI is useful for staging the tumor and for follow-up after treatment.

The osteoblastic component of the tumor has low signal intensity on all sequences. The nonmineralized component has low signal intensity on TI-weighted images and high signal intensity on T2- weighted images. Bone marrow extension is best seen on TI-weighted images, where the loss of the high signal intensity of normal bone marrow can be appreciated. Soft tissue extension is best seen on T2-weighted images, whereas tumor and muscle may have the same signal intensity on T1-weighted images. CT is better than MRI in demonstrating matrix mineralization but is less accurate in detecting skip lesions and bone marrow and soft tissue extension.

Decreased signal intensity of the nonmineralized mass on T2-weighted MR images is thought to represent fibrosis or sclerosis of the tumor. Persistent high signal intensity may be a result of either nonresponding tumor or necrotic tumor, reactive granulation tissue, or hemorrhage.

Administration of gadolinium cannot help distinguish viable tumor from reactive inflammation, since both enhance, but a lack of enhancement indicates tumor necrois.

CT findings of a positive response to treatment include:

  • marked decrease in size or complete disappearance of the soft tissue mass,
  • increased calcification of the mass,
  • improved delineation of the margins,
  • formation of a peripheral rim of calcification.

In the last two decades, the development of aggressive chemotherapy has significantly improved the survival of patients with osteosarcoma, and imaging studies are being used to evaluate the tumor's response to treatment.