Osteosarcomas are malignant bone forming tumors and the second most common primary bone tumor after multiple myeloma. They account for ~20% of all primary bone tumors 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 tumor 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% taking place 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 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 tumors typically occur in the metaphyseal regions of long bones, and have a striking predilection for the knee, with up to 60% occurring there
- secondary tumors, 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 the degree of differentiation, location within the bone, and histological variants 3.
These subtypes vary in imaging findings, demographics and biological behavior, and include:
- intramedullary: ~80%
- surface or juxtacortical: ~10-15%
- extraskeletal: ~5%
Macroscopically osteosarcomas are bulky tumors where a heterogeneous cut surface demonstrates areas of hemorrhage, fibrosis and cystic degeneration. Their extension within the medullary cavity is often much more extensive than the bulky part of the tumor 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 fibrocystic 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:
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
- tumor matrix ossification/calcification
- variable: reflects a combination of the amount of tumor 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 utilized in assisting biopsy and staging. CT adds little to plain radiography and MRI in the direct assessment of the tumor. 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 an essential tool to determine accurate local staging and assessment for limb-sparing resection, particularly for evaluation of intraosseous tumor extension and soft-tissue involvement. Evaluation of the growth plate is also essential as up to 75-88% of metaphyseal tumors do cross the growth plate into the epiphysis 4.
- soft tissue non-mineralised component: intermediate signal intensity
- mineralised/ossified components: low signal intensity
- peritumoral edema: intermediate signal intensity
- scattered regions of hemorrhage will have a variable signal (see aging blood on MRI)
- enhancement: solid components enhance
- soft tissue non-mineralised component: high signal intensity
- mineralised/ossified components: low signal intensity
- peritumoral edema: 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 tumor, followed by wide resection of the bone and insertion of an endoprosthesis. The outcome depends 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.
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 chance of seeding. A poorly planned track that crosses compartments can result in a more extensive resection, potentially with poor outcomes for the patient.
- 1. Greenspan A. Orthopedic imaging, a practical approach. Lippincott Williams & Wilkins. (2004) ISBN:0781750067. Read it at Google Books - Find it at Amazon
- 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
- 3. Kumar V, Abbas AK, Fausto N et-al. Robbins and Cotran pathologic basis of disease. W B Saunders Co. (2005) ISBN:0721601871. Read it at Google Books - Find it at Amazon
- 4. Murphey MD, Robbin MR, Mcrae GA et-al. The many faces of osteosarcoma. Radiographics. 17 (5): 1205-31. Radiographics (abstract) - Pubmed citation
- 5. Yarmish G, Klein MJ, Landa J et-al. Imaging characteristics of primary osteosarcoma: nonconventional subtypes. Radiographics. 2010;30 (6): 1653-72. doi:10.1148/rg.306105524 - Pubmed citation
- 6. Picci P, Sangiorgi L, Rougraff BT et-al. Relationship of chemotherapy-induced necrosis and surgical margins to local recurrence in osteosarcoma. J. Clin. Oncol. 1995;12 (12): 2699-705. Pubmed citation
Related Radiopaedia articles
The differential diagnosis for bone tumors is dependent on the age of the patient, with a very different set of differentials for the pediatric patient.
- bone-forming tumors
- cartilage-forming tumors
- chondromyxoid fibroma
- juxtacortical chondroma
- fibrous bone lesions
- bone marrow tumors
- other bone tumors or tumor-like lesions
- aneurysmal bone cyst
- benign fibrous histiocytoma
- giant cell tumor of bone
- Gorham massive osteolysis
- haemophilic pseudotumour
- intradiploic epidermoid cyst
- intraosseous lipoma
- musculoskeletal angiosarcoma
- musculoskeletal hemangiopericytoma
- primary intraosseous hemangioma
- post-traumatic cystic bone lesion
- simple bone cyst
- impending fracture risk