Osteosarcoma
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Direct answer
Osteosarcoma, the commonest primary malignant bone tumour outside myeloma in young people, is a high-grade tumour that makes malignant osteoid, most often in the metaphysis near the knee. Its classic radiograph shows a mixed sclerotic and lytic lesion with a sunburst periosteal reaction and Codman triangle, and alkaline phosphatase is often raised. Treatment is neoadjuvant chemotherapy followed by limb-salvage resection, and spread to the lungs is the main threat.
What you must remember
- Age and site: the second decade of life, in the metaphysis of long bones, with the distal femur and proximal tibia the commonest sites.
- Histology: malignant cells that produce osteoid, which is the defining feature of the tumour.
- Radiograph: a mixed sclerotic and lytic lesion, with sunburst periosteal reaction and Codman triangle from aggressive periosteal lifting.
- Biochemistry: serum alkaline phosphatase is often raised, though a normal value does not exclude the diagnosis.
- Staging: chest CT for lung metastases, MRI of the whole bone for local extent and skip lesions, and a bone scan for distant bone metastases.
- Chemotherapy: a neoadjuvant regimen of high-dose methotrexate, doxorubicin and cisplatin, with response judged on the resected tumour.
- Surgery: limb-salvage resection with an endoprosthesis or allograft; rotationplasty is an option in selected young children with distal femoral tumours.
- Radioresistance: radiotherapy has little role and is reserved for palliation.
A treatment pathway from biopsy to surveillance
A child with a painful knee mass and a sunburst lesion on X-ray needs a planned sequence rather than a single operation. The order of steps decides both the chance of cure and the chance of keeping the limb.
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Biopsy first: A core or incisional biopsy confirms the diagnosis, and the biopsy tract is planned so that it can be removed with the tumour.
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Stage the disease: Chest CT looks for lung nodules, MRI defines the local extent and any skip lesions in the same bone, and a bone scan finds metastases in other bones.
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Give neoadjuvant chemotherapy: Several cycles of methotrexate, doxorubicin and cisplatin shrink the primary tumour and treat micrometastases before surgery.
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Resect with limb salvage: Wide resection with reconstruction using an endoprosthesis or allograft preserves the limb when the margins and neurovascular structures allow.
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Grade the response: The pathologist measures tumour necrosis in the resected specimen, and a good response of at least 90 percent necrosis guides the postoperative regimen.
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Follow up for years: Chest imaging continues regularly because lung relapse is the commonest failure, and late effects of treatment are monitored.
Where the exam tries to catch you
Most lost marks here come from getting the sequence wrong or comparing osteosarcoma with the wrong tumour.
- Trap: operating first and giving chemotherapy only if the margins are positive Correct: neoadjuvant chemotherapy comes before surgery, and the response to it shapes the postoperative regimen.
- Trap: choosing radiotherapy as the main treatment Correct: osteosarcoma is largely radioresistant, so surgery and chemotherapy carry the cure.
- Trap: believing a normal alkaline phosphatase excludes osteosarcoma Correct: the value is often raised but can be normal, so the diagnosis rests on imaging and biopsy.
Frequently asked questions
Which site is most common for osteosarcoma?
Osteosarcoma most commonly arises in the metaphysis of the distal femur, followed by the proximal tibia and proximal humerus. These sites reflect the high growth activity of the metaphysis during adolescence.
What does Codman triangle indicate in osteosarcoma?
Codman triangle is a triangular shadow formed where an aggressive tumour lifts the periosteum off the cortex. In osteosarcoma it signals rapid growth, although other aggressive lesions can produce it as well.
Why is neoadjuvant chemotherapy given before surgery in osteosarcoma?
Neoadjuvant chemotherapy is given before surgery in osteosarcoma to shrink the primary tumour and treat micrometastases early. It also lets the pathologist judge the response on the resected tumour, which guides further treatment.
How is osteosarcoma staged before treatment?
Osteosarcoma is staged with chest CT for lung metastases, MRI of the primary to define local extent and skip lesions, and a bone scan to find distant bone metastases. Some centres add PET-CT in selected cases.