Osteosarcoma
Epidemiology
Most common primary malignant bone tumour of childhood/adolescence (in adults, metastases and myeloma are more common overall)
- Age: Bimodal peaks
- Primary: Adolescents (10–20 years)
- Secondary: Older adults with predisposing factors
- Slight male predominance
- Genetic association:
- Li-Fraumeni
- RB mutation
Common Locations
- Metaphysis of long bones
- Distal femur > proximal tibia > proximal humerus
- Secondary osteosarcoma: often axial skeleton (pelvis, spine) or diaphyseal; arises in Paget disease, irradiated bone, infarcts; usually older adults and worse prognosis.
Histology
- Malignant spindle cells producing immature osteoid
- Several subtypes:
- Conventional
- Intramedullary, high grade, classic around the knee.
- Subtypes: osteoblastic, chondroblastic, fibroblastic
- Telangiectatic
- purely lytic, expansile, fluid–fluid levels
- mimics ABC but is more aggressive, enhances nodular solid, and/or extensive soft tissue components.
- Small-cell (resembles Ewing sarcoma)
- Parosteal
- Low-grade, dense ossified mass stuck on posterior distal femur, radiolucent cleavage plane with cortex.
- Periosteal
- Intermediate grade, diaphyseal surface lesion with chondroid matrix and perpendicular spicules.
- Conventional
Imaging Features
Regarding conventional type.
X-ray / CT
- Aggressive, mixed lytic and sclerotic lesion
- Ill-defined margins
- Cloud-like osteoid matrix
- Aggressive periosteal reaction:
- Sunburst pattern
- Codman's triangle
- Cortical destruction and soft tissue mass common
- Skip lesions (intramedullary metastases in same bone) may be seen
MRI
- T1: Low signal
- T2: high signal tumour/necrosis and fluid, but with low-signal bands/areas corresponding to mineralised osteoid.
- Post-contrast: Heterogeneous enhancement
- Crucial for:
- Local staging
- Intraosseous extent
- Neurovascular involvement
- Soft tissue and joint invasion
Differential diagnosis
| Diagnosis | Key Differences |
|---|---|
| Ewing sarcoma | Diaphyseal or metadiaphyseal, permeative pattern, large soft-tissue mass, lamellated (“onion-skin”) periosteal reaction, usually no osteoid matrix. Systemic symptoms more common. |
| Osteomyelitis | Systemic symptoms, sequestrum, sinus tracts. However, imaging can be very similar (aggressive periosteal reaction + soft tissue swelling), so clinical picture and marrow signal pattern matter. |
Management
- Limb-sparing surgery preferred over amputation when possible
- Neoadjuvant chemotherapy → wide surgical resection → adjuvant chemotherapy
- Biopsy after imaging; biopsy tract must be placed so it can be completely excised at definitive surgery.
- Prognosis depends on:
- Response to chemo
- Presence of metastases (esp. lung)
- Tumour location and subtype
- Parosteal/periosteal types have better prognosis
Staging workup
- Local MRI of affected limb
- Whole bone imaging to evaluate skip lesions
- Chest CT for pulmonary metastases
- Bone scan or PET for metastatic workup
Exam Points
- Most common malignant bone tumour in children/adolescents
- Classic features: metaphyseal lesion with sunburst periosteal reaction and osteoid matrix
- Most common site of metastasis: lungs, often with calcified “cannonball” metastases.
- Telangiectatic osteosarcoma mimics ABC → always consider in aggressive lesions with fluid levels
