A 28-year-old male presents with a 3-month history of progressive left knee pain and swelling. Plain radiographs show a mixed lytic and sclerotic lesion in the distal femur with cortical thickening and periosteal reaction. There is no soft tissue mass. Which investigation is most appropriate to confirm the diagnosis and assess local staging?

## Investigation of Choice for Bone Tumor Local Staging **Key Point:** MRI is the gold standard for local staging of bone tumors because it provides superior soft tissue contrast, shows intramedullary extent, assesses neurovascular involvement, and detects skip lesions. ### Why MRI is Superior for Bone Tumors | Aspect | MRI | CT | Bone Scan | PET-CT | |--------|-----|----|-----------|---------| | **Soft tissue extension** | Excellent | Good | Poor | Moderate | | **Intramedullary extent** | Excellent | Fair | Poor | Poor | | **Cortical breakthrough** | Excellent | Good | Poor | Poor | | **Neurovascular involvement** | Excellent | Fair | Poor | Fair | | **Skip lesions** | Can detect | Limited | Poor | Limited | | **Staging purpose** | Local staging | Local staging | Systemic | Systemic | **High-Yield:** The imaging hierarchy for bone tumors is: 1. **Plain radiographs** → Initial diagnosis and characterization 2. **MRI** → Local staging (soft tissue, marrow, neurovascular) 3. **CT chest/abdomen** → Metastatic staging 4. **PET-CT** → Systemic staging and prognostic assessment **Clinical Pearl:** MRI with gadolinium contrast is essential because: - Gadolinium enhancement helps differentiate tumor from edema - Shows vascularity and necrotic areas - Improves detection of soft tissue component - Guides surgical planning regarding margins and neurovascular structures **Mnemonic: MRIS** — MRI for local staging, Radiographs for initial diagnosis, Imaging (CT) for metastases, Staging (PET) for prognosis. ### Role of Other Investigations - **CT chest/abdomen:** Used AFTER local staging is confirmed; assesses for pulmonary and visceral metastases (especially important in osteosarcoma and Ewing sarcoma) - **PET-CT:** Useful for detecting skeletal and soft tissue metastases; prognostic tool; not for local staging - **Bone scan:** Outdated for primary tumor assessment; may show increased uptake but lacks anatomic detail [cite:Robbins 10e Ch 26]