A 28-year-old male presents with a 3-month history of progressive left knee pain and swelling. Plain radiographs show a mixed lytic-sclerotic lesion in the distal femur with cortical destruction and an ill-defined margin. There is no periosteal reaction. Which investigation is most appropriate to confirm the diagnosis and assess for metastases?

## Investigation of Choice for Suspected Osteosarcoma ### Clinical Context The imaging findings—mixed lytic-sclerotic lesion, cortical destruction, ill-defined margin, and absence of periosteal reaction—are highly suggestive of **osteosarcoma**, the most common primary malignant bone tumor in young adults. ### Why PET-CT with 18F-FDG is Correct **Key Point:** PET-CT with 18F-FDG is the investigation of choice for staging and assessing metastatic disease in osteosarcoma. 1. **High sensitivity and specificity** for detecting skeletal and pulmonary metastases 2. **Whole-body imaging** in a single study—critical because 10–20% of osteosarcoma patients have occult pulmonary or skeletal metastases at presentation 3. **Prognostic value**: SUV max correlates with tumor aggressiveness and response to neoadjuvant chemotherapy 4. **Guides treatment**: Presence of metastases alters staging (Stage IIB vs. Stage III) and influences chemotherapy intensity **High-Yield:** PET-CT is superior to bone scan (Tc-99m MDP) because: - Bone scan has lower specificity (osteoblastic activity is non-specific) - PET-CT detects pulmonary metastases better (bone scan does not image lungs effectively) - PET-CT is now standard of care in major oncology centers for osteosarcoma staging ### Role of Other Investigations | Investigation | Role | Limitation | |---|---|---| | **MRI of knee** | Local tumor extent, soft tissue involvement, surgical planning | Does NOT assess distant metastases | | **CT chest** | Detects pulmonary metastases | Misses skeletal metastases; requires separate imaging | | **Bone scan (Tc-99m MDP)** | Older modality for skeletal metastases | Lower sensitivity/specificity than PET-CT; does not image lungs | **Clinical Pearl:** In osteosarcoma, the lungs are the most common site of metastasis (80–90% of metastatic disease). PET-CT detects both pulmonary and skeletal metastases, making it superior to bone scan alone. ### Staging Workflow for Osteosarcoma ```mermaid flowchart TD A[Suspected osteosarcoma on plain X-ray]:::outcome --> B[MRI of primary site]:::action B --> C[Assess local extent & soft tissue involvement]:::outcome C --> D[PET-CT for staging]:::action D --> E{Metastases detected?}:::decision E -->|Yes| F[Stage IIB or III]:::outcome E -->|No| G[Stage IIA]:::outcome F --> H[Neoadjuvant chemo + surgery]:::action G --> H ``` **Tip:** Remember the sequence: **Plain X-ray → MRI (local) → PET-CT (staging)**. MRI is for surgical planning; PET-CT is for staging and metastasis detection.