A 14-year-old girl with biopsy-confirmed osteosarcoma of the proximal humerus is being staged for treatment. After MRI of the affected limb, which is the most appropriate next investigation to assess for metastatic disease?

Explanation

## Staging Investigations in Osteosarcoma **Key Point:** High-resolution CT of the chest is the investigation of choice for detecting pulmonary metastases in osteosarcoma, as the lungs are the most common site of distant spread (present in 10–20% at diagnosis). ### Metastatic Pattern in Osteosarcoma **High-Yield:** Osteosarcoma has a strong predilection for **early pulmonary metastases**. Approximately 80% of patients have micrometastases at diagnosis, even if chest imaging appears normal. This is why aggressive chemotherapy is given even in apparently localized disease. ### Staging Investigations Hierarchy | Investigation | Sensitivity for Lung Mets | Specificity | Clinical Role | |---|---|---|---| | **CT chest (high-resolution)** | 90–95% | High | Gold standard for pulmonary staging | | **Bone scan (Tc-99m)** | 70–80% | Moderate | Skeletal metastases (less common) | | **PET-FDG** | 85–90% | High | Metabolic activity; supplementary | | **Skeletal survey** | <50% | Low | Outdated for staging | **Clinical Pearl:** High-resolution CT detects nodules as small as 2–3 mm, which is critical because even microscopic lung disease affects prognosis and chemotherapy planning. Standard chest X-ray misses 30–40% of pulmonary metastases. **Mnemonic: CHEST CT FIRST** — Confirms Early Spread, Tiny nodules detected, Critical for Treatment planning. ### Staging Sequence After Diagnosis ```mermaid flowchart TD A[Histologically confirmed osteosarcoma]:::outcome --> B[MRI of primary site]:::action B --> C[High-resolution CT chest]:::action C --> D{Pulmonary metastases present?}:::decision D -->|Yes| E[Stage IV disease]:::outcome D -->|No| F[Assess for skeletal metastases]:::action F --> G[Bone scan or PET if clinically indicated]:::action G --> H[Complete staging + treatment planning]:::outcome ``` **Warning:** Do NOT use skeletal survey or standard chest X-ray for staging in osteosarcoma. These have poor sensitivity and will miss critical metastatic disease that affects treatment intensity and prognosis.