## Pulmonary Staging in Osteosarcoma ### Why High-Resolution CT Chest Is the Gold Standard **Key Point:** High-resolution CT (HRCT) chest is the most sensitive investigation for detecting pulmonary metastases in osteosarcoma. It can identify lesions as small as 2–3 mm, which is critical because 15–20% of patients have occult lung metastases at diagnosis. **High-Yield:** Pulmonary metastases are the most common site of distant spread in osteosarcoma. HRCT chest must be performed in all newly diagnosed cases to: 1. Detect occult metastases (changes prognosis and chemotherapy intensity). 2. Guide neoadjuvant chemotherapy decisions. 3. Plan surgical resection of primary tumor and any resectable lung lesions. ### Sensitivity Comparison for Pulmonary Metastases | Investigation | Sensitivity for Lung Mets | Best Use | |---|---|---| | **HRCT chest** | 90–95% (gold standard) | Initial staging, small nodules | | **Chest X-ray** | 50–60% | Gross metastases only | | **PET-CT** | 70–80% | Metabolically active lesions; may miss small nodules | | **Bone scan** | Not useful for lungs | Skeletal metastases only | **Clinical Pearl:** A patient with osteosarcoma and no pulmonary metastases on HRCT has a 5-year survival of ~60–70% with chemotherapy and surgery. Those with pulmonary metastases at diagnosis have <20% survival, making early detection critical. ### Timing and Sequence of Investigations ```mermaid flowchart TD A[Osteosarcoma suspected on plain film]:::outcome --> B[MRI of primary site]:::action B --> C{Local extent defined?}:::decision C -->|Yes| D[HRCT chest for staging]:::action D --> E{Pulmonary mets?}:::decision E -->|No| F[Proceed to chemotherapy]:::action E -->|Yes| G[Assess resectability]:::decision G --> H[Neoadjuvant chemo + surgical planning]:::action ``` **Mnemonic:** **CHEST** — **C**omputed tomography, **H**igh-resolution, **E**ssential for **S**taging, **T**iming before chemo. [cite:Harrison 21e Ch 297] 
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