## Diagnostic Approach to Osteosarcoma **Key Point:** Tissue biopsy with histopathological examination is the gold standard and only definitive investigation for confirming osteosarcoma diagnosis. Histology demonstrates malignant osteoid production by neoplastic cells. ### Role of Each Investigation | Investigation | Purpose | Timing | |---|---|---| | **Tissue biopsy** | Definitive diagnosis via histology | After imaging, before treatment | | **MRI** | Local staging, soft tissue involvement | Before biopsy | | **CT chest** | Detect pulmonary metastases | After diagnosis confirmed | | **PET scan** | Assess metabolic activity, distant metastases | Supplementary, not primary diagnostic | **High-Yield:** While MRI is essential for local staging and surgical planning, it cannot diagnose osteosarcoma—only histology can. The classic histological finding is **malignant osteoid** (unmineralized bone matrix) produced directly by neoplastic cells, distinguishing it from reactive bone formation. **Clinical Pearl:** Biopsy should be performed by the orthopedic surgeon or interventional radiologist using a needle technique (core needle or open) that does not contaminate tissue planes, as biopsy tract contamination affects surgical margins and prognosis. **Mnemonic: BIOPSY FIRST** — Before Imaging, Osteosarcoma Proves Staging; Yet histology is definitive. ### Sequence of Investigations ```mermaid flowchart TD A[Clinical suspicion + Plain X-ray findings]:::outcome --> B[MRI of affected limb]:::action B --> C{Imaging consistent with malignancy?}:::decision C -->|Yes| D[Tissue biopsy]:::action D --> E[Histopathology confirms osteosarcoma]:::outcome E --> F[CT chest + staging]:::action F --> G[Treatment planning]:::action ``` **Warning:** Do NOT proceed to CT chest or PET for staging before histological confirmation. Premature staging investigations waste time and resources if diagnosis is not confirmed. 
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