A 16-year-old boy presents with progressive pain and swelling around the knee for 3 months. Plain radiographs show a metaphyseal lesion of the distal femur with mixed lytic and sclerotic areas, cortical destruction, and a sunburst pattern. Which investigation is most appropriate to confirm the diagnosis of osteosarcoma?

## Investigation of Choice for Osteosarcoma Diagnosis ### Role of Core Needle Biopsy **Key Point:** Core needle biopsy with histopathology is the gold standard confirmatory investigation for osteosarcoma. It provides definitive tissue diagnosis by demonstrating malignant osteoid formation, which is the hallmark of osteosarcoma. **High-Yield:** Histopathology shows: - Malignant spindle cells (osteoblasts) - Osteoid and bone formation by neoplastic cells - High mitotic activity and necrosis - Absence of normal bone architecture ### Why Biopsy Over Imaging Alone? While plain radiographs and MRI are excellent for **localization and staging**, they cannot differentiate osteosarcoma from other aggressive bone lesions (Ewing's sarcoma, metastatic disease, infection). Only histopathology confirms malignancy and cell type. **Clinical Pearl:** Biopsy technique matters—core needle biopsy is preferred over fine needle aspiration (FNA) because it provides adequate tissue architecture for diagnosis. Open biopsy is avoided to prevent tumor seeding. ### Diagnostic Sequence in Osteosarcoma | Investigation | Purpose | Timing | |---|---|---| | Plain radiographs | Initial detection, pattern recognition | First | | MRI | Local staging, soft tissue involvement | Before biopsy | | **Core needle biopsy** | **Histological confirmation** | **Before treatment** | | CT chest | Metastatic screening | After diagnosis | | Serum markers (ALP, LDH) | Prognostic, not diagnostic | Baseline | **Warning:** Do not delay biopsy waiting for "perfect" imaging. Once clinical and radiological suspicion is high, proceed to biopsy for tissue confirmation. ### Other Investigations (Not Confirmatory) - **Serum ALP and LDH:** Elevated in ~90% of osteosarcoma but non-specific; useful for prognosis and monitoring, not diagnosis. - **CT chest:** Essential for staging (lung metastases in 20% at presentation) but comes AFTER diagnosis. - **MRI:** Excellent for local staging and soft tissue extent, but cannot confirm malignancy without tissue.