A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the distal femur. He reports the pain is worse at night and does not respond well to NSAIDs. On examination, there is a firm, fixed mass over the distal femoral metaphysis with warmth and overlying skin erythema. Plain radiographs show a mixed lytic and sclerotic lesion with cortical destruction and a sunburst pattern of new bone formation. The lesion extends into the soft tissues with a Codman triangle visible. What is the most likely diagnosis?

Explanation

## Diagnosis: Osteosarcoma ### Clinical Presentation **Key Point:** Osteosarcoma is the most common primary malignant bone tumor in children and adolescents, typically arising in the metaphyseal regions of long bones during periods of rapid skeletal growth. ### Characteristic Features of This Case | Feature | Finding | Significance | |---------|---------|---------------| | Age | 16 years | Peak incidence 10–25 years; often during growth spurts | | Location | Distal femur | Most common site (around the knee in 50% of cases) | | Pain character | Night pain, NSAID-resistant | Suggests malignancy rather than mechanical pain | | Physical findings | Firm, fixed mass with warmth | Rapid growth and local inflammation | ### Radiological Hallmarks **High-Yield:** The classic radiological triad of osteosarcoma: 1. **Sunburst (radiating spicules)** — perpendicular new bone formation along blood vessels 2. **Codman triangle** — periosteal reaction with lifting of periosteum and new bone formation at the margin 3. **Mixed lytic and sclerotic lesion** — aggressive bone destruction with concurrent osteoid/bone production **Clinical Pearl:** The sunburst pattern and Codman triangle are not pathognomonic but are highly suggestive of osteosarcoma when seen in a young patient with a metaphyseal lesion around the knee. ### Pathological Basis **Key Point:** Osteosarcoma is a high-grade malignant tumor of mesenchymal origin that produces osteoid and bone. Histologically, it shows: - Pleomorphic spindle cells with high mitotic activity - Abnormal mitotic figures - Direct production of osteoid/bone by tumor cells (not just reactive bone) - Often associated with p53 mutations and Rb pathway alterations ### Metastatic Potential **Warning:** At presentation, ~20% of patients already have pulmonary metastases (the most common site). Staging with chest CT is mandatory. ### Management Approach ```mermaid flowchart TD A[Suspected Osteosarcoma]:::outcome --> B[Confirm with MRI + Biopsy]:::action B --> C[Staging: Chest CT, Bone scan/PET]:::action C --> D[Neoadjuvant Chemotherapy]:::action D --> E[Surgical Resection]:::action E --> F[Adjuvant Chemotherapy]:::action F --> G[5-year survival ~70% with treatment]:::outcome ``` **High-Yield:** Neoadjuvant chemotherapy followed by wide surgical resection and adjuvant chemotherapy is the standard of care. This multimodal approach has improved 5-year survival from <20% (surgery alone) to ~70%. ### Mnemonic: OSTEOSARCOMA Risk Factors **O** — Osteoblastic tumors; Overactive growth (adolescence) **S** — Skeletal dysplasias (Paget disease, hereditary retinoblastoma) **T** — Tall stature, rapid growth **E** — Extremities (distal femur, proximal tibia, proximal humerus) **O** — Often metaphyseal **S** — Second malignancy risk (post-radiation or chemotherapy) **A** — Around the knee (50% of cases) **R** — Rb and p53 mutations **C** — Codman triangle and sunburst on X-ray **O** — Osteoid production by tumor cells (histological hallmark) **M** — Metastases to lungs (20% at presentation) **A** — Aggressive, high-grade malignancy