A 16-year-old boy presents with a 3-month history of progressive pain and swelling around the left knee. He reports the pain is worse at night and does not improve with NSAIDs. On examination, there is a firm, warm swelling over the distal femur with restricted knee flexion. Plain radiographs show a mixed lytic and sclerotic lesion at the metaphyseal region of the distal femur with cortical destruction and a Codman triangle. Serum alkaline phosphatase is elevated at 850 U/L. What is the most likely diagnosis?

Explanation

## Diagnosis: Osteosarcoma ### Clinical Presentation **Key Point:** The classic presentation of osteosarcoma is a teenager with pain and swelling around the knee (distal femur or proximal tibia accounts for ~50% of cases), unresponsive to conservative management. ### Radiological Features | Feature | Finding in This Case | |---------|----------------------| | Location | Metaphyseal region (distal femur) | | Pattern | Mixed lytic and sclerotic lesion | | Cortical involvement | Yes, with cortical destruction | | Codman triangle | Present (periosteal reaction) | | Soft tissue mass | Firm, warm swelling | **High-Yield:** Codman triangle is a periosteal reaction seen when tumor lifts the periosteum; it is NOT pathognomonic but highly suggestive of malignancy, especially in a teenager. ### Biochemical Marker **Clinical Pearl:** Elevated alkaline phosphatase (ALP) reflects osteoblastic activity and bone formation by tumor cells. ALP >500 U/L in a young patient with a metaphyseal bone lesion strongly suggests osteosarcoma. ### Age and Location **Mnemonic:** **OSTEOSARCOMA = Teenagers + Knee** — Peak incidence is 10–25 years; ~50% occur around the knee joint (distal femur or proximal tibia). This patient is exactly in the target demographic. ### Pathological Features - Highly aggressive, malignant tumor of bone-forming mesenchyme - Produces osteoid and bone (hence "osteo-sarcoma") - Rapid growth with early soft tissue and vascular invasion - High propensity for early pulmonary metastases (80% at presentation if untreated) ### Next Steps in Diagnosis 1. **MRI of the primary site** — for soft tissue extension and surgical planning 2. **Chest CT** — to exclude pulmonary metastases 3. **Biopsy** — open or core needle biopsy for histological confirmation 4. **Staging:** TNM staging and prognostic factors (tumor size, grade, response to neoadjuvant therapy) [cite:Robbins 10e Ch 26]