A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the distal femur. On examination, there is a firm, tender mass over the distal femur with warmth and overlying skin erythema. The patient reports a history of a minor knee injury 2 months ago. Plain radiographs show a mixed lytic and sclerotic lesion at the distal femoral metaphysis with cortical destruction and a Codman triangle. Serum alkaline phosphatase is elevated at 280 U/L (normal: 30–120). 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 metaphysis of long bones during periods of rapid skeletal growth. ### Characteristic Features | Feature | Osteosarcoma | Ewing Sarcoma | GCT | Osteomyelitis | |---------|--------------|---------------|-----|---------------| | **Age** | 10–25 years (peak: 15–20) | 10–25 years | 20–40 years | Any age | | **Site** | Metaphysis (distal femur, proximal tibia, proximal humerus) | Diaphysis/metadiaphysis | Epiphysis (post-fusion) | Variable | | **Radiographic** | Mixed lytic/sclerotic, Codman triangle, sunburst, periosteal reaction | Permeative lytic, onion-skin periostitis | Eccentric, lytic, soap-bubble | Periosteal reaction, sequestra | | **Alkaline phosphatase** | Elevated (60–80% of cases) | Normal or mildly elevated | Normal | Normal | | **Pain duration** | Weeks to months | Weeks to months | Months to years | Acute onset | ### High-Yield Diagnostic Criteria **High-Yield:** The **distal femur** is the single most common site for osteosarcoma (40% of all cases), followed by proximal tibia (15–20%) and proximal humerus (10%). This patient's location is pathognomonic. **Key Point:** The **Codman triangle** (periosteal new bone formation at the edge of the lesion) and **sunburst pattern** (radiating spicules of new bone) are classic but not pathognomonic radiographic signs of osteosarcoma. **Clinical Pearl:** Elevated serum alkaline phosphatase in a young patient with a metaphyseal bone lesion is a strong clue to osteosarcoma, as the tumor actively produces osteoid and bone matrix. ### Pathophysiology ```mermaid flowchart TD A[Adolescent with rapid skeletal growth]:::outcome --> B[Malignant transformation of osteoblasts]:::action B --> C[Aggressive metaphyseal lesion]:::outcome C --> D{Radiographic pattern?}:::decision D -->|Mixed lytic/sclerotic + Codman| E[Osteosarcoma]:::action D -->|Permeative + onion-skin| F[Ewing sarcoma]:::action E --> G[Elevated ALP, LDH]:::outcome G --> H[MRI + Biopsy for confirmation]:::action ``` ### Why This Case Is Osteosarcoma 1. **Age:** 16 years—peak incidence for osteosarcoma. 2. **Site:** Distal femoral metaphysis—most common location. 3. **Radiographs:** Mixed lytic/sclerotic with Codman triangle—classic for osteosarcoma. 4. **Biochemistry:** Elevated alkaline phosphatase—reflects osteoid production by tumor cells. 5. **Timeline:** 3-month progressive course—typical for osteosarcoma. **Warning:** Do NOT confuse the preceding trauma (knee injury) as the cause. Osteosarcoma is NOT caused by trauma; trauma may simply draw attention to a pre-existing lesion. ### Next Steps in Management - **MRI of primary site** — assess soft tissue extension and marrow involvement. - **Chest CT** — detect pulmonary metastases (present in ~20% at diagnosis). - **Biopsy** — tissue diagnosis (open or core needle biopsy). - **Staging:** TNM or Enneking system. - **Treatment:** Neoadjuvant chemotherapy + wide surgical resection ± adjuvant chemotherapy. [cite:Robbins 10e Ch 27]