A 14-year-old girl from Mumbai presents with a 2-month history of pain and a firm, warm swelling over the anterior aspect of her knee. Plain radiographs reveal a metaphyseal lesion with a 'sunburst' pattern and Codman's triangle. What is the most common histological type of osteosarcoma?

Explanation

## Histological Classification of Osteosarcoma **Key Point:** Osteosarcoma is classified into several subtypes based on location and behaviour. The conventional (intramedullary) type is by far the most common, accounting for >90% of all cases. ### Osteosarcoma Subtypes | Subtype | Frequency | Location | Prognosis | Peak Age | |---------|-----------|----------|-----------|----------| | **Conventional (intramedullary)** | >90% | Medullary cavity, metaphysis | Poor (5-yr survival ~60–70%) | 10–25 years | | **Parosteal** | 5–10% | Surface of bone, lateral femur | Better (5-yr survival ~80–90%) | 20–40 years | | **Periosteal** | 1–2% | Periosteal surface | Intermediate | 15–30 years | | **Extraskeletal** | <1% | Soft tissues, no bone origin | Variable | 20–60 years | ### Conventional Osteosarcoma: Key Features **High-Yield:** Conventional osteosarcoma is: - **Intramedullary** (within the bone marrow cavity) - Arises in the **metaphyseal region** of long bones - Highly aggressive with rapid local growth and early metastasis - Presents with classic radiological signs: **sunburst pattern** and **Codman's triangle** - Peak incidence: adolescents during growth spurts (10–25 years) **Mnemonic:** **COIN** — **C**onventional **O**steosarcoma **I**ntramedullary **N**eoplasm — the most common type, occurring within the bone. ### Why Conventional is Most Common 1. **Arises during rapid bone growth** — adolescence is the peak risk period 2. **Metaphyseal origin** — fastest osteoblastic activity 3. **Aggressive biology** — high mitotic rate and early vascular invasion 4. **No protective periosteal barrier** — intramedullary location allows rapid spread ### Distinguishing Features of Other Subtypes **Parosteal Osteosarcoma:** - Arises on the **surface** of bone (lateral femur is classic) - Older age group (20–40 years) - **Lower grade** histologically - Better prognosis (80–90% 5-year survival) - Slower growth, often diagnosed late **Periosteal Osteosarcoma:** - Arises from **periosteal surface** - Intermediate grade and prognosis - Rare (1–2% of cases) **Extraskeletal Osteosarcoma:** - Arises in **soft tissues** without bone origin - Extremely rare (<1%) - Older age group **Clinical Pearl:** When a teenager presents with a metaphyseal bone tumour around the knee with sunburst pattern and Codman's triangle, assume conventional osteosarcoma until proven otherwise. The radiological signs are pathognomonic for the aggressive intramedullary variant.