A 14-year-old girl from Delhi presents with a 4-month history of progressive pain and swelling of the left knee. Imaging shows a metaphyseal lesion with aggressive features. Biopsy confirms high-grade osteosarcoma. Which of the following is the most common histological type of osteosarcoma?

Explanation

## Histological Types of Osteosarcoma **Key Point:** Conventional (intramedullary) osteosarcoma is the most common type, accounting for ~90% of all osteosarcomas. It arises within the medullary cavity and is highly aggressive. ### Classification and Frequency | Type | Frequency | Age | Grade | Prognosis | Key Features | | --- | --- | --- | --- | --- | --- | | **Conventional (intramedullary)** | ~90% | 10–25 years | High | Poor (5-yr OS ~70%) | Arises in medullary cavity; metaphyseal; aggressive | | **Parosteal** | ~8–10% | 20–40 years | Low | Better (5-yr OS ~90%) | Surface lesion; slow-growing; lower grade | | **Periosteal** | ~1–2% | 15–30 years | Intermediate | Intermediate | Arises on bone surface; intermediate grade | | **Secondary** | <1% | >40 years | High | Poor | Arises in pre-existing lesion (Paget's, radiation) | **High-Yield:** Conventional osteosarcoma = 90% of cases = high-grade = poor prognosis. This is the "typical" osteosarcoma tested in NEET PG. ### Conventional Osteosarcoma 1. **Location:** Metaphysis of long bones (especially around the knee). 2. **Growth:** Rapid, intramedullary expansion. 3. **Grade:** High-grade malignancy (G3–G4). 4. **Age:** Peak incidence 10–25 years (adolescent growth spurt). 5. **Prognosis:** 5-year overall survival ~70% with chemotherapy + surgery (improved from ~10% with surgery alone). **Clinical Pearl:** When a question mentions "osteosarcoma" without further qualification, it refers to conventional osteosarcoma. Parosteal and periosteal variants are rare and usually specified in the stem. **Mnemonic:** **"COPP"** — Conventional (90%) > Other types (parosteal, periosteal, secondary) — helps recall the dominance of conventional type. ### Why Conventional is Most Common - Arises during the rapid growth phase of adolescence. - Occurs at sites of maximum bone turnover (metaphysis). - High-grade malignancy reflects aggressive biological behaviour. - Parosteal and periosteal variants are surface lesions and are much rarer.