## Histological Subtypes of Osteosarcoma **Key Point:** Conventional (intramedullary) osteosarcoma is the most common subtype, accounting for ~75% of all osteosarcomas. It arises within the medullary cavity of the bone. ### Classification of Osteosarcoma Subtypes | Subtype | Frequency | Key Features | Prognosis | | --- | --- | --- | --- | | **Conventional (intramedullary)** | ~75% | Arises in medullary cavity; mixed spindle cell, osteoid, and bone formation | Standard | | **Chondroblastic** | ~10% | Prominent cartilage component; may mimic chondrosarcoma | Similar to conventional | | **Fibroblastic** | ~10% | Prominent fibrous stroma; may mimic fibrosarcoma | Similar to conventional | | **Telangiectatic** | ~3-5% | Cystic/hemorrhagic appearance; may mimic aneurysmal bone cyst | Slightly worse | | **Small cell** | <2% | Small round cells; may mimic Ewing sarcoma | Worse | | **Surface (parosteal/periosteal)** | ~5-10% | Arises on bone surface; lower grade | Better than conventional | **High-Yield:** The distinction between conventional osteosarcoma (intramedullary, high-grade) and surface osteosarcomas (parosteal/periosteal, lower-grade) is clinically important for prognosis and treatment planning. **Mnemonic:** **CCFTS** = Chondroblastic, Conventional (most common), Fibroblastic, Telangiectatic, Small cell — in rough order of frequency. **Clinical Pearl:** Conventional osteosarcoma is the "default" type when no modifier is specified; it is highly aggressive and requires chemotherapy + surgery. 
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