Primary Bone Tumors
A boards-focused walk through primary bone tumors, sorted by the three clues that crack every vignette — patient age, location within the bone, and radiographic buzzwords — with osteosarcoma and Ewing sarcoma vignettes and the mandatory biopsy-before-treatment workup. Emphasizes imaging signs, genetics, RANKL/PGE2 mechanisms, and next-best-step decisions.
Framework: age + location + X-ray pattern
Primary bone tumors are named for the tissue they form — osteoid, cartilage, fibrous tissue, or 'small round blue cells' — and split into benign vs malignant. On Step 1, three clues solve almost every vignette:
- Age of the patient
- Location within the bone — epiphysis vs metaphysis vs diaphysis
- Radiographic pattern — margins, periosteal reaction, and matrix
Context matters: in an adult >40, an isolated bone lesion is far more often metastasis (breast, prostate, lung, kidney, thyroid) or multiple myeloma than a primary tumor. But boards love the classic primaries of childhood and young adulthood, most of which cluster around the knee (distal femur / proximal tibia) — the site of greatest bone turnover.
- Osteochondroma — most common benign tumor; metaphysis; bony stalk with a cartilage cap continuous with the medullary cavity. Rare transformation → chondrosarcoma.
- Osteoid osteoma — men <25; night pain relieved by NSAIDs/aspirin (nidus makes PGE2); small (<2 cm) lucent nidus with sclerotic rim in cortex.
- Osteoblastoma — like osteoid osteoma but >2 cm, in the vertebrae (posterior elements); larger, and pain is less reliably relieved by aspirin.
- Giant cell tumor (osteoclastoma) — 20–40 yrs; epiphysis around the knee; 'soap-bubble' lytic lesion; locally aggressive; multinucleated giant cells (neoplastic stromal cells express RANKL).
- Enchondroma — cartilage in small bones of hands/feet; Ollier disease (multiple) / Maffucci (+ hemangiomas).
- Osteoma — skull/facial bones; Gardner syndrome.
- Osteosarcoma — most common primary bone sarcoma. Bimodal: adolescents (growth spurt) and elderly (Paget disease, prior radiation, bone infarct). Metaphysis around the knee. Codman triangle + sunburst spiculation. Risk: Li-Fraumeni (TP53), hereditary retinoblastoma (RB1). ↑ALP.
- Ewing sarcoma — boys <15; diaphysis of long bones/pelvis; 'onion-skin' periosteal reaction; t(11;22) → EWSR1-FLI1; small round blue cells, CD99+, PAS+ (glycogen). May mimic osteomyelitis (fever, ↑ESR). Aggressive but chemo- and radiosensitive.
- Chondrosarcoma — adults >40; axial skeleton/pelvis, proximal femur/humerus; malignant cartilage with ring-and-arc calcification.
- Chordoma — notochord remnants; sacrum or clivus; physaliphorous cells.
Side-by-side comparison
| Tumor | Peak age | Site in bone | Classic imaging | Key association |
|---|---|---|---|---|
| Osteochondroma | 10–20s | Metaphysis | Exostosis, marrow-continuous cap | Most common benign |
| Giant cell tumor | 20–40 | Epiphysis (knee) | 'Soap-bubble' lytic | Locally aggressive; RANKL |
| Osteoid osteoma | <25, M | Cortex/diaphysis | Lucent nidus + sclerosis | Night pain, NSAID-relieved |
| Osteosarcoma | 10–20 & elderly | Metaphysis (knee) | Codman triangle, sunburst | TP53, RB1, Paget |
| Ewing sarcoma | <15, M | Diaphysis | 'Onion-skin' | t(11;22) EWSR1-FLI1 |
| Chondrosarcoma | >40 | Axial/pelvis | Lytic + arc calcification | Malignant cartilage |
Vignette: A 15-year-old boy has 6 weeks of worsening knee pain and a tender, firm mass over the distal femur. X-ray shows a destructive metaphyseal lesion with periosteal new bone lifted into a triangle at its margin and radiating 'sunburst' spicules.
- Diagnosis: Osteosarcoma — Codman triangle + sunburst, metaphysis around the knee.
- Next best step: MRI of the whole affected bone for local staging (marrow extent, skip lesions), then biopsy — must precede any treatment and be placed along the future resection tract; add CT chest for lung mets.
- Treatment: neoadjuvant chemotherapy → limb-sparing wide resection → adjuvant chemotherapy (osteosarcoma is radioresistant).
Vignette: An 11-year-old boy has 3 weeks of thigh pain with low-grade fever and leukocytosis; ESR is elevated. X-ray shows a diaphyseal femoral lesion with layered 'onion-skin' periosteal reaction. Biopsy: sheets of small round blue cells, CD99+, PAS+; cytogenetics show t(11;22).
- Diagnosis: Ewing sarcoma — systemic signs mimic osteomyelitis, but diaphyseal onion-skin + genetics seal it.
- Next best step: confirm with biopsy, stage (MRI, CT chest, marrow), then treat with multiagent chemotherapy + local control (surgery ± radiation) — highly chemo- and radiosensitive.
- Ewing 11 → 22: '11 + 11 = 22' recalls the t(11;22) translocation of Ewing sarcoma.
- Onion vs Sun: onion-skin = Ewing; sunburst + Codman triangle = osteosarcoma.
- Epiphysis = Giant: the epiphyseal primary tumor is the Giant cell tumor; osteosarcoma/osteochondroma are metaphyseal, Ewing is diaphyseal.
- Osteoid Osteoma → aspirin: night pain relieved by NSAIDs/aspirin (nidus PGE2).
- Plain X-ray first. Read aggressiveness: narrow zone of transition + sclerotic margin = benign; wide zone of transition, 'moth-eaten'/permeative bone, aggressive periosteal reaction, soft-tissue mass = malignant.
- MRI defines local/marrow extent; CT chest screens for pulmonary metastases (osteosarcoma, Ewing).
- Biopsy is mandatory before treating a suspected sarcoma and should be done at a sarcoma center along the planned resection axis — a poorly placed biopsy tract can forfeit limb salvage.
- Incidental osteochondroma: observe; excise if painful, growing after skeletal maturity, or cap >1.5–2 cm (chondrosarcoma risk).
- ↑ALP (osteosarcoma) and ↑LDH (Ewing) carry prognostic weight.
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