primary bone tumor

**Definition and epidemiology** - Primary bone tumors originate in bone or its constituent tissues. Metastases to bone (breast, prostate, lung, kidney, thyroid) are far more common than primary tumors in adults. - Age is a major clue. Children and adolescents: osteosarcoma, Ewing sarcoma, osteoid osteoma. Adults over 40: multiple myeloma, chondrosarcoma. Ages 20-40: giant cell tumor.

**Benign tumors** - **Osteochondroma** is the most common benign bone tumor. It is a cartilage-capped bony outgrowth at the metaphysis of long bones, seen in males under 25. Malignant transformation to chondrosarcoma is rare. Multiple hereditary exostoses involve EXT1/EXT2 mutations. - **Osteoid osteoma** is a small (<2 cm) nidus in the cortex, usually of the femur or tibia. It causes night pain that is relieved by NSAIDs (the tumor makes prostaglandins). Imaging shows a radiolucent nidus with surrounding sclerosis. - **Osteoblastoma** is similar but larger (>2 cm), often involves the spine, and is not relieved by NSAIDs. - **Giant cell tumor** (osteoclastoma) occurs at the epiphysis of long bones (distal femur or proximal tibia) in ages 20-40. Imaging shows a "soap bubble" appearance. Histology shows multinucleated giant cells and stromal cells that express RANKL. Denosumab is a treatment option. - **Enchondroma** is a benign cartilage tumor in the small bones of the hands. Multiple enchondromas occur in Ollier disease, and with hemangiomas in Maffucci syndrome.

**Malignant tumors** - **Osteosarcoma** is the most common primary malignant bone tumor in children and adolescents (it is the most common overall if myeloma is excluded). It arises at the metaphysis of long bones, most often the distal femur or proximal tibia (around the knee). - Risk factors: Paget disease of bone in older adults, radiation, retinoblastoma (RB1 mutation), and Li-Fraumeni syndrome (TP53). - Imaging: Codman triangle (periosteal elevation) and "sunburst" pattern. - Histology: malignant osteoid-producing spindle cells. - Treatment: neoadjuvant chemotherapy, surgery, then adjuvant chemotherapy. The lungs are the main site of metastasis. - **Ewing sarcoma** is a small round blue cell tumor that occurs in the diaphysis of long bones in children and adolescents. - Genetics: t(11;22) EWS-FLI1. - Imaging: "onion skin" periosteal reaction. - Histology: Homer-Wright rosettes, PAS-positive glycogen, and CD99 positivity. - Presentation: pain, fever, and elevated ESR (can mimic osteomyelitis). - **Chondrosarcoma** is a malignant cartilage tumor in adults over 40. It involves the pelvis, proximal femur, and shoulder (axial and proximal sites). It responds poorly to chemotherapy and radiation, so surgery is the treatment. - **Multiple myeloma** is a plasma cell neoplasm. It causes lytic "punched-out" lesions, hypercalcemia, renal failure, anemia, and bone pain (CRAB). Workup shows an M spike on SPEP and Bence Jones proteins in urine. Smear shows rouleaux formation and histology shows "fried egg" plasma cells.

**Location pearls (high-yield)** - Epiphysis: giant cell tumor (and chondroblastoma). - Metaphysis: osteosarcoma, osteochondroma. - Diaphysis: Ewing sarcoma, fibrous dysplasia, and multiple myeloma (lesions can occur in marrow-rich bones).

**Radiographic clues** - Aggressive lesions have a wide zone of transition, cortical destruction, and periosteal reaction (Codman triangle, sunburst, or onion skin). - Benign lesions have a sharp, sclerotic margin and a narrow zone of transition.

**Workup** - Plain radiograph first, then MRI for local staging, then CT chest and bone scan or PET for metastases. - Biopsy comes after imaging, and should be planned with the surgeon so the tract can be excised later. - Alkaline phosphatase and LDH may be elevated (osteosarcoma and Ewing sarcoma).