Conventional Osteosarcoma

Conventional osteosarcoma accounts for 75-85% of all osteosarcoma.' It has a predominantly metaphyseal origin and the most common site is the distal femur followed by the proximal tibia, proximal humerus and pelvis. The tumor may grow up the metaphysis in the medullary canal and extend through the epiphysis into the joint. Adolescents are most affected as the tumor usually occurs before the growth plate is closed.
The clinical picture is of rapidly progressing pain, swelling and fever. The tumor is often large at presentation. Alkaline phosphatase may be two to three times the normal value.
Conventional osteosarcoma may appear sclerotic, lytic or mixed on plain x-ray. Cortical destruction may lead to periosteal reaction or Codman's triangles. Extension outside the bone into the soft tissue is common. MRI delineates the soft tissue extension, cellular areas, and necrosis. Ligaments and tendons often form a scaffolding for the tumor to grow on.
Microscopically, osteoid production is essential for diagnosis. Conventional osteosarcoma is generally a grade 3 or 4 tumor with nuclear atypia, hyperchromasia and a high mitotic rate. There are three histological variants of conventional osteosarcoma: osteoblastic, chondroblastic, and fibroblastic. Osteoblastic osteosarcoma has abundant osteoid ranging from a lace like matrix to thickened trabecular bone. Chondroblastic osteosarcoma has cartilage production and the fibroblastic form has a spindle cell stroma with focal osteoid.
Treatment of osteosarcoma is complex. A biopsy must be performed for diagnosis. A chest CT scan and whole body bone scan are performed to rule out metastases. Preoperative chemotherapy is undertaken before resection. Post-operative chemotherapy completes the regime. Osteoblastic, chondroblastic and fibroblastic subtypes all have similar prognoses based on tumor size, extent and location. Metastases are a poor prognostic sign.

' Bloem, JL and HM Kroon, Osseous Lesions, Radiolooic Clinics of North America, 31(2):261-277, March 1993. Bullough, Peter, Orthopaedic Pathology (third edition), Times Mirror International Publishers Limited, London, 1997. Huvos, Andrew, Bone Tumors: Diagnosis. Treatment and Prognosis, W.B. Saunders, Co., 1991.

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