Pathology

Causes

The exact cause of OS is not known, but genetics may play an important role. Children and adults with other hereditary abnormalities, including exostoses (bony growths), retinoblastoma, Ollier's disease, osteogenesis imperfecta, polyostotic fibrous dysplasia, and Paget's disease, have an increased risk for developing osteosarcoma.

This form of cancer has also been linked to exposure to ionizing irradiation associated with radiation therapy for other types of cancer (i.e., Hodgkin's disease and non-Hodgkin's lymphoma).

The tumor is solid, hard, irregular ("fir-tree," "moth-eaten" or "sun-burst" appearance on X-ray examination) due to the tumor spicules of calcified bone radiating in right angles. These right angles form what is known as Codman's triangle (Image 9.12). Surrounding tissues are infiltrated.

Image 9.12 X-ray features of tumors: Codman's triangle

Note! Codman triangle (previously referred to as Codman's triangle) is the triangular area of new subperiosteal bone that is created when a lesion, often a tumour, raises the periosteum away from the bone. It is a type of periosteal reaction. The main causes for this sign are osteosarcoma, Ewing's sarcoma, etc.

Microscopically: The characteristic feature of osteosarcoma is presence of osteoid (bone formation) within the tumour. Tumor cells are very pleomorphic (anaplastic), some are giant, numerous atypical mitoses. These cells produce osteoid describing irregular trabeculae (amorphous, eosinophilic/pink) with or without central calcification (hematoxylinophilic/blue, granular) - tumor bone. Tumor cells are included in the osteoid matrix. Depending on the features of the tumour cells present (whether they resemble bone cells, cartilage cells or fibroblast cells), the tumour can be subclassified. Osteosarcomas may exhibit multinucleated osteoclast-like giant cells.