Causes

Most of the time, rather than starting in bones, cancer cells spread (metastasize) to the bones from other areas of cancer in the body. This is called secondary or metastatic bone cancer. This means that the cancer originated in a different place but has now migrated to the bones. For example, lung cancer commonly spreads from the lungs to the bones.
In general, no one knows for certain what causes most primary bone cancer. Adults with Paget's disease of bone, which involves abnormal development of new bone cells, may be at increased risk of osteosarcoma.
In a few cases, bone cancers may have a hereditary component, such as in:

Li-Fraumeni syndrome:- This condition is characterized by an increased risk of many different cancers, including osteosarcoma, leukemia, breast cancer, ovarian cancer and others.
Rothmund-Thomson syndrome:- This condition causes short stature, skeletal problems and rashes, and increases risk of bone cancer.
Hereditary retinoblastoma:- Children with this rare cancer of the eye have an increased risk of osteosarcoma.
Multiple exostoses:- Children with this inherited condition that causes cartilage bumps to form on your bones have an increased risk of chondrosarcoma.

Radiation is occasionally associated with bone cancer. Exposure to radiation from an X-ray won't harm you. But heavy doses of radiation, such as radiation therapy given for other cancers, can increase your risk of developing bone cancer, especially if you receive the therapy at a young age. Still, radiation therapy is becoming more and more sophisticated, which may lead to fewer of such side effects. For example, doctors today are better able to regulate doses of radiation and more precisely target the tumor being treated.