When cells break away from a cancerous tumor (a primary tumor) they can travel to other parts of the body through the bloodstream or the lymph vessels. Moving through the bloodstream or lymphatic system, cancer cells can lodge in an organ at a distant location and establish a new (secondary) tumor. Different types of tumors seem to prefer to spread to particular sites in the body, such as the liver or the skeleton. Many cancer types noted above commonly spread to the bone.

Secondary tumors that have spread to bone (bone mets) through the process known as bone metastasis are not the same as primary bone cancer that starts in the bone (sarcoma). A tumor that has metastasized to bone is made up of abnormal cancer cells from the original tumor site and not of bone cells. For example, lung cancer that spreads to the bone consists of lung cancer cells. In this case, bone metastasis would be called metastatic lung cancer.

When cancer cells spread to the bone, they commonly lodge in the spine, rib cage, pelvis, limbs, and skull.

Cancer cells that spread to bone can cause damage in two ways:

• The tumor may wear away bits of bone, creating small holes called osteolytic lesions. This process can make bones fragile and weak so that they break or fracture easily.
• The tumor may stimulate bone to form and build up abnormally. These areas of new bone are called osteoblastic or osteosclerotic lesions. They are weak and unstable and may break or collapse. They also can be painful.

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