Cancer cells can travel from the original tumor (primary tumor) to other parts of the body along the blood or lymphatic system. Very often, cancer cells are destroyed by the lymphatic system before they can cause any damage to the body. However, some cancer cells not only survive, but also spread to other places or tissues in the body and continue to grow and form new tumors (i.e. secondary tumors), a process called cancer metastasis. This process is called cancer metastasis. If cancer cells metastasize to the bone and become a new tumor, it is considered cancer bone metastasis. The cancer cells in cancer bone metastasis are not bone cancer cells, but are made up of cancer cells from the primary tumor (e.g. cancer cells from breast, lung or prostate cancer). Which cancers are prone to bone metastasis? Bone metastases are common in patients with breast, lung and prostate cancers. Other types of cancers such as kidney, pancreatic, rectal, stomach, thyroid and ovarian cancers may also cause bone metastases. About half of the patients with these cancers will develop metastases. Where do bone metastases usually affect the bones? The spine is the most common site of cancer bone metastases, however, other locations such as the pelvis, hip, thigh and skull can also be affected by cancer bone metastases. How does cancer metastasis damage bone? To understand how cancer metastasis affects bone, it is important to understand the structure and role of normal bone. The outermost layer of normal bone is the bone cortex, which is very hard and supports the entire skeleton of the body. In the middle of the bone is a layer of soft spongy tissue called bone marrow. The bone marrow is the living cell tissue responsible for the production and storage of red blood cells, white blood cells and platelets. Red blood cells carry blood from the lungs to the body; white blood cells fight infection; and platelets clot blood and control bleeding. Normal bones also store and release many important minerals such as calcium, magnesium, phosphorus and sodium to keep the body functioning properly. Bone metastases can damage bone in three ways: 1) through abnormal bone erosion, a process known as resorption, which creates small holes in the bone, resulting in osteolytic lesions. 2) by triggering abnormal bone growth. 2. triggering abnormal bone growth, which results in new bone growing elsewhere, leading to osteosclerotic lesions. 3. Cancer metastases can destroy bone, making it weak and prone to breakage (fractures). Cancer bone metastases can affect any of the important functions of the bones and are also the most common cause of pain in cancer patients.