The skeleton is a common target for the spread of cancer (called metastasis). Cancer cells separate from an existing tumor—known as the primary cancer—and use the blood stream or lymphatic system as a highway to travel through the body. These cancerous cells can settle in different places and begin new tumors. A single tumor is referred to as a metastasis, while multiple tumors are referred to as metastases.

Breast, prostate, and lung cancer are common cancers that metastasize to the bones. Other cancers can also spread there, including thyroid and kidney cancers. Metastatic cancer takes the name of the original location, not the area to which it spreads. For instance, breast cancer that spreads to the bones would still be referred to as breast cancer. Cancer that forms in the cells of the bone is considered bone cancer. Metastatic cancer in the bone is more common than bone cancer itself.

Why Does Cancer Spread to Bone?

Bone metastases are most often found in the spine. Other common sites are the bones of the hips, skull, ribs, and the upper arm and legs. There are several reasons why some cancers target the skeleton. Some cancerous cells contain proteins that allow them to adhere to specific parts of the body, including the bones. In other cases, bones have characteristics that encourage some cancers to grow. If the primary cancer is already spreading to other organs, the chances of developing bone metastases are higher. There is also a higher risk of bone metastases when tumors are bigger and have found their way to the lymph nodes.

Symptoms of Bone Metastases

Cancerous cells can damage bones and make them weak and brittle or even dissolve them. There are a host of symptoms that come with the spread of cancer to the skeleton.

  • Pain. The most common symptom of bone metastases, pain can start out intermittently and ease with movement. As the metastases grow, the pain can become constant and increase during activity.
  • Fractures. Bones can also be weakened by the cancer, causing them to break. This can happen during normal activity, not just as the result of a fall. Bone metastases are often first discovered when a cancer patient breaks a bone performing an act that would generally not result in a broken bone.
  • Numbness. If the tumors are on the spine, they can cause pressure on the spinal cord. The first sign of this can be back pain, but numbness and weakness can also result.
  • Problems Urinating. Another symptom of spine metastases include problems urinating or fecal incontinence. Any of these signs in a person with known cancer is considered an emergency.
  • Paralysis. Pressure on the spinal cord can also cause paralysis. What parts of the body the paralysis strikes depends on the location of the pressure. The legs are the most common, but the arms and other parts of the body can also be affected.
  • Hypercalcemia. Metastatic cancer in the skeleton can cause calcium to leak into the bloodstream. A high level of calcium in the blood is called hypercalcemia, which can cause a variety of problems including constipation and dehydration. Weakness, loss of energy and confusion can also result from this condition. In serious cases, it can even cause coma or death.