The spread of cancer from its site of origin to another location in the body is called metastasis. Cancer cells can spread, or metastasize, through the blood and lymph systems. Bone metastases usually occur by way of the bloodstream. A cancer cell may break away from the original location in the body and travel in the circulatory system until it gets lodged in a small capillary network in bone tissue. Cancer may also spread to bone by erosion from the adjacent cancer, though this occurs less frequently than spread by the bloodstream.
Bone is one of the most common locations in the body to which cancer metastasizes. The major cancer types that tend to metastasize to bone include multiple myeloma, breast, prostate, lung, kidney, and thyroid cancers. Bone metastases may cause pain, may make the bones more susceptible to fractures, and may cause increased levels of calcium in the blood.
Bone metastases result in lesions or injury to the bone tissue. There are two types of lesions: lytic lesions, which destroy bone material; and blastic lesions, which fill the bone with extra cells. Normal bone is constantly being remodeled, or broken down and rebuilt. Cancer cells that have spread to the bone disrupt the balance between the activity of osteoclasts (cells that break down bone) and osteoblasts (cells that build bone).
Bone metastases generally occur in the central parts of the skeleton, although they may be found anywhere in the skeletal system. Common sites for bone metastases include the back, pelvis, upper leg, ribs, upper arm, and skull. More than 90% of all metastases are found in these locations.
Complications Associated with Bone Metastases
Complications that are associated with bone metastases include pain, bone loss, hypercalcemia, and decreased blood cell production.
Pain: A common complication, and often the initial symptom of bone metastases, is bone pain. The thick membrane that covers each bone, called the periosteum, has many nerves, making it a highly sensitive tissue. Damage or pressure to this tissue caused by bone metastases may result in a great deal of pain. Bone pain can be debilitating to the point that it compromises a patient’s ability to manage normal day-to-day activities. Bone pain due to metastases may be hard to differentiate from ordinary low back pain or arthritis. The most notable difference is that pain due to bone metastasis is typically more constant, even at night.
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Bone loss: Bone loss occurs when there is decreased calcification or reduced density of the bones. The result is weak bones that are at increased risk of fracture. Bone loss in the leg or hip bones can be a serious condition because these are major weight-bearing bones and fractures in these areas are often debilitating.
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Hypercalcemia: An increased level of calcium in the bloodstream is called hypercalcemia. This disorder results from the destruction of bone associated with metastases. Hypercalcemia can be a life-threatening condition.
Symptoms of hypercalcemia may include:
|Excessive thirst||Extreme muscle weakness|
|Dry mouth or throat||Irregular heart beat|
These many signs and symptoms make it difficult for doctors to correctly diagnose hypercalcemia. Consequently, they are commonly attributed to either the cancer treatment or the malignancy itself. This disorder can be severe and difficult to manage. Severe hypercalcemia is a medical emergency requiring immediate treatment.
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Decreased blood cell production: Bone metastases can also cause a decrease in blood cell production. The multiplication of cancer cells in the bone marrow eventually crowds out and suppresses the normal production of blood cells. This may cause a significant decrease in red blood cells, platelets, and white blood cells, which can cause anemia, abnormal bleeding, and neutropenia, respectively. Anemia decreases the body’s ability to transport oxygen to the body’s organs. Anemia may cause patients to experience tiredness, fatigue, shortness of breath, and/or a reduced tolerance to activity. Neutropenia, or the depletion of white blood cells, compromises the immune system and makes patients more susceptible to infection.