|Lara Bonner Millar, MD|
|The Abramson Cancer Center of the University of Pennsylvania|
Bone metastases, also called "bone mets," are deposits of cancers within the bone, which occurs when a cancer spreads from somewhere else in the body. This is different than a primary bone tumor, which starts in the bone. Bone metastases develop in many people with cancer at some point in the course of their disease. Some cancers that are more likely to spread to the bone include breast, prostate, and lung cancer. Let's use an example to better understand this latter concept: a lung cancer is first formed in the lung tissue, but tumor cells can break off from the original mass and travel through the bloodstream or lymph system to other areas of the body, including the bone. This spreading of the tumor is known as "metastasis". When a lung cancer metastasizes to the bone, this "bone cancer" is actually lung cancer cells.
It sounds odd, but if the pathologist took a biopsy of the bone lesion and looked at it under a microscope, it would look like lung cancer cells. It is important to understand the difference between primary bone tumors and bone metastases because they are treated differently.
Cancers that originate in the bone are called primary bone cancers and are much less common than bone metastases. Certain types of primary bone cancers include osteosarcoma, Ewing’s sarcoma, and multiple myeloma, and they are addressed elsewhere on OncoLink. Primary bone cancers may be curable when identified early, whereas the presence of bone metastases indicates that the cancer is not curable, though that does not mean it is not treatable.
How Are Bone Metastases Detected?
When a patient with cancer complains of pain in the bone(s), the healthcare team needs to figure out whether or not it is due to a bone metastasis. Metastases can occur in any bone in the body, but are most often found in bones near the center of the body. The spine is the most common site of bone metastasis. Other common sites are the pelvis (hip), upper leg bone (femur), upper arm bone (humerus), ribs, and the skull. Imaging studies such as bone scan, x-rays, CT scan, or MRI, may be used, depending on the particular situation. Besides pain, bone metastases can weaken bones and even result in fractures. Fractures that are the result of bone metastases are called "pathologic" fractures. Some bone metastases result in calcium leaching from the bones into the bloodstream, and can cause high calcium levels in the blood.
How Are Bone Metastases Treated?
Once a bone metastasis is identified, there are multiple potential treatment options. Your healthcare team will discuss with you which treatment is most appropriate for your situation. Treating bone metastases is important because treatment can improve symptoms and quality of life. Treatment options depend of the type of cancer, location and extent of the metastases, and the patient’s overall health. Most doctors believe the most important treatment for bone metastases is treatment directed against the primary cancer (the original cancer). This is usually done with systemic therapies. Systemic therapies enter the bloodstream and can reach cancer cells that have spread throughout the body. Systemic therapies include chemotherapy or hormone therapies that are taken by mouth or injected. There are also drugs called bisphosphonates that can help make diseased bones stronger and help prevent fractures. Bisphosphonates may be used along with the chemotherapy or hormone therapy for bone metastasis. These treatments are discussed in more detail below.
Another form of systemic treatment is radionuclide therapy. A radionuclide is an element that is radioactive. These drugs are injected into a vein and concentrate in the areas of bone that contain cancer. The radiation they give off kills the cancer cells and alleviates pain. The treatment is given over one day. Samarium and strontium are the most used radionuclides for bone metastases. They are more effective for "blastic" metastases, those that stimulate bone growth; prostate cancer metastases are often blastic. Most metastases from other types of cancers are "lytic" meaning they break down bone, and would not be treated with radionuclides. This treatment can initially cause pain flare before the pain gets better and is not used in people with low blood counts.
Certain treatments are directed at the metastases itself. This includes radiation therapy. Radiation is frequently offered when patients have bony pain from a metastasis that is not relieved with pain medication. Radiation is typically delivered five days a week over a period of 1-2 weeks. The goal of the treatment is to reduce pain. About 80-90% of patients will experience at least partial improvements in their pain; however sometimes there is a delay of weeks after treatment before maximum pain relief is achieved.
If a weight-bearing bone has become weakened by the metastasis to the point where it may fracture, an orthopedic surgeon may place a rod within that bone in order to stabilize it and prevent a fracture from forming.
Metastases to the spine may be treated with radiation, but sometimes if there is a fracture in the spine, a procedure called kyphoplasty or vertebroplasty may be needed. This involves injection of cement to stabilize the bone. In situations where cancer in the spine grows in such a way that is puts pressure on the spinal cord, it is called a "cord compression." Symptoms include pain, muscle weakness, sensory loss, or incontinence. Typically patients will be treated with either surgery followed by radiation, or radiation alone. If left untreated, spinal cord compression can result in spinal cord injury or paralysis. For this reason, cord compression is considered an oncologic emergency and new or worsening back pain should be evaluated promptly.
Bisphophonates are a type of medications that are effective in preventing fractures from metastases. This class of drug is commonly prescribed as treatment for osteoporosis (a form of bone thinning not related to cancer). Bisphosphonates can help bones affected by cancer by allowing them hold onto calcium, which makes them stronger. This slows down bone damage caused by the cancer, reduces high blood calcium levels (hypercalcemia), and decreases the risk of fracture. Bisphosphonates are more effective for lytic than blastic metastases. They have been studied in many cancers including breast, prostate and lung cancer. The bisphosphonates used as treatment for bone metastasis are given intravenously, usually every 3 to 4 weeks. The most commonly used drugs are zoledronate (Zometa®) and pamidronate (Aredia®). Possible side effects include tiredness, nausea, vomiting, lack of appetite, low blood calcium levels and a rare side effect called osteonecrosis of the jaw (ONJ). Because of the success in using bisphosphonates in metastatic breast cancer, researchers are also interested in determining whether bisphosphonates have a role in breast cancer prevention.
A newer medication, called denosumab (Xgeva®), is also being used to prevent further bone damage from cancer cells. This medication works a little differently – it is a type of targeted therapy (monoclonal antibody) and works by targeting a specific protein that is necessary for the bone destruction to occur. By inhibiting this protein, called RANKL, denosumab inhibits the breakdown of bone and, in turn, reduces the chance of developing a fracture in the affected bone. This medication has side effects similar to the bisphosphonates, though it has a higher likelihood of causing low blood calcium, so patients are asked to take calcium and vitamin D supplements while on this treatment.
This article is intended to be an introduction to bone metastasis and potential treatments. To learn more, visit some of the links below.
References & Further Reading