Understanding Bone Metastases—When Cancer Spreads to the Bones
Cancer that has developed in one place can spread and invade other parts of the body. The process of spreading is called metastasizing. If a tumor spreads to the bone, it is called bone metastasis.
Cancer cells that have metastasized to the bone can damage the bone and cause symptoms. Various treatments are available to control the symptoms and the spread of bone metastases. To better comprehend what happens in metastasis, it helps to understand the anatomy of the bones.
Bone is a type of connective tissue made up of minerals, such as calcium and phosphate, and a type of protein called collagen. The outer layer of bone is called the cortex. The spongy center of bone is called bone marrow.
Bone is alive and continually repairs and renews itself through a process called remodeling. Two kinds of cells contribute to this process:
Osteoblasts are bone-forming cells.
Osteoclasts are cells that break down, or reabsorb, bone.
Below are some of the functions bones perform:
The skeleton provides structural support.
Bones store and release, as needed, minerals that the body needs to function, such as calcium.
Bone marrow produces and stores blood cells. These include red blood cells, white blood cells, and platelets. Red blood cells transport oxygen from the lungs to the rest of the body. White blood cells fight infections. Platelets help the blood clot.
When cancer cells invade the bone, any or all of the functions of the bone may be affected.
How cancer spreads to the bone
When cells break away from a cancerous tumor, they can travel through the bloodstream or lymph vessels to other parts of the body. Cancer cells can lodge in an organ at a distant location and establish a new tumor. The original tumor that cells break away from is called the primary tumor. The new tumor that the traveling cells create is called the secondary tumor. Secondary tumors in the bone are called bone metastases.
Different types of tumors seem to prefer to spread to particular sites in the body. For example, many types of cancer commonly spread to the bone. The bone is a common site of metastasis for these cancers:
Bone metastases are not the same as cancer that starts in the bone. Cancer that starts in the bone is called primary bone cancer. There are different types of primary bone cancers, such as osteosarcoma. A tumor that has metastasized to bone is not made of bone cells. Bone metastases are made up of abnormal cancer cells that arise from the original tumor site. For example, lung cancer that spreads to the bone is made of lung cancer cells. In this case, bone metastasis would be called metastatic lung cancer.
Cancer cells that spread to the bone commonly lodge in these places:
Cancer cells that spread to bone are harmful in two ways:
The tumor may eat away areas of bone, creating holes called osteolytic lesions. This process can make bones fragile and weak, so that they break or fracture easily. These areas may be painful.
The tumor may stimulate bone to form and build up abnormally. These areas of new bone are called osteosclerotic or osteoblastic lesions. They are weak and unstable and may break or collapse. They can also be painful.
Symptoms of bone metastases
Bone metastases can cause the following symptoms:
Bone pain. Pain is the most common symptom of bone metastasis. It's usually the first symptom you notice. At first, the pain may come and go. It tends to be worse at night or with bed rest. Eventually, the pain may increase and become severe. Not all pain indicates metastasis. Your doctor can help distinguish between pain from metastasis and aches and pains from other sources.
Broken bones. Bone metastasis can weaken bones, putting them at risk for breaking. In some cases, a fracture is the first sign of bone metastasis. The long bones of the arms and legs and the bones of the spine are the most common sites of fracture. A sudden pain in the middle of the back may indicate a cancerous bone breaking and collapsing.
Numbness or weakness in the legs, trouble urinating or having a bowel movement, or numbness in the abdomen. These are all signs that the spinal cord may be compressed. When cancer metastasizes to the spine, it can squeeze the spinal cord. The pressure on the spinal cord may cause these symptoms, as well as back pain. If you have these symptoms, you should tell a doctor or nurse right away. If untreated, they can cause paralysis.
Loss of appetite, nausea, thirst, constipation, tiredness, or confusion. These are all signs that there may be high levels of calcium in the blood. Bone metastases can cause a release of calcium into the bloodstream. This condition is called hypercalcemia. If you have these symptoms, you should tell a doctor or nurse right away. If untreated, they may cause a coma.
Other symptoms. If bone metastasis affects your bone marrow, you may have other symptoms related to decreased blood cell counts. Your red blood cell levels may drop, causing anemia. Signs of anemia are tiredness, weakness, and shortness of breath. If white blood cells are affected, you may develop infections. Signs of infection include fevers, chills, fatigue, or pain. If the number of platelets drops, bruising or abnormal bleeding may occur.
It is important for you to discuss any of these symptoms with your doctor. Detecting and treating bone metastasis early can help reduce complications.
How doctors find and diagnose bone metastasis
In some cases, your doctor may find bone metastasis before you have symptoms. In some cancers, where bone metastasis is common, your doctor may order tests to make sure the cancer has not spread to the bones, before recommending treatment. When you have symptoms of bone metastasis, doctors can do the following tests to find the cause:
Computed tomography scan
Magnetic resonance imaging scan
Following is a description of the tests:
Bone scan. A bone scan can usually detect bone metastasis earlier than an X-ray can. The scan looks at the entire skeleton and allows the doctor to monitor the health of all the bones in the body, including how they are responding to treatment.
In a bone scan, the doctor injects the patient with a low amount of radioactive material. The amount is much lower than that used in radiation therapy. The radioactive substance is attracted to diseased bone cells throughout the body. Diseased bone can appear on the bone scan image as darker, dense areas. Conditions other than metastasis, such as arthritis, infections, or previous fractures that have healed, may also be picked up on a bone scan. However, the patterns these conditions produce are often different from those produced by cancer. Additional tests can help distinguish among these other conditions.
Computed tomography (CT) scan. The CT scan provides X-ray images so doctors can examine cross sections of organs and bones in the body. X-rays provide only one perspective per image; the CT scanner takes many pictures as it rotates around the body. A computer combines the images into one picture to show if cancer has spread to the bones.
Laboratory tests. Bone metastasis can cause a number of substances to be released into the blood in amounts that are higher than normal. Two such substances are calcium and an enzyme called alkaline phosphatase. Blood tests for these substances can help diagnose bone metastasis. Doctors can also measure the levels of these chemicals over time to monitor a person's response to treatment. Elevated levels of these substances can indicate other medical conditions besides metastasis.
Magnetic resonance imaging (MRI). An MRI scan uses radio waves and strong magnets instead of X-rays to provide pictures of bones and tissues. MRI provides cross-sectional images of the body, as a CT scan does. It is particularly useful in looking at the spine and spinal cord, as well as joints. Frequently, MRI helps to further evaluate a bone mass seen on X-ray.
X-rays. Radiographic examination, called X-rays, can show where in the skeleton the cancer has spread. X-rays also show the general size and shape of the tumor or tumors. It's common for more than one metastasis to be found.
Positron emission tomography (PET). A PET scan uses a type of sugar that is radioactive. This specific sugar is injected into the blood. Cancer cells absorb large amounts of the sugar, compared to normal cells. After the injection, the patient lies on a table in a PET scanner where the whole body is imaged. A special camera takes pictures of the radioactive areas found in the body. A PET scan is not very detailed, but can sometimes identify tumors too small to be seen on other tests. If an abnormal area is seen, your doctor will likely order another test, such as a CT scan or MRI, to get more information. New technology combines PET and CT scans to provide more detailed images all at once.
Biopsy. Your doctor may recommend a bone biopsy to confirm the presence of bone metastasis, particularly when imaging tests and blood tests suggest, but don't confirm, metastasis.
How bone metastasis is treated
In addition to treating the cancer, these treatment options are available for bone metastasis:
Radiation therapy and radiopharmaceuticals
Chemotherapy and hormone therapy
Other treatments, including physical therapy and drugs
A description of each treatment follows:
These drugs slow the progression of abnormal bone destruction and formation that bone metastases cause. The drugs do the following:
Decrease risk for fractures
Reduce bone pain
Lower high blood calcium levels
Slow bone damage that metastases cause
Different types of bisphosphonates are available, such as:
Each has somewhat different effects. Bisphosphonates in cancer treatment are usually given through an IV (intravenous) line every three to four weeks. The oral forms are not well absorbed and can irritate the gastrointestinal tract. The side effects of bisphosphonates are usually mild and do not last long. Some of the most common are:
Lack of appetite
Early studies with bisphosphonates focused on the use of the drugs in people with breast cancer, prostate cancer, and multiple myeloma. Researchers are examining bisphosphonates to treat bone metastases from other types of cancer. Researchers are also exploring whether bisphosphonates can prevent the development or recurrence of bone metastases.
Doctors sometimes give this drug, which is injected under the skin every four weeks, in place of a bisphosphonate. Doctors may also prescribe denosumab if bisphosphonates stop working. Denosumab can help prevent or delay problems like fractures in patients with bone metastases.
Radiation therapy amd radiopharmaceuticals
Radiation therapy uses high-energy ionizing radiation to injure or destroy cancer cells. Typically, radiation is administered once a day in 10 treatments over a two-week period. It may take two to three weeks for the full effects of this treatment to occur. Side effects of radiation may include skin changes in the area being treated and, rarely, a temporary increase in symptoms of bone metastasis. Radiation is also useful in easing pain and killing tumor cells in bone metastases. It may be used to prevent a fracture and to treat spinal cord compression.
Radiopharmaceutical therapy is another type of radiation. This approach involves injecting a radioactive substance, such as strontium-89 or samarium-153, into a vein. The substance is attracted to areas of bone containing cancer. Providing radiation directly to the bone in this way destroys active cancer cells in the bone and can ease symptoms. It is especially useful if many bones are involved. Two important side effects are decreased blood counts with increased risk for bleeding, and, rarely, leukemia.
Chemotherapy and hormone therapy
Chemotherapy (chemo) drugs are used to kill cancer cells throughout the body. They may be taken orally or given intravenously. Chemo is the main treatment for many metastatic cancers. Hormone therapy uses drugs to prevent hormones from forming or acting on cells to promote cancer growth. The goals of either of these treatments in people with bone metastases are to control the tumor's growth, reduce pain, and reduce the risk for skeletal fractures.
In immunotherapy, doctors give you man-made immune system proteins systematically to kill cancer cells. Immunotherapy also boosts the immune system.
Surgeons perform surgery for bone metastases to prevent or treat a bone fracture. The surgery can involve removing most of the tumor or stabilizing the bone to prevent or manage a fracture, or both. Metal rods, plates, screws, wires, or pins may be surgically inserted to strengthen or provide structure to the bone damaged by metastasis.
Other treatments for bone metastases and their symptoms include physical therapy and drug and nondrug approaches to controlling pain. Doctors use many different drugs or combinations of drugs to treat pain from bone metastases. Nonsteroidal anti-inflammatory agents are the principal drug type used to treat the pain that bone metastases cause. They stop prostaglandins, the substances that seem responsible for much of your bone pain. It is important to take these medicines with food or milk to protect the stomach. Nondrug approaches to managing pain include the use of heat and cold, relaxation techniques, and therapeutic beds or mattresses.
Clinical trials explore ways to better manage bone metastases.