Bone Metastases: When Cancer Spreads to the Bones

Cancer that has started in one place can spread and invade other parts of the body. This spreading is called metastasizing. If a tumor spreads to the bone, it is called bone metastasis.

Cancer cells that have spread to the bone can damage the bone and cause symptoms. Different treatments are available to control the symptoms and the spread of bone metastases. To better understand what happens in metastasis, it helps to understand the anatomy of the bones.

Bone basics

Bone is a type of connective tissue made up mostly 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 always repairing and renewing itself through a process called remodeling. Two kinds of cells help with 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 gives structural support.

• Bones store and release, as needed, minerals that the body needs to work properly, such as calcium.

• Bone marrow makes and stores blood cells. These include red blood cells, white blood cells, and platelets. Red blood cells bring 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 these bone functions 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 start 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 cancer seem to spread to certain 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:

• Breast

• Kidney

• Lung

• Prostate

• Thyroid

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 start 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 often stay in these places:

• Limbs

• Pelvis

• Rib cage

• Skull

• Spine

Cancer cells that spread from other parts of the body can form 2 main types of bone tumors:

• The tumor may eat away areas of bone, creating holes called osteolytic lesions. This 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 (osteosclerotic or osteoblastic lesions) 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 often the first symptom you notice. At first, the pain may come and go. It tends to be worse at night or with bed rest. Over time, the pain may grow and become severe. Not all pain means metastasis. Your doctor can help tell the difference 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 break (fracture) is the first sign of bone metastasis. The most common sites of fracture are the long bones of the arms and legs, and the bones of the spine. A sudden pain in the middle of your back may mean that a cancerous bone is breaking and collapsing.

• Numbness or weakness in the legs, trouble urinating or having a bowel movement, or numbness in the belly. 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, it can cause paralysis.

• Loss of appetite, nausea, thirst, constipation, tiredness, or confusion. These are all signs that you may have high levels of calcium in your 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, it may cause a coma.

• Other symptoms. If bone metastasis affects your bone marrow, you may have other symptoms related to lower 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 get 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. Finding 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 your bones, before recommending treatment. When you have symptoms of bone metastasis, doctors can do the following tests to find the cause:

• Bone scan. A bone scan can often find bone metastasis earlier than an X-ray can. The scan looks at your whole skeleton. It allows the doctor to check the health of all the bones in your body, including how they respond to treatment. In a bone scan, the doctor injects you with a low level of radioactive material. The amount is much lower than that used in radiation therapy. The radioactive substance is attracted to diseased bone cells all over the body. This helps diseased bone show up more clearly on the bone scan image.

• CT scan. This imaging test shows detailed images of any part of the body, including the bones. It is more detailed than a regular X-ray. It uses a combination of X-rays and computer technology to make cross-sectional images of the body. These images are combined into 1 detailed picture to show if cancer has spread to the bones.

• MRI. An MRI scan uses radio waves and strong magnets, instead of X-rays, to make pictures of bones and tissues. MRI provides cross-sectional images of the body, as a CT scan does. It is very useful in looking at the spine and spinal cord, as well as joints. Often, an MRI helps to further check a bone mass seen on an X-ray.

• X-rays. An X-ray image 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 1 metastasis to be found.

• PET scan. This imaging test uses a type of sugar that is radioactive. This sugar is injected into your blood. Cancer cells absorb large amounts of the sugar, compared to normal cells. After the injection, you lie on a table in a PET scanner, while your whole body is imaged. A special camera takes pictures of the radioactive areas found in your body. A PET scan is not very detailed, but can sometimes find tumors too small to be seen on other tests. If an abnormal area is seen, your doctor will likely order another test for more information. This may be a CT scan or MRI. New technology combines PET and CT scans for more detailed images all at once.

• Lab tests. Bone metastasis can cause many 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 check your response to treatment. Higher levels of these substances can be a sign of other health problems besides metastasis.

• Biopsy. Your doctor may recommend a bone biopsy to be sure there is bone metastasis. A sample of bone is removed and checked under a microscope. This is often done when imaging tests and blood tests suggest, but don't confirm, you have metastasis.   

How bone metastasis is treated

In addition to treating the cancer, these treatment options are available for bone metastasis:

• Bisphosphonates (medicines that slow down bone cells called osteoclasts) 

• Denosumab (another medicine that slows down osteoclasts)

• Radiation therapy and radiopharmaceuticals (radioactive medicines)

• Chemotherapy, hormone therapy, targeted therapy, or immunotherapy to treat the cancer

• Surgery

• Other treatments, including physical therapy and pain medicines

Bisphosphonates

These medicines slow the progression of abnormal bone destruction and formation that bone metastases cause. The medicines do the following:

• Decrease your 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:

• Pamidronate

• Zoledronic acid

Each has somewhat different effects. Bisphosphonates in cancer treatment are often given through an IV (intravenous) line every 3 to 4 weeks. They are also available as pills that you swallow. But the pills are not well absorbed and can irritate the GI (gastrointestinal) tract. The side effects of bisphosphonates are usually mild and don’t last long. Some of the most common side effects are:

• Tiredness

• Nausea

• Vomiting

• Lack of appetite

• Bone pain

Researchers are also trying to see if bisphosphonates can prevent bone metastases from starting, or from coming back.

Denosumab

Doctors sometimes give this medicine in place of a bisphosphonate. It is given either by IV or injected under the skin every 4 weeks.

Doctors may also prescribe denosumab if bisphosphonates stop working. Denosumab can help prevent or delay problems like fractures in people with bone metastases.

Radiation therapy and radiopharmaceuticals

Radiation therapy uses high-energy ionizing radiation to hurt or destroy cancer cells. Radiation is often helpful in easing pain and killing tumor cells in bone metastases. It may also be used to help prevent fractures and to treat spinal cord compression. It may take 2 to 3 weeks for the full effects of this treatment to occur. Side effects of radiation may include skin changes in the area being treated. In rare cases, it may cause a short-term increase in symptoms of bone metastasis. 

Radiopharmaceutical therapy is another type of radiation. This approach involves injecting a radioactive substance into a vein. The substance is attracted to areas of bone that have cancer. Giving radiation directly to the bone in this way destroys active cancer cells in the bone and can ease symptoms. It is often very useful if many bones are affected. Two important side effects are lower blood counts and greater risk for bleeding. In rare cases, there is a greater risk of leukemia.

Chemotherapy, hormone therapy, targeted therapy, and immunotherapy

These different types of medicine treatments are often used for advanced cancers. Depending on the type of cancer you have, these treatments might be used to help treat cancer that has spread anywhere in the body. They can sometimes help treat bone metastases by helping to control tumor growth, reduce pain, and lower the risk for bone fractures.

Surgery

Surgeons sometimes 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. Another example is kyphoplasty. The surgeon injects bone cement into 1 of the bones in the spine to help keep it from collapsing.

Other therapies

Other treatments for bone metastases and their symptoms include physical therapy, and controlling pain with or without medicine. Doctors use many different medicines or combinations of medicines to treat pain from bone metastases. The main types of medicines used to treat this pain are NSAIDs (nonsteroidal anti-inflammatory drugs). These include aspirin and ibuprofen. They stop the substances (prostaglandins) that seem responsible for much bone pain. Other ways to manage pain without medicine include using heat and cold, relaxation methods, and therapeutic beds or mattresses.

Researchers are studying new ways to better manage bone metastases. These new methods are tested in clinical trials.