Bone Metastases: When Cancer Spreads to the Bones

Cancer that has started in one place can spread to and invade other parts of the body. This spread is called metastasis. 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 cells that build new bone. 

• Osteoclasts are cells that break down, or reabsorb, old bone.

Here are some of the things bones do:  

• The skeleton gives structural support.

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

• Bone marrow makes and stores blood cells. These include red blood cells, white blood cells, and platelets. Red blood cells bring oxygen 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 cancer tumor, they can move through the bloodstream or lymph vessels to other parts of the body. Cancer cells can settle 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 forms is called the secondary tumor. Secondary tumors in the bone are called bone metastases.

Different types of cancer tend 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

• Bladder

What are bone metastases?

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 started 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. In adults, metastatic bone cancer is much more common than primary bone cancer.

Cancer cells that spread to the bone often stay in these places:

• Limbs (upper arm and upper leg bones)

• Pelvis

• Rib cage

• Skull

• Spine

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

• The tumor may eat away areas of bone. This creates holes called osteolytic lesions. This can make bones fragile and weak. So the bones can break or fracture easily. These areas may be painful.

• The tumor may cause the bone to form and build up abnormally. These areas of new bone are called osteosclerotic or osteoblastic lesions. These are hard, but they're weak and unstable. They 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 is usually  worse at night or with bed rest. Over time, the pain may become severe. Still, not all pain means metastasis. Your healthcare provider can help tell the difference between pain from metastasis and aches and pains from other causes.

Broken bones

Bone metastasis can weaken bones. This puts your bones at risk for breaking. In some cases, a break (fracture) is the first sign of bone metastasis. The most common sites where bones may break are the long bones of the arms and legs, and the bones of the spine. For instance, sudden pain in the middle of your back may mean that a bone is breaking or collapsing.

Nerve problems

Numbness or weakness in the legs, trouble urinating or having a bowel movement, or numbness in the belly are all signs that the spinal cord may be compressed. When cancer metastasizes to the spine, it can squeeze or compress 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 healthcare provider 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, tell a your healthcare provider 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 that are caused by 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 your platelets are low, you may have bruising or abnormal bleeding.

It is important for you to discuss any of these symptoms with your healthcare provider right away. Finding and treating bone metastasis early can help reduce complications.

How doctors find and diagnose bone metastasis

In some cases, your healthcare provider may find bone metastasis before you have symptoms. In some cancers, where bone metastasis is common, your healthcare provider may do tests to make sure the cancer has not spread to your bones, before recommending treatment. When you have symptoms of bone metastasis, healthcare providers can use 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 healthcare provider to check the health of all the bones in your body. In a bone scan, you will get an injection with 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 one 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.

• 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. Healthcare providers can also measure the levels of these chemicals over time to check your response to treatment. But remember, higher levels of these substances can be a sign of other health problems besides metastasis.

• Biopsy. Your healthcare provider may suggest a bone biopsy to be sure a change 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

Bone metastases are treated with the same treatments used to treat the primary cancer. For instance, metastatic prostate cancer in the bone may be treated with hormone therapy. In addition to treating the primary 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)

• Surgery

• Tumor ablation

• Bone cement

• Other treatments, including physical therapy and pain medicines

Bisphosphonates

These medicines slow down the abnormal bone destruction and formation that bone metastases cause. The medicines help 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 medicine has somewhat different effects. Bisphosphonates in cancer treatment are often given through a small, flexible tube called an IV (intravenous) line every three to four weeks. They are also available as pills that you swallow. But the pills are not well absorbed and can irritate the gastrointestinal (GI) tract. The side effects of bisphosphonates are usually mild and don’t last long. Some of the most common side effects are:

• Fever

• Tiredness

• Nausea

• Vomiting

• Lack of appetite

• Bone pain

Denosumab

Healthcare providers sometimes give this medicine in place of a bisphosphonate. It is injected under the skin every four weeks. Side effects are much the same as those caused by bisphosphonates.

Healthcare providers may also give 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 strong ionizing X-rays to damage 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 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. 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. Side effects are rare. 

Surgery

Surgery may be done 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 inserted to strengthen or provide structure to the bone damaged by metastasis. 

Tumor ablation

This treatment involves putting a probe right into a tumor. A CT scan might be used to guide the probe. Chemicals, electricity, heat, or cold is then passed through the probe to destroy the tumor.

This treatment may be an option to treat a bone tumor that's causing problems. If a hole is left behind, it might be filled with bone cement (below). 

Bone cement

A quick-setting cement can be put into bone using a needle. This can help stabilize the bone or strengthen it.

If the cement is put into the spinal bones, it's called vertebroplasty or kyphoplasty. The bone cement is injected into one of the bones in the spine to help keep it from collapsing. It's called cementoplasty if it's used to treat other bones.

This treatment may be used after other treatments that have been used to destroy the tumor in the bone.

Other therapies

Other treatments for bone metastases and their symptoms include physical therapy and pain control with or without medicine. Healthcare providers 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 nonsteroidal anti-inflammatory drugs (NSAIDs). These include aspirin and ibuprofen. They stop substances called prostaglandins that seem responsible for much bone pain. Other ways to manage pain without medicine include using heat and cold, relaxation, and therapeutic beds or mattresses.