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's called bone metastasis.

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

Bone is a type of connective tissue made up mostly of minerals, like calcium, 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 in a process called remodeling. Two kinds of cells help with this:

• 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 protect vital organs, for instance, the ribs protect the lungs and heart.
• Bones store and release minerals, like calcium, that the body needs to work correctly.
• 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.

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 far from where it started 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 parts of the body. These cancers commonly spread to the bones:

• Breast
• Kidney
• Lung
• Prostate
• Thyroid
• Bladder
• Ovary
• Melanoma

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, like osteosarcoma and Ewing sarcoma.

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 (primary) tumor site. For instance, lung cancer that spreads to the bone is made of lung cancer cells. In this case, the 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 affect these places:

• Limbs (upper arm and upper leg bones)
• Pelvis (hipbones)
• 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. They 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. They're hard, but they're weak and unstable. They may break or collapse. They can also be painful.

Bone metastases can cause these symptoms:

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's usually worse at night or with rest. Over time, the pain may become severe. Still, not all pain means metastasis. Your doctor can help tell the difference between pain from metastasis and aches and pains from other causes.

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.

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 doctor right away. If untreated, it can cause paralysis.

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 problem is called hypercalcemia. If you have these symptoms, tell your doctor or nurse right away. If untreated, high calcium levels may cause a coma.

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 bruise or bleed very easily.

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

In some cases, your doctor may find bone metastasis before you have symptoms. In some cancers, where bone metastasis is common, tests might be done to make sure the cancer has not spread to your bones before a treatment plan is made.

When you have symptoms of bone metastasis, these are some of the tests that can be used:

• Bone scan. A bone scan can often find bone metastasis earlier than an X-ray can. The scan looks at your whole skeleton, so all the bones in your body can be checked for cancer. In a bone scan, a mildly radioactive tracer is put into your blood through a vein. The tracer is attracted to diseased bone cells all over your body. This helps diseased bone show up more clearly on the scan.
• CT scan. This test shows detailed images of any part of the body, including the bones. It's more detailed than a regular X-ray. It uses a combination of X-rays and computer technology to make cross-sectional images of the inside of the body. These images are combined into detailed pictures 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 makes cross-sectional images of the inside of the body. It's very useful in looking at the spine and spinal cord, as well as joints. Often, an MRI helps to get a better look at the outline of a bone mass seen on an X-ray.
• X-rays. An X-ray 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's radioactive. This sugar is put 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 machines combine 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. Blood tests can be done to see if there are higher amounts than normal. Two such substances are calcium and an enzyme called alkaline phosphatase. Blood tests for these substances can help diagnose bone metastasis. These levels can also be used to 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, too, not just metastasis.
• Biopsy. Your doctor may suggest a bone biopsy to be sure a change is bone metastasis. A small piece of bone is removed and tested for cancer cells. This is often done when imaging tests and blood tests suggest, but don't confirm, you have metastasis.

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. Along with treatment for the primary cancer, these treatments can be used for bone metastasis:

• Bisphosphonates (medicines that slow down bone cells called osteoclasts, which break down bone)
• Denosumab (another medicine that slows down osteoclasts)
• Radiation therapy and bone-targeted radiopharmaceuticals (radioactive medicines)
• Surgery
• Tumor ablation
• Bone cement
• Other treatments, including physical therapy and pain medicines

These medicines slow down the abnormal bone breakdown that bone metastases cause. They help to:

• Decrease your risk for fractures
• Reduce bone pain
• Lower high blood calcium levels
• Slow bone damage that metastases cause

Bisphosphonates include:

• Pamidronate
• Zoledronic acid

Bisphosphonates in cancer treatment are often given through a small, flexible tube called an I.V. (intravenous) every 3 to 12 weeks. They can also be taken 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 or joint pain

Sometimes this medicine is used in place of a bisphosphonate. It's given as a shot (injection) under the skin every 4 weeks. Side effects may include rash, muscle or joint pain, nausea, constipation, and diarrhea.

Denosumab might be given if bisphosphonates stop working. It can help prevent or delay problems like fractures in people with bone metastases.

Both bisphosphonates and denosumab might cause a rare side effect called osteonecrosis of the jaw (ONJ). Talk with you doctor about your risk of ONJ.

Radiation therapy uses strong beams of X-rays to damage or destroy cancer cells. Radiation often helps ease pain and kill 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 to get the full effects of this treatment. 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 (such as pain) of bone metastasis.

Radiopharmaceutical therapy targeted to bones is another type of radiation. This treatment involves putting a radioactive substance into your blood through a vein. The substance is attracted to areas of bone that have cancer. Giving radiation right to the bone in this way kills active cancer cells there and can ease symptoms. It's very useful if many bones are affected. Side effects are rare.

Surgery may be done to prevent or treat a bone fracture. The surgery can involve removing most of the tumor, stabilizing the bone to prevent or manage a fracture, or both. Metal rods, plates, screws, wires, or pins may be put in to strengthen or provide structure to the bone damaged by metastasis.

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 from the inside.

This treatment may be a choice to treat one or two tumors that are causing problems. If a hole is left behind, it can be filled with bone cement (below).

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

If 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 treatments for bone metastases and their symptoms include physical therapy and pain control with or without medicine. Many different medicines or combinations of medicines can be used to treat pain from bone metastases. The main types of medicines used to treat this pain are nonsteroidal anti-inflammatory drugs (NSAIDs), like 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.