Understanding Bone Metastasis
What is metastasis?
When cancer spreads from the part of the body where it started (its primary site) to other parts of the body it’s called metastasis. Metastasis can happen when cells break away from a cancer tumor and travel through the bloodstream or through lymph vessels to other parts of the body. (Lymph vessels are much like blood vessels, except they carry a clear fluid called lymph back toward the heart.) Cancer cells that travel through the blood or lymph vessels can spread to other organs or tissues in distant parts of the body.
Many of the cancer cells that break off from the original tumor die without causing any problems. But some settle in a new area. There, they grow and form new tumors. When cancer spreads, we say that it metastasizes. If there’s only a single tumor, it’s called a metastasis or a metastatic tumor. When there are 2 or more metastatic tumors, it’s called metastases.
Sometimes metastatic tumors are found on tests done when the primary cancer is first diagnosed. In other cases, the metastasis is found first, causing the doctor to look for the place that the cancer started.
Different cancers tend to spread to different sites, but some of the most common sites of metastasis are the bones, liver, brain, and lungs.
What is bone metastasis?
A bone metastasis is an area of bone that contains cancer that spread there from somewhere else.
Cancer can spread to any bone in the body, but metastases are most often found in bones near the center of the body. The spine is the most common site. Other common sites are the hip bone (pelvis), upper leg bone (femur), upper arm bone (humerus), ribs, and the skull.
Once cancer has spread to the bones or to other parts of the body it’s rarely able to be cured. Still, it often can be treated to shrink, stop, or slow its growth. Even if a cure is no longer possible, treating the cancer may be able to help you live longer and feel better.
How does bone metastasis cause bone changes and other problems?
Bone is the supporting framework of the body. Bones are made of a network of fibrous tissue called matrix, minerals such as calcium that attach to the matrix and give the bone its strength and hardness, and 2 main kinds of bone cells are osteoblasts and osteoclasts.
Knowing a little about these 2 kinds of cells can help you understand how bone metastases grow, and how some medicines work to treat bone metastases. The osteoblast is the cell that forms new bone, and the osteoclast is the cell that dissolves old bone. When these cells are both working right, new bone is always forming while old bone is dissolving. This helps keep the bones strong.
Cancer cells can affect the bones by interfering with osteoblasts and osteoclasts:
- Often, the cancer cells make substances that turn on the osteoclasts. This leads to bone being broken down without new bone being made. This weakens the bones. The holes that develop when parts of bones dissolve are called osteolytic (OS-tee-oh-lit-ik) or lytic (LIT-ik) lesions. Lytic lesions are so weak that they can cause the bone to easily break.
- Sometimes, the cancer cells release substances that turn on the osteoblasts. This leads to new bone being made without breaking down the old bone broken down first. This makes areas of the bones harder, a condition called sclerosis. The areas of bone where this occurs are called osteoblastic or blastic lesions. Although these blastic areas are harder, the structure of the bone is not normal and these areas actually break more easily than normal bone.
Bone metastasis can cause other problems as well:
- When cancer spreads to the bones of the spine, it can press on the spinal cord. This can cause nerve damage that may even lead to paralysis if not treated.
- As cancer cells damage the bones, calcium from the bones is released into the blood. This can lead to problems caused by high blood calcium levels (hypercalcemia [HI-per-kal-SEE-me-uh]).
Why do cancers metastasize to bones?
For cancer cells to spread to other parts of the body, they have to go through many changes:
- They have to be able to break away from the original (primary) tumor and get into the bloodstream or lymph system, which can carry them to another part of the body.
- At some point they need to attach to the wall of a blood or lymph vessel and move through it, out into a new organ.
- They then need to be able to grow and thrive in their new location.
All the while, the cancer cells need to be able to avoid attacks from the body’s immune system. Going through all these steps means the cells that start new tumors may no longer be exactly the same as the ones in the tumor where they started, But they will still be called the same name. For instance, breast cancer that spreads to the bone is called metastatic breast cancer, not bone cancer.
What’s the difference between primary bone cancer and bone metastasis?
Some cancers start in the bone, rather than spreading to the bones from somewhere else. Cancers that start in the bone are called primary bone cancers. These cancers are very different from bone metastases. Bone metastasis is much more common than primary bone cancers, especially in adults.
Chow E, Finkelstein JA, Sahgal A, Coleman RE. Metastatic cancer to the bone. In: DeVita VT, Lawrence TS, Rosenberg SA, eds. DeVita, Hellman, and Rosenberg’s Cancer: Principles & Practice of Oncology. 9th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2011: 2192-2204.
Coleman RE, Holen I. Bone metastasis. In: Niederhuber JE, Armitage JO, Doroshow JH, Kastan MB, Tepper JE, eds. Abeloff’s Clinical Oncology. 5th ed. Philadelphia, Pa: Elsevier; 2014: 739-763.
Last Medical Review: May 2, 2016 Last Revised: May 2, 2016