How Does Cancer Spread?

Cancer can spread from where it started (the primary site) to other parts of the body. When cancer cells break away from a tumor, they can travel to other areas of the body through either the bloodstream or the lymph system. This process is called metastasis.

How do cancer cells spread to new parts of the body?

When cancer cells break away from a tumor, they can travel to other parts of the body, usually through the bloodstream (circulatory system) or the lymph system.

For cancer cells to spread to new parts of the body, they must:

• Escape from the original tumor and invade nearby tissue.
• Attach to the wall of a nearby blood or lymph vessel, move through it, and travel along it to a new organ or lymph node.
• Stop in the new organ or lymph node, invade its wall, and move into the tissue.
• Find ways to grow and thrive in their new location, including creating new blood vessels (angiogenesis).
• Avoid attacks from the body’s immune system.

The nervous system can be affected when cancer spreads, too. It’s possible for cancer cells to enter nerves or release substances that can affect how nerves function to protect the body. In these cases, the cancer cells invade the outer part of the nerve (called the sheath) and spread up toward the skin or deeper into the body. This is called perineural spread or invasion.

Why are cancer cells able to spread?

Cancer cells are physically different from normal cells in ways that help them spread. They are:

• Less “sticky,” so they can break away from the tumor more easily.
• Better at moving and spreading through body tissues.
• Able to make proteins that break down surrounding tissue, helping them invade nearby areas.

What determines where cancer might spread?

The type of cancer and its location affects where it might be most likely to spread. The location is important because most cancer cells that break free from the primary tumor travel through the blood or lymph system until they get trapped in the next “downstream” organ or set of lymph nodes. For example, breast cancer often spreads to the lymph nodes under the arm (axillary lymph nodes) because these lymph nodes are closest to the breast. Similarly, many cancers spread to the lungs because blood passes through the lungs as it circulates throughout the body.

The type of cells that the cancer grows in matters, too. This is known as the tumor microenvironment. It can vary for different cancer types and is affected by:

• How much blood flow it has
• How many nutrients are there
• Whether it has certain hormone receptors
• What kinds of immune cells are there, like macrophages, neutrophils, and natural killer (NK) cells

Some areas of the body are easier for certain cancer cells to grow in. For example, prostate cancer often spreads to the bones because bone tissue provides conditions that help those cancer cells survive and grow.

Certain gene changes (mutations) in cancer cells can also affect where they can spread, making it easier for cancer cells to grow in certain organs.

Where does cancer most often spread?

In addition to lymph nodes, the lungs, liver, bones, and brain are common places certain cancers might spread to. But different types of cancer tend to spread to different places. Here are some common types of cancer and the places they most often spread:

• Bladder cancer may spread to the bones, liver, or lungs
• Breast cancer may spread to the bones, brain, liver, or lungs
• Colorectal, ovarian, stomach, and pancreatic cancers may spread to the liver, lungs, or peritoneum (lining of the abdominal cavity)
• Kidney cancer may spread to the adrenal glands, bones, brain, liver, or lungs
• Lung cancer may spread to the adrenal glands, bones, brain, liver, or the other lung
• Melanoma may spread to other areas of the skin, to muscle tissue, or to the bone, brain, liver, or lungs
• Prostate cancer may spread to the adrenal glands, bones, liver, or lungs
• Thyroid cancer may spread to the bone, liver, or lungs
• Uterine (endometrial) cancer may spread to the bone, liver, lung, peritoneum, vagina