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.

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 or the lymph system. (Lymph vessels are much like blood vessels, except they carry a clear fluid and immune system cells through the body.)

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

• Break away or escape from the original tumor and enter the bloodstream or lymph system.
• Attach to the outside wall of a blood or lymph vessel, move through the vessel wall, and flow with the blood or lymph to a new organ or lymph node and invade it.
• Find ways to grow and thrive in their new location.
• Avoid attacks from the body’s immune system.

Cancer cells that travel through the bloodstream may reach distant organs. If they travel through the lymph system, the cancer cells may end up in lymph nodes (small, bean-sized collections of immune cells) or they could spread to other organs.

The nervous system can be affected when a 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. Either way, most of the escaped cancer cells die or are killed before they can start growing somewhere else. But one or two might settle in a new area, begin to grow, and form new tumors. This spread of cancer to a new part of the body is called metastasis.

What determines where a cancer might spread?

The type of cancer a person has affects where it might be most likely to spread.

The location of the cancer matters because most cancer cells that break free from the primary tumor are carried in the blood or lymph system until they get trapped in the next “downstream” organ or set of lymph nodes. This explains why breast cancer often spreads to underarm lymph nodes, but rarely to lymph nodes in the belly. Likewise, many cancers commonly spread to the lungs. This is because the heart pumps blood from the rest of the body through the lungs’ blood vessels before sending it elsewhere.

The type of cells that the cancer grew from matters, too. Some organs or areas in the body have tissues that certain cancer cells are able to survive and grow in better than others. This is often called the tumor microenvironment and can be different for different cancer types. For example, prostate cancer often spreads to the bones because bone tissue provides the kind of environment prostate cancer cells prefer. Certain gene changes (mutations) in cancer cells might also affect where they can spread. Some gene changes make it easier for cancer cells to live 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
• 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