Why cancer cells tend to spread to certain parts of the body?

Where a cancer starts often plays a role in where it will spread. Most cancer cells that break free from the original tumor are carried in the blood or lymph until they get trapped in the next “downstream” organ or set of lymph nodes. Once the cells are there, they can start new tumors. This explains why breast cancer often spreads to underarm lymph nodes, but rarely to lymph nodes in the groin. Likewise, there are many cancers that 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 liver is a common site of spread for cancer cells that start in the colon because blood from the intestines flows into the liver.

Cancer cells often break away from the main (primary) tumor and travel through the blood and/or lymph system, but they don’t always settle in and start new tumors. Most of the time, the cells that broke away die. When cancer does spread to other organs and start to form new tumors, it’s because of certain genetic changes in the cells that scientists are now starting to understand. Someday, doctors may be able to tell if a person’s cancer is the type that will spread to other organs by looking for these genetic changes. Research is also focusing on treatments that block or target these genetic changes so the cancer cells can’t spread and grow.

Sometimes the patterns of spread cannot be explained by where things are in the body. Some cancer cells are able to find and invade certain sites far away from where they started. For example, advanced prostate cancer often moves into the bones before spreading to other organs. This “homing” pattern may be caused by substances on the cancer cell surfaces that stick to cells in certain organs.

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