The type of cancer and where it starts often determines where it will spread. Most tumor cells that have broken free from the original tumor are carried in the blood or lymph circulation until they get trapped in the next "downstream" organ or set of lymph nodes. This explains why breast cancer often spreads to axillary (underarm) lymph nodes, but rarely to lymph nodes in the groin. Likewise, the lung is a common site of spread (metastasis) for many cancers. This is because the heart pumps blood from the rest of the body through the lung's blood vessels before sending it elsewhere. The liver is a common site of metastasis for cancer cells that start in the stomach and intestines because blood from the intestines flows into the liver.
Doctors have learned that cancer cells often break away from the main (primary) tumor and circulate in the blood. But in most cases they don't settle in any one place. Over time, they die. When the cancer does spread to other organs it is because of certain genetic changes in the cells that scientists are beginning to recognize. Someday, they 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 treatment that blocks or targets genetic changes so the cancer cells cannot spread and grow.
Sometimes the patterns of spread cannot be explained by the way the body is made. Some cancer cells are able to find and invade certain sites. For example, advanced prostate cancer often invades 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. In other cases, cells of some organs release hormone-like factors that actually cause cancer cells to grow faster.
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