Cancer spreads when cancer cells break off and get into nearby blood or lymph vessels. Most cancer cells that have broken away from the place where they started are carried in these vessels until they end up in the next "downstream" organ or set of lymph nodes. This explains why breast cancer often spreads to the underarm lymph nodes but rarely to lymph nodes in the groin. Many cancers spread to the lungs because the heart pumps blood from the rest of the body through the lung's blood vessels before sending it elsewhere.
Sometimes the pattern of spread is not explained by the way the body is made. Some cancer cells seem to "home in" on certain places, perhaps because of substances on their surfaces that stick to cells in these organs. In other cases, the organ itself may release substances that cause the cancer cells to grow faster.
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