Do We Know What Causes Metastatic Cancer?

How Cancer Cells Spread

Metastasis is the end result of a process that involves many steps. Cancer cells travel from the place they started through the blood or lymph vessels to other parts of the body.

• Step 1 is when some of the cancer cells begin to grow fast and become more likely to spread.
• Step 2 is when the tumor begins to grow its own blood vessels to supply blood to the tumor. This allows it to grow faster.
• Step 3 is when the fast-growing cancer cells begin to break away from the place where they started. They also change so that they can break into the walls of the blood vessels or lymph vessels to travel to other tissues.
• Step 4 involves the cancer cells being able to live in the blood stream. Most cancer cells that get into the blood or lymph vessels are killed by the immune system. But some can survive.
• Step 5 happens when these cells are able to attach to distant organs or lymph nodes.
• Step 6 is a key step in which the new tumors are able to form new blood vessels to carry food and oxygen to the cancer cells.
• Step 7 is when the cancer cells are able to keep growing in their new "home" and avoid the body's attempts to reject or destroy them.

Why Cancer Cells Tend to Spread to Certain Parts of the Body

Most cancer cells that have broken away from the place where they started are carried in the blood or lymph vessels until they get trapped in the next "downstream" lymph node or small blood vessel. 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 body structure. 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.

Source: http://www.cancer.org