How a cancer spreads
This page tells you about how a cancer spreads. There is information about
The main reason cancer can be difficult to cure is that it can spread to a different part of the body from where it started. The cancer that grows where it first started in the body is called the 'primary cancer'. The place a cancer spreads to and then starts growing is called the 'secondary cancer' or 'metastasis'.
In order to spread, some cells from the primary cancer must break away, travel to another part of the body and start growing there. Cancer cells do not stick together as well as normal cells. They also may produce substances that stimulate them to move. But how do cancer cells travel through the body?
There are three main ways a cancer spreads
The cancer grows directly into nearby body tissues. There is more about this in the page on how a cancer grows.
In order to spread, the cancer cell must first become detached from the primary tumour. It must then slip through the wall of a blood vessel to get into the bloodstream.
When it is in the bloodstream, it is swept along by the circulating blood until it gets stuck somewhere, usually in a very small blood vessel called a capillary.
Then it must slip through the wall of the capillary and into the tissue of the organ close by. There it must start to multiply to grow a new tumour.
As you can see, this is a complicated journey. Most cancer cells do not survive it. Probably, out of many thousands of cancer cells that reach the blood circulation only one will survive to form a secondary cancer or metastasis.
Some cancer cells are probably killed off by the white blood cells in our immune system. Others cancer cells may die because they are battered around by the fast flowing blood.
Cancer cells in the circulation may try to stick to platelets to form clumps to give themselves some protection. This may also help them to be filtered out in the next capillary network they come across so they can then move into the tissues to start a secondary tumour.
The way a cancer spreads through the lymphatic system is very similar to the way it spreads through the bloodstream. The cancer cell must become detached from the primary tumour. Then it travels in the circulating lymph fluid until it gets stuck in the small channels inside a lymph node. There it begins to grow into a secondary cancer.
Whether it is in the blood or the lymph, the spreading cancer cell stops at the first place it gets stuck. In the bloodstream, this is often the first capillary network it comes across. The blood flow from most body organs goes next through the capillaries in the lungs. So not surprisingly, the lungs are a very common place for cancer to spread to.
The blood from the organs of the digestive system goes through the capillaries of the liver before going back to the heart and then to the lungs. So it is common for digestive system cancers to spread to the liver. In fact, the liver is the second most common area of cancer spread.
Some cancers show unexpected patterns of spread. For example, prostate cancer often spreads to the bones. Scientists are still investigating why this happens.
During cancer surgery, it is routine for the surgeon to remove the main lymph nodes close to the area of the cancer. For example, the surgeon operating to remove a breast cancer will remove at least some of the lymph nodes from under the arm. These are the first lymph nodes through which lymph draining from the breast flows. The surgeon does this because the first lymph nodes draining an organ are the most likely ones to contain cancer cells.
Micrometastases are metastases (cancer spread) that are too small to be seen. If there are individual cells, or even small areas of growing cells elsewhere in the body, there is no scan detailed enough to spot them.
For a few tumours, blood tests can detect proteins released by the cancer cells. These may give a sign that there are metastases too small to show up on a scan. But for most cancers, there is no blood test that can say whether a cancer has spread or not.
For most cancers the doctor can only say whether it is likely or not that a patient has micrometastases. This 'best guess' may be based on
- Previous experience of many other patients treated in the same way. Doctors naturally collect and publish this information to help each other.
- Whether cancer cells are found in the blood vessels in the tumour removed during surgery (for example in testicular cancer). If they are found then cancer cells are more likely to have reached the bloodstream and spread to somewhere else in the body.
- The grade of the cancer - the higher the grade, the more aggressive the cancer and the more likely that cells have spread.
- Whether lymph nodes that were removed at operation contained cancer cells (for example in breast cancer or bowel cancer). This is direct evidence that cancer cells have broken away from the original cancer. But there is no way of knowing whether any have spread further.
This information is important. If the doctor thinks it is likely that there are micrometastases, they may offer further treatment with chemotherapy, radiotherapy or hormone therapy. This is called 'adjuvant treatment'. The aim is to kill the cancer deposits before they grow big enough to be seen on a scan.
Some doctors call this 'belt and braces' treatment. In other words, the treatment is to try to make sure the cancer does not come back. No one can know for sure if all the cancer cells have been destroyed when someone has finished treatment. It is this uncertainty that can make cancer difficult to cope with for many people, even if they seem to have been successfully treated.
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