How cancer can spread
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The place where a cancer starts in the body is called the primary cancer or primary site. Cells from the primary site may break away and spread to other parts of the body. These escaped cells can then grow and form other tumours, which are known as secondary cancers or metastases.
Cancer cells can spread to other parts of the body through the bloodstream or lymphatic system. There they can start to grow into new tumours.
Cancers are named according to where they first started developing. For example, if you have bowel cancer that has spread to the liver, it's called bowel cancer with liver metastases or secondaries. It is not called liver cancer. This is because the cancerous cells in the liver are actually cancerous bowel cells. They are not liver cells that have become cancerous.
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 don't stick together as well as normal cells do. They may also produce substances that stimulate them to move.
The diagram below shows a tumour in the cells lining a body structure such as the bowel wall. The tumour grows through the layer holding the cells in place (the basement membrane).
Some cells can break away and go into small lymph vessels or blood vessels called capillaries in the area.
When the cancer cells go into small blood vessels they can then get into the bloodstream. They are called circulating tumour cells (or CTCs).
Researchers are currently looking at using circulating tumour cells to diagnose cancer and avoid the need for tests such as biopsies. They are also looking at whether they can test circulating cancer cells to predict which treatments will work better.
The circulating blood sweeps the cancer cells along until they get stuck somewhere. Usually they get stuck in a very small blood vessel such as a capillary.
Then the cancer cell must move through the wall of the capillary and into the tissue of the organ close by. The cell can multiply to form a new tumour if the conditions are right for it to grow and it has the nutrients that it needs.
This is quite a complicated process and most cancer cells don't survive it. Probably, out of many thousands of cancer cells that reach the bloodstream, only a few will survive to form a secondary cancer.
Some cancer cells are probably killed off by the white blood cells in our immune system. Others cancer cells may die because they get 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. Platelets are blood cells that help the blood to clot. This may also help the cancer cells to move into the surrounding tissues.
The lymphatic system is a network of tubes and glands in the body that filters body fluid and fights infection. It also traps damaged or harmful cells such as cancer cells.
Cancer cells can go into the small lymph vessels close to the primary tumour and travel into nearby lymph glands. In the lymph glands, the cancer cells may be destroyed but some may survive and grow to form tumours in one or more lymph nodes. Doctors call this lymph node spread.
Micrometastases are areas of cancer spread (metastases) that are too small to see. Some areas of cancer are too small to show up on any type of scan.
For a few types of cancer, blood tests can detect certain proteins released by the cancer cells. These may give a sign that there are metastases in the body that are 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, doctors can only say whether it is likely or not that a cancer has spread. Doctors base this on a number of factors:
- previous experience – doctors collect and publish this information to help each other
- whether there are cancer cells in the blood vessels in the tumour removed during surgery – if cancer cells are found then the cancer is more likely to have spread to other parts of the body
- the grade of the cancer (how abnormal the cells are) – the higher the grade, the more quickly the cancer grows and the more likely for cells to spread
- whether lymph nodes removed during an operation contained cancer cells – if the lymph nodes contained cancer cells this shows that cancer cells have broken away from the original cancer (but there is no way of knowing whether the cells have spread to any other areas of the body)
This information is important in treating cancer. Doctors may offer extra treatment, such as chemotherapy, radiotherapy, biological therapy or hormone therapy if they suspect of micrometastases. The extra treatments may increase the chance of curing the cancer.
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