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Primary and secondary cancer

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'. Cancer that has spread to another part of the body is called metastatic cancer. 

 

How a cancer spreads

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 do. They may also produce substances that stimulate them to move. But how do cancer cells travel through the body?

There are three main ways a cancer spreads

 

Local spread

The cancer grows directly into nearby body tissues. There is more about this in the page on how a cancer grows.

 

Through the blood circulation

In order to spread, the cancer cell must first become detached from the primary cancer. It must then move through the wall of a blood vessel to get into the bloodstream.

Diagram showing how cancer cells 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 move back 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.

 

Through the lymphatic system

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.

 

Why cancers spread where they do

Whether it is in the blood or the lymph, the moving 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 from the organ to 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.

Cancer cells often get trapped in the group of lymph nodes closest to the tumour. During cancer surgery, the surgeon may remove the main lymph nodes close to the area of the cancer. For example, a surgeon operating to remove a breast cancer will remove one or more of the lymph nodes from under the arm. These are the first lymph nodes that the lymph draining from the breast goes to and so are most likely to contain any escaping cancer cells.

The first nodes that lymph draining from a tumour reaches are called the sentinel lymph nodes. After removing sentinel nodes, the surgeon will send them off to the lab to see if they contain cancer cells. Finding out whether the cancer has spread into the lymph nodes gives the doctor some information about whether you need further treatment.

 

Micrometastases

Micrometastases are areas of cancer spread (metastases) that are too small to see. If there are individual cells, or even small areas of growing cells elsewhere in the body, no scan is detailed enough to show them.

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 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 the following factors.

  • 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 removed during an operation contained cancer cells (for example in breast cancer or bowel cancer). 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 they have spread to any other areas of the body

This information is important. If the doctor thinks it is likely that there are micrometastases, they may offer further treatment such as chemotherapy, radiotherapy, biological therapy or hormone therapy. Treatment after surgery is called adjuvant treatment. The aim is to kill the areas of cancer cells 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. But no one can know for sure if all the cancer cells have been destroyed when someone has finished treatment. Or if the cancer has spread in the first place. 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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Updated: 24 July 2013