How cancer can spread

Primary and secondary cancer

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 cells can then grow and form other tumours. These are called secondary cancers or metastases.

How cancer can spread to other areas of the body

Cancer cells can spread to other parts of the body. They do this 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, bowel cancer that has spread to the liver is called bowel cancer with liver metastases or secondaries. It is not called liver cancer. This is because the cancerous cells in the liver are cancerous bowel cells. They are not liver cells that have become cancerous.

To spread, some cells from the primary cancer must break away. They then 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. This is the basement membrane.

Spread through the bloodstream

Cancer cells can go into small blood vessels and then get into the bloodstream. They are called circulating tumour cells (or CTCs).

Researchers are looking at using circulating tumour cells to diagnose cancer. This is instead of taking a tissue sample (biopsy). They hope to be able to test circulating cancer cells to predict which treatments people need. They are also looking to detect circulating tumour DNA (ctDNA) and monitor treatment.

The circulating blood sweeps the cancer cells along until they get stuck somewhere. Often 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

• it has the nutrients that it needs

This is quite a complicated process and most cancer cells don't survive it. Many thousands of cancer cells that reach the bloodstream. Only a few survive to form a secondary cancer.

The white blood cells in our immune system find and kill some cancer cells. Others cancer cells might 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. This gives them some protection. Platelets are blood cells that help the blood to clot. This could also help the cancer cells to move into the surrounding tissues.

Spread through the lymphatic system

The lymphatic system is a network of tubes and glands in the body. It 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. They then can travel into nearby lymph glands. In the lymph glands, the cancer cells might die. But some may survive and grow to form tumours in one or more lymph nodes. This is called lymph node spread.

This 2 minute video is about the lymphatic system.

Micrometastases

Micrometastases are areas of cancer spread (metastases) that are too small to see. They are too small to show up on any type of scan.

For a few types of cancer, blood tests can detect certain proteins that the cancer cells release. These are sometimes called tumour markers. These may show 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 they find cancer cells in the blood vessels then the cancer is more likely to have spread to other parts of the body

• the grade of the cancer, this is how abnormal the cells are. The higher the grade, the more quickly the cancer grows and the more likely for cells to spread

• whether there are cancer cells in the lymph nodes removed during an operation. If the lymph nodes contain 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. You might have extra treatment if doctors suspect there are micrometastases. This treatment might include:

• chemotherapy

• radiotherapy

• targeted treatment

• hormone therapy

The extra treatments might increase the chance of curing the cancer.