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About Cancer

Reviewed February 2021

What is cancer?

What is cancer?

Normal cells have a life cycle. They divide and grow throughout the body to replace worn out tissue, heal wounds and maintain healthy organs. 

Cancer starts when cells in the body become abnormal. They start growing and multiplying out of control. When they do this, they sometimes form a mass called a tumour. Cancer cells make it hard for your body to work properly.

Some cancers grow only in the place where they started (localized). Other cancers may grow and invade the normal tissue around them (local invasion).

Another name for cancer cells is malignant. Malignant cells can grow into other tissues and spread around the body.

Sometimes tumours are made out of cells that are not cancerous (not malignant). They cannot invade the tissue around them or spread around the body. These are called benign tumours. 

Watch the Canadian Cancer Society's video What is Cancer? 

What is metastasis?

Cancer cells can sometimes break loose from the original (primary) tumour. Metastasis is when these cells get carried to other parts of the body and start growing. The tumour in the new site is called a secondary tumour.  

Cancers do not spread in a completely random way. Metastasis mainly happens through the lymphatic system or the circulatory system.

  • Lymphatic System: The most common way for cancer to spread is through the lymphatic system, which carries lymph fluid throughout the body. Often, a surgeon will remove the tumour and the lymph nodes nearby, even if there is no sign of cancer in the lymph nodes. This is done in case even one cancer cell has broken away from the tumour and travelled into the lymph node.  
  • Circulatory System: Your circulatory system carries blood throughout your body. Cancer cells can break off from the tumour and travel through the bloodstream. Tumours that spread by the blood almost always do so through the veins, not the arteries.

Some parts of the body accept cancer cells more easily than others. For example, cancers rarely metastasize to the skin. But, cancers often spread to the liver and lungs.

When one type of cancer spreads to another part of the body, it does not become another type of cancer. For example, if a person with colon cancer has a metastasis in the lung, the tumour growing in the lung is the same cancer as in the colon.

What causes cancer?

Many genes in the human body help control how cells divide and grow. When changes occur in your genes (called mutations), they may lose control over the cells. This can cause cancer.

Gene mutations can be caused by aging, chemicals, radiation, hormones, other things in the body, and the environment. Because most cancers happen by chance (randomly) and after a cell has had many gene mutations, most cancers are seen in people over the age of 50.

Most cancers are not hereditary (inherited) and caused by a gene mutation you got from a parent. For information about hereditary risk, see our Hereditary Cancer Program page.

Can I prevent cancer?

There are lifestyle choices you can make that can lower your risk of getting cancer. For example, exercising more or eating a healthier diet can lower your cancer risk. For information on cancer prevention, please visit our Prevention site.

There is information about healthy living after cancer in our Life After Cancer.

The American Institute for Cancer Research has helpful recommendations to lower your risk of getting cancer.

Types of cancer & Staging

Types of cancer

Cancers are usually named after the part of the body where they first start growing. The name does not change even if the cancer spreads to another part of the body.

For example, if breast cancer spreads (metastasizes) to the lung, it is called breast cancer with lung metastases. If chemotherapy is the best treatment, breast cancer drugs are used to treat the lung metastases. Other cancers, such as leukemia (a cancer involving the blood), may not mention a tumour site in the name.

Cancers are also different in how fast they grow, how they spread, and how they react to different treatments. This is why it is important to correctly diagnose a cancer so that treatment starts as soon as possible.


Staging describes the cancer. It is based on how much cancer is in the body, where it was first diagnosed, if the cancer has spread and where it has spread to.

Staging can be complicated and hard to understand. If you have questions, please talk to your BC Cancer health care team.

The stage of the cancer is used to plan the treatment. The TNM classification system is the most common staging system.

  • T stands for tumour: This describes the size of the main (primary) tumour. It describes if the tumour has grown into nearby tissues. This part of staging is usually a number between 1 and 4. A higher number usually means the tumour is bigger and may have grown into nearby tissues.

  • N stands for lymph nodes: This describes if the cancer has spread into the lymph nodes near the tumour.  Lymph nodes are part of your body's lymphatic system.
    • N0: cancer has not spread to nearby lymph nodes
    • N1, N2 or N3: cancer has spread into nearby lymph nodes. This may also describe the size of the lymph nodes and how many lymph nodes have cancer.

  • M stands for metastasis: This describes if the cancer has spread to parts of the body farther away from the tumour. 
    • M0: cancer has not spread to other parts of the body.
    • M1: cancer has spread to other parts of the body

Stage grouping

The TNM system is used to give an overall stage for the cancer, from 0 to 4.  You will most often see the stages as Roman numerals (I, II, III, and IV) instead of numbers (1, 2, 3, and 4).  Below, we give them as numbers to make them easier to understand.

A lower number usually means a better prognosis (how well your treatment will work and how long you will live).

Sometimes a stage has sub-stages. For example, a certain type of cancer may have a Stage 2A and a Stage 2B.

For most cancers, the stages are:

  • Stage 0: also called carcinoma in situ.  This is not cancer but the cells may become cancer in the future (pre-cancerous cells).
  • Stage 1: the tumour is usually small and has not grown outside of the place it started.

  • Stages 2 and 3: the tumour is larger or it has grown outside of the place it started.

  • Stage 4: the cancer has spread to a distant part of the body (it has metastasized).
Not all types of cancer use the TNM system. Some cancers use risk groups and other types use grades.

If you are looking for staging information for a specific type of cancer, please go to our Types of Cancer page. 


Cancer treatment includes surgery, systemic therapy (drug therapy) and radiation therapy.

The type(s) of treatment or the order of treatment will be different for each patient.  Treatment depends on the location of the tumour, the stage of the cancer, and BC Cancer management manual.

You may hear the term "investigational treatment". This means that the treatment is being studied in a clinical trial to find out if it is a safe and effective way to treat cancer.

"Supportive care", "alternative" or "complementary" are also terms you may hear. 

Conventional therapies have been proven to cure cancers or slow down their growth. Supportive care helps patients who are using conventional treatments. Alternative or complementary therapies have not usually undergone rigorous scientific testing to see if they are safe or effective.

If you have questions about complementary or alternative therapies, visit our webpage: Complementary & Alternative Therapies.

Our Coping with Cancer section has information on nutrition, emotional, financial and practical support, how to deal with symptoms and side effects, and information about palliative care.

Surgery is often the first step in cancer treatment and can be used to diagnose and treat cancer. More than half of the people diagnosed with cancer will have some type of surgery. To find out if surgery is used to diagnose or treat a specific cancer, please go to our Types of Cancer pages.


Depending on the size and location of a tumour, surgery can: 

  • Remove pre-cancerous tumours (tumours that may become cancer).
  • Completely remove a tumour. In this case, surgery can cure the cancer.
  • Make the tumour smaller so that other cancer treatments, like systemic therapy or radiation, can work better.

For people who have cancer that cannot be cured, surgery may help with pain or other symptoms.


Some cancer surgeries are done at the BC Cancer - Vancouver. However, many more are done by surgeons at hospitals throughout the province. Surgical oncologists are specialists in using surgery to treat cancer. 


If you have questions about surgery, please talk to your health care team.

For more information see BC Cancer Library surgery pathfinder


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SOURCE: About Cancer ( )
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