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What is breast cancer?

Breast cancer is cancer that starts in the breast tissue. Breast cancer occurs when abnormal cells in the breast grow in an uncontrolled way.

Breast cancer can develop at any age. It is most common in women but also affects a small number of men each year.

Breasts are made up of lobules and ducts surrounded by fatty and connective tissue. Lobules produce breast milk and ducts carry milk to the nipple

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What does a breast look like on the inside?

Cancer is the name for a group of diseases that develop when the body’s cells grow in an uncontrolled way and spread into the body’s tissues. Breast cancer is cancer that starts in the breast tissue

Breasts are made up of lobules and ducts surrounded by fatty and connective tissue:

  • lobules produce breast milk
  • ducts carry milk to the nipple.

Illustration of a woman's breast including showing ductal carcinoma in situ & invasive breast cancer

Illustration of a woman's breast including showing ductal carcinoma in situ & invasive breast cancer

Close to the breasts are a number of lymph nodes. These are part of the lymphatic system and help to protect the body from infection. The closest lymph nodes to the breast are in the armpit (axillary nodes). There are also lymph nodes under the breastbone (internal mammary nodes) and in the neck (supraclavicular nodes).

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How does breast cancer develop?

Breast cancer can start in the ducts or lobules of the breast

Sometimes cancer cells stay in the ducts and lobules of the breast. This is called non-invasive breast cancer. If the cancer cells spread into the surrounding tissue, this is called invasive breast cancer. The site where the cancer starts is called the primary cancer.

Sometimes, breast cancer cells travel in the bloodstream or lymphatic system to other parts of the body such as the bones or liver. This is called metastatic breast cancer (or secondary or advanced breast cancer).

The lymphatic system

The lymphatic system is a network of tiny vessels that collect fluid and waste products from the body’s tissues.

This fluid is called lymph. Lymphatic vessels take the lymph to small glands called lymph nodes where substances that could be harmful to the body, such as bacteria or cancer cells, are trapped and removed. This helps to protect the body from infection. The lymph then passes back into the blood.

Lymph nodes are small, rounded glands that can range in size from about 1 mm to 25 mm.

There are lymph nodes all around the body, including the armpit, groin, stomach, chest and neck. The number of lymph nodes varies in different people. There are usually around 15–30 lymph nodes in the armpit.
 

Illustration of lymph nodes near the breast

Illustration of lymph nodes near the breast

Why are lymph nodes important in breast cancer?

The lymph nodes in the armpit (axilla) drain lymph fluid from nearby areas, including the breast.

The lymph nodes in the armpit are often the first place that cancer cells spread to outside the breast. About one in three women with breast cancer have cancer cells in the lymph nodes in their armpit when their breast cancer is diagnosed.

If there are cancer cells in the lymph nodes in the armpit, it may be possible to feel a lump in the armpit. However, many women are not aware of any changes. Cancer cells in the lymph nodes can usually only be seen under a microscope and cannot be felt or seen on X-rays or scans.

Lymph nodes can become enlarged for many reasons, including having a cold or an infection. If there is a lump in the armpit, this does not necessarily mean that cancer has spread to the armpit.