How cancer develops in the breast

‘Cancer’ is the name for a group of diseases that develop when the body has collections of abnormal cells.

Normally, cells in our bodies grow and divide in a controlled way. Sometimes, this process gets out of control and the cells become damaged or abnormal. When this happens, they can spread into surrounding tissues or other parts of the body. 

Breasts are made up of small sacs and tubes:

• lobules (in women, these produce breast milk)
• ducts (in women, the ducts carry milk to the nipple)

Other tissue surrounds these sacs and tubes.

Everyone has breast tissue, but the ducts and lobules have no real function in men. 

Breast cancer usually starts in the ducts or lobules of the breast. In ‘invasive’ breast cancers, the cells containing cancer may (or may not) have spread to:

• the breast tissue around the ducts and lobules
• lymph nodes in the breast or armpit
• other parts of the body. 

In non-invasive breast cancers, the cells stay within the breast.

Primary and secondary breast cancer

The place in the body where the cancer starts is the ‘primary’ cancer. Cancer that begins in the breast is called breast cancer.

Over time, the cells may grow out of control and spread outside the breast and local lymph nodes. If the cells spread into surrounding breast tissue and other parts of the body, the cancer changes from ‘primary’ to ‘secondary’.

Secondary cancer is also called ‘advanced’ or 'metastatic' breast cancer.

About cancer cells and receptors

All cells in our body have proteins called ‘receptors’ in them or on them. These receptors look for substances in the blood so they can grow and reproduce. Abnormal cells grow and reproduce uncontrollably. They form tumours and potentially spread to other parts of the body.

Breast cancer has multiple subtypes. These are determined by the receptors that sit on or in the cells and control how the cells behave.

• Hormone receptor-positive ('HR+') breast cancer
• HER2-positive ('HER2+') breast cancer
• HER2-low breast cancer
• Triple negative breast cancer (breast cancer that is ER/PR/HER2-negative)
• Triple positive breast cancer (breast cancer that is ER/PR/HER2-positive)

Find out about Types and subtypes of breast cancer.