What is breast cancer? 

Breast cancer occurs when abnormal cells in the breast grow in an uncontrolled manner. It occurs in both men and women, but women are at greater risk due to their breast development and lifelong exposure to oestrogens. Cells that grow abnormally can form tumours. Breast cancer occurs when breast tumours spread. There is generally a long period between breast tissue changes and the development of breast cancer.

How does breast cancer start ?

DNA plays a central role in developing all cancers, including breast cancer. Like other cells in the body, breast cells contain DNA which provides instructions for cell growth and division. When a cell divides, its DNA is copied precisely. But occasionally, a mistake can cause a genetic change known as a “mutation”. This may then be passed on to other cells, leading to abnormal cell growth. Over time mutations accumulate, and their combined effects lead to a malignant (cancerous) breast tumour. The more often a cell divides, the greater the risk of mutations occurring and accumulating.

Breast anatomy


The breast is made up of the following main components: 

  • Lobules are part of the glandular system; they are glands that produce breast milk. Lobules are found in groups which together form a lobe. 
  • Ducts are small tubes that carry breast milk from the lobules to the nipple. 
  • The nipple is where breast ducts come together to allow for milk ejection through a small opening in the skin. 
  • Fat (adipose) and connective tissue (stroma) are supportive components of the breast that surround the ducts and lobules. 
  • Blood vessels (not shown) allow for a constant supply of blood and nutrients to the breast to keep the tissue alive.  
  •  Lymph vessels and nodes are part of the immune system and transport lymph fluid containing various immune cells. It functions to help fight any infections and take away waste materials from the breast. In breast cancer, cancerous cells can enter the lymph vessels and start to grow in the lymph nodes. This is known as lymph node involvement 

Breast cancers most commonly arise in the ducts that transport milk from the lobules to the nipple. These are known as ductal cancers. Some breast cancers can develop in the lobules, which are known as lobular cancers.  

There are other places breast cancer can arise, but these are rarer and can be read about in the ‘What are the most common types of breast cancer?’ section below.  

What are the most common types of breast cancer?

Breast cancer is a diverse group of diseases. It may be invasive, which means the cancer cells have spread outside the ducts or lobules into the surrounding tissue or non-invasive (also known as in situ), meaning the cancer cells have not yet spread into the surrounding tissues.  

Non-invasive breast cancer is an early form of breast cancer, which remains localised to the ducts or lobules of the breast. The most common form is ductal carcinoma in situ (DCIS); if left untreated DCIS may become invasive.  Another common type of non-invasive breast cancer is lobular carcinoma in situ (LCIS). This type of carcinoma is not cancerous (so does not spread), although its presence is correlated with an increased chance of developing cancer at a later stage.    

The most common types of invasive breast cancer include:    

  • Invasive breast cancer of no special type (NST), formally known as invasive ductal carcinoma, where cancer cells have grown through the lining of the milk ducts into surrounding breast tissue. It is called NST as when cells are viewed under a microscope, they have no special features. It accounts for around 70% of all invasive breast cancers.    
  • Invasive lobular breast cancer is where cancer cells in the lobules (milk-producing glands) have spread into surrounding breast tissue. It accounts for around 15% of all invasive breast cancers.    
  • Inflammatory breast cancer is a rare, fast-growing type of breast cancer caused by breast cancer cells blocking the small lymph channels in the breast and skin. In this type of breast cancer, the skin of the breast looks red and inflamed. It accounts for around 1-5% of invasive breast cancers.    
  • Paget’s disease of the breast is a rare type of breast cancer that causes change to the skin of the nipple, similar to eczema.  
  • Hormone and HER2 receptors. The breast cancer subtypes listed above can also be further broken down based on their expression of specific proteins which can help inform treatment regimes. 

Breast cancers are usually tested for the presence of hormone receptors for oestrogen (ER+) and progesterone (PR+), and also the protein human epithelial growth factor 2 (HER2). Tumours positive for oestrogen and progesterone receptors are known as hormone receptor-positive (HR+). More information on exactly what this means can be found below.

Breast cancers that are positive for oestrogen (ER+) and/or progesterone (PR+) receptors are stimulated to grow by the binding of these hormones to their receptors. Meanwhile, breast cancers that are HER2 positive have higher than normal levels of this protein which promotes tumour growth. Around 80% of breast cancers are oestrogen receptor-positive.

If a breast cancer is positive for oestrogen, progesterone or HER2 receptors, it means they can be treated using therapies that target these hormones and proteins to help slow the growth of the tumour (for example by lowering their levels or blocking their effects). 

A breast cancer that does not have oestrogen, progesterone or HER2 receptors is known as triplenegative breast cancer. It is more common in younger women and tends to be more aggressive and more difficult to treat.Around 15% of breast cancers are triple-negative. 

What “causes” breast cancer ?

There is no simple cause of breast cancer. A variety of risk factors come together to make you more, or less, susceptible. Some of these risk factors are inherited, some are incurred throughout your life and others are present in the environment in which you live.

Naturally occurring oestrogens (and some other hormones in our bodies) can influence the risk of developing breast cancer, mainly because of their ability to increase rates of cell division and promote the growth of oestrogen-responsive tumours.

Modern-day living is increasing our risk of getting breast cancer, with those born in earlier generations or in less well-developed regions having a lower risk of developing the disease.

For more information visit our susceptibility page.

Last Review: June 2023

Next Review: June 2024

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