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.
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.
The breast is made up of the following main components:
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.
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:
Breast cancers are usually tested for the presence of hormone receptors for oestrogen and progesterone, and also the protein human epithelial growth factor 2 (HER2). Tumours positive for these factors are known as hormone receptor positive. More information on exactly what this means can be found below.
Breast cancers that are positive for oestrogen and/or progesterone 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 70-75% 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 triple–negative 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.
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.
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Last Review: June 2023
Next Review: June 2024
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