What breasts are made of, and where and how cancer starts.

Breast tissue

Breast tissue is found in the breast, upper chest and the armpit. Each breast contains 15-20 glands called lobes, where milk is produced. These lobes are connected to the nipple by tubes called ducts. The structure of the lobes and ducts is a bit like the branches of a tree. Breast cancer usually begins within the lobes.

Much of the rest of the breast is fatty tissue. The breast and armpit also contain lymph nodes and vessels carrying lymph fluid, which are part of the immune system. Breast cancer can sometimes spread to other areas of the body through this lymph system or blood vessels.

It’s quite normal for breasts to differ slightly in shape and size and vary during periods and pregnancy. They also change as we get older, for example by becoming softer. Being breast aware means getting to know how your breasts look and feel normally, so you can look out for any unusual changes and get them checked by your doctor.

The female breast

How do cancers start?

Our bodies are made up of trillions of cells. You have different cells for different parts of your body – for example skin cells look and work differently from liver cells. Each cell contains DNA, which instructs the cell on how to look and behave.

Sometimes, our bodies need to make new cells to replace old ones or repair damage. To do this, an existing cell makes an extra copy of all its DNA and then splits into two, with each of the new cells receiving a complete set of DNA instructions. This process can happen many times over and many cells can be doing this at any one time.

Normally when a cell splits, a number of checks happen to make sure that the DNA has been copied correctly and the new cells have everything they need to work properly. If a mistake is found, the cells die.

However, if these safety checks fail, the new cells may survive with mistakes in them. Often, a mistake in DNA (known as a mutation) does not cause any problems. However, sometimes the mistake can make the new cells behave strangely, splitting into abnormal cells at a fast rate, forming a tumour.

Not all tumours are cancerous. If the tumour cells don’t have - and are very unlikely to gain - the ability to grow into neighbouring tissue, they are called benign tumours. Benign tumours often don’t need treatment and are not cancer.

However, if the tumour cells have the ability to invade neighbouring tissues, they are cancerous. Cancers are usually treated, otherwise they may grow and spread, which can be life-threatening.

Our genes, our lifestyle and our surrounding environment can all affect our risk of developing breast cancer.

How does breast cancer grow and spread?

If untreated, breast cancers can grow bigger, taking over more surrounding breast tissue. Breast cancer that has not spread beyond the breast or armpit is known as primary, or early, breast cancer. However, sometimes breast cancer cells break away from the original cancer and enter the blood or lymph vessels. Travelling though these vessels, cancer cells may settle in other areas of the breast or in the lymph nodes of the breast tissue, forming new tumours. They may also spread to other areas of the body where they can form new tumours, called secondary breast cancer.

Breast cancer treatments

There are many treatments for primary breast cancer and these often successfully get rid of the disease. There are also treatments for secondary breast cancer that help to control the disease and slow its growth and spread.

More information

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Breast Cancer Now’s health information is produced following best practice guidelines developed by the Patient Information Forum. 

Find out more about how we develop our health information and the Patient Information Forum.

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