What is breast cancer?

Breast cancer is when abnormal cells in the breast begin to grow and divide in an uncontrolled way and eventually form a growth (tumour).

Breast cancer starts in the breast tissue, most commonly in the cells that line the milk ducts of the breast. It is the most common cancer in the UK. It mainly affects women, but men can get it too. 

The breasts

Breasts are made up of:

  • fat
  • glandular tissue divided into lobes (each containing lots of lobules)
  • a network of ducts or milk ducts
  • connective tissue

The ducts spread from the lobes towards the nipple.

Diagram showing detail in the lobe

Breast size and density

One breast is usually smaller than the other. Your breasts may feel different at different times in the month. It is common for breasts to feel lumpy just before your period.  

Younger women have more glandular tissue than fat in their breasts, making them dense.

Your breasts also change during pregnancy. You may notice that they become bigger and feel more tender.

After your periods stop (menopause) fat gradually replaces the glandular tissue, which is less dense.

Where it starts

Breast cancer most commonly starts in the cells that line the ducts of the breast.

Around 70 out of 100 (around 70%) of breast cancers have no special features when the cells are looked at under the microscope. They are called invasive breast cancer no special type (NST). Invasive means the cancer cells have spread outside the ducts and into the surrounding breast tissue. 

Almost 15 in every 100 breast cancers (almost 15%) are invasive lobular carcinoma. This means that the cancer started in the lobules of the breast and spread outside the lobules into the surrounding breast tissue.


Lymph nodes

There is a network of lymph glands (also called lymph nodes) close to the breast. They are part of the lymphatic system that runs throughout the body. The lymph nodes and lymph vessels contain a yellow fluid called lymph that flows through the lymphatic system. It collects waste products and drains into veins for the waste to be removed. 

Cancer cells that have broken away from the breast tissue can be carried by the lymph fluid to nearby lymph nodes. If you have cancer, but no cancer cells in any of your lymph glands, your cancer is less likely to have spread.

The diagram shows the network of lymph glands around the breast.

Diagram showing the network of lymph nodes in and around the breast
Lymph nodes around the breast

The lymph glands in the armpit (axilla) are called the axillary lymph glands. There is also a chain of lymph nodes that runs up the centre of the chest, close to the breastbone. This is called the internal mammary chain.

Who gets it

Breast cancer is more common in women than men. In 2017, around 54,700 women and around 390 men were diagnosed in the UK. 

1 in 7 women in the UK develop breast cancer during their lifetime. It is more common in older women. 

Breast cancer risk can be affected by age, family history and lifestyle factors such as obesity and smoking.

How common it is?

Breast cancer is the most common cancer in the UK. Around 55,200 people are diagnosed with breast cancer every year in the UK. That is around 150 people a day. 

15 out of 100 (15%) of all newly diagnosed cancers in the UK are breast cancer.