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 most commonly starts 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 lobules produce milk for breastfeeding and connect to the ducts. The ducts spread from the lobes towards the nipple.
Breast size and density
One breast is usually smaller than the other. Your breasts may feel different at different times of 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.
Lymph nodes
There is a network of lymph glands close to the breast. These are also called lymph nodes. They are part of the 
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 them into veins.
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 below shows the network of lymph glands around the breast.
The lymph glands in the armpit are called the axillary lymph glands. There are also lymph nodes in the centre of the chest, close to the breastbone. These are called the internal mammary chain.
Where breast cancer starts
Breast cancer can start in different parts of the breast. Most commonly it starts in the cells that line the ducts of the breast. This is invasive breast cancer or invasive ductal carcinoma. Invasive means the cancer cells have spread outside the ducts where they started and into the surrounding breast tissue.
They can also start in the lobules of the breast. This means that the cancer has spread outside the lobules and into the surrounding breast tissue. This is invasive lobular breast cancer.
There are also other rarer types of breast cancer.
Ductal and lobular carcinoma in situ
This means that some cells in the lining of the ducts or lobules have started to turn into cancer cells. But they have not started to spread into the surrounding breast tissue.
Who gets it?
Breast cancer is more common in women than men. Around 55,500 women and around 370 men are diagnosed in the UK each year.
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. This includes obesity and smoking.
How common it is?
Breast cancer is the most common cancer in the UK. Around 55,900 people are diagnosed with breast cancer every year in the UK. That is more than 150 people a day.
15 out of 100 (15%) newly diagnosed cancers in the UK are breast cancer.
