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.
Breasts are made of fat, connective tissue, and gland tissue divided into lobes.
A network of ducts spreads 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 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.
After the menopause the glandular tissue is gradually replaced by fat, which is less dense.
Where it starts
Breast cancer most commonly starts in the cells that line the ducts of the breast.
9 out of 10 of these cancers have no special features when the cells are looked at under the microscope. They are called invasive breast cancer (NST). NST stands for No Special Type.
Around 1 in 10 breast cancers (10%) are invasive lobular carcinoma. This means that the cancer started in the cells that line the lobules of the breast.
There are other rarer types of breast cancer.
Nearby 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 released from the breast tissue can be trapped in the nearby lymph nodes.
The diagram shows the network of lymph glands 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 2015, around 54,800 women and around 370 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. In 2015, around 55,100 people were diagnosed with breast cancer 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.