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Breast anatomy and how cancer starts

What are breasts made of?

The breast is highly complex. It goes through more changes than any other part of the human body – from birth, puberty, pregnancy and breastfeeding, right through to menopause.

Breast tissue extends from the collarbone, to lower ribs, sternum (breastbone) and armpit. Each breast contains 15-20 glands called lobes, where milk is produced in women who are breastfeeding. These lobes are connected to the nipple by 6-8 tubes called ducts which carry milk to the nipple.

The breast and armpit contain lymph nodes and vessels carrying lymph fluid and white blood cells. Much of the rest of the breast is fatty tissue.

Drawing of female breast anatomy showing the lymph nodes, nipple, areola, chest wall, ribs, muscle, fatty tissue, lobe, ducts, and lobules.
Drawing of female breast anatomy showing the lymph nodes, nipple, areola, chest wall, ribs, muscle, fatty tissue, lobe, ducts, and lobules.

How does cancer start in the breast?

The breast, like any other part of the body, consists of billions of microscopic cells. These cells usually multiply in an orderly fashion – new healthy cells continue to divide and replace the ones that have died.

However, sometimes cells develop abnormalities (mutations). This occurs when the genes that usually check that cells are replicating correctly fail to detect mutations. When this happens, abnormal cells continue to divide and multiply, sometimes growing quite rapidly.

At this stage, a growth may not be cancerous. It could be a “non-invasive tumour” which remains contained in the duct or the lobe.

A tumour is considered cancerous once it is able to invade surrounding tissue. These cancers require treatment because if they continue to grow and spread, they could become life-threatening.

Breast cancer usually begins in the ducts that carry milk to the nipple or within the smaller structures of the lobes. A small number of cancers start in other tissues of the breast; these are called sarcomas and lymphomas. Although many types of breast cancer can cause a lump in the breast, not all do.

How does cancer spread beyond the breast?

Breast cancer can spread when cells break away from the main tumour and are transported to other parts of the body. This can happen via the lymphatic system or the bloodstream.

Lymph vessels are like small veins, except that they carry a clear fluid called lymph away from the breast. Lymph contains tissue fluid and waste products, as well as immune system cells. Breast cancer cells can enter nearby lymph vessels and begin to grow in lymph nodes. Lymph vessels draw up the lymph fluid from the body and pump it towards the chest where it ultimately drains into the bloodstream.

If cancer cells have spread to one or more lymph nodes, there is a higher chance that they could have spread (metastasised) to other sites in the body. Usually, a surgical biopsy is needed to remove one or more lymph nodes and determine whether cancer is present.