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Breast anatomy and how cancer starts

What are breasts made of?

The breast is highly complex and goes through the most changes of any other part of the human body – from birth, puberty, pregnancy and breast feeding through to menopause. Most changes in the breast are related to hormonal produced in the breast at different stages in a woman’s life.

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 also contain lymph nodes and vessels carrying lymph fluid and white blood cells, which are part of the immune system. 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.

Boys and girls begin life with similar breast tissue. However, at puberty high testosterone and low estrogen levels stop breast development in men.

How does cancer start in the breast?

The breast, like any other part of the body, consists of billions of microscopic cells with specific functions. 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, instead of abnormal cells dying, they continue to divide and multiply, sometimes growing quite rapidly.

Still this growth may not be cancerous; it could be what’s called a non-invasive tumour where it remains contained in the duct or in the lobe. Some breast lumps are not cancer, such as cyst or fatty tumours which are benign.

A tumour is considered cancerous if it is able to invade surrounding tissue. These cancers might 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 smaller structures of the lobes. Although many types of breast cancer can cause a lump in the breast, not all do. A small number of cancers start in other tissues of the breast; they are called sarcomas and lymphomas and are not really thought of as breast cancers.

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 via the lymphatic system and the bloodstream.

Lymph vessels are like small veins, except that they carry a clear fluid called lymph (instead of blood) 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 the cells could have spread (metastasised) to other sites in the body. Usually, a surgical biopsy to remove one or more lymph nodes will be needed to know whether the cancer is present. In Australia, there is no standard blood test for detecting whether breast cancer has spread.

How is research helping?

  • Tracing breast cancer back through its family tree to pinpoint the exact place and time it pops into existence is still a work in progress. If, as many researchers suspect, the root cause of breast cancer is failure to control cell division in progenitor cells (similar to stem cells), then understanding more about the fundamental workings of these cells, like what processes drive them to divide, could provide new ways to treat cancers in all its forms. NBCF has funded Dr Samantha Oaks who is seeking to identify and understand progenitor cells in the breast and their role in the first stages of breast cancer development.