The breasts and lymphatic system

This page tells you about the structure of the breasts and about the lymphatic glands.

The breasts

The breasts are made up of

  • Fat
  • Connective tissue
  • Gland tissue divided into lobes

A network of ducts spreads from the lobes towards the nipple.

 Diagram showing the lobes and ducts of a breast

Breast size and density

It is very common for breasts to be different sizes from one another. They may also feel different at different times of the month – for example, just before a period they can feel lumpy.

Breast density changes as a woman ages. Younger women have more glandular tissue in their breasts, which makes them dense. Once a woman goes through menopause, the glandular tissue is gradually replaced by fat, which is less dense. It is more difficult to read a mammogram if the breast tissue is dense so mammograms are not as reliable for younger women.

Specialists have found that older women who take hormone replacement therapy (HRT) have denser breasts than would be expected for their age, which can make their mammograms less accurate.

Most women are aware of the common risk factors when it comes to breast cancer. However, many are not aware of the importance of breast density, which has been described as one of the most significant risk factors of all. Women with dense breasts have a higher proportion of fibroglandular and supportive tissue, whereas those with less dense breasts have a higher proportion of fat.

Unfortunately, breast density cannot be measured by look or feel, only by mammography, where dense breast tissue appears white and fatty breast tissue appears dark.

Breast density can be categorised into

  • Mostly fatty – accounts for less than 10% of women in Ireland
  • Scattered density – accounts for about 40% of women
  • Heterogeneously dense less than 40% of women and
  • Extremely dense less than 10% of women

Breast density is one of the strongest independent risk-factors for developing breast cancer, stronger even than age or family history, with women with ‘extremely dense’ breast tissue being four-to-six times more likely to develop breast cancer compared to women with ‘mostly fatty’ breasts. High breast density can also have the effect of ‘masking’ breast cancers, as they also show up as white on a mammogram, and are thus more difficult to see.

Breast density cannot be measured by look or feel, only by mammography, where dense breast tissue appears white and fatty breast tissue appears dark. Breast density can be categorised into ‘

10% of women between the ages of 40 – 74 have extremely dense breast, 35% have heterogeneously dense breast. These are the women who are at risk of a cancer not being picked up by mammogram as dense breast look white on a mammogram and cancer looks white on a mammogram.

Currently, women in Ireland going through the BreastCheck mammographic screening programme are not routinely informed about their breast density when receiving their results, and many are unaware of this important risk-factor.

For more information on breast density, please see –

  • Breast density advocate and breast cancer survivor Siobhain Freeney’s website here
  • Mayo Clinic website here
  • Medical Independent article here


Lymph nodes and the lymphatic system

Our bodies have a network of lymph vessels and lymph nodes which is part of the body’s immune system. It collects fluid, waste material, and other things such as viruses and bacteria that are in the body tissues, outside the bloodstream.

In the same way that veins collect and carry blood through the body, lymph vessels carry a watery yellow fluid called lymph. Lymph flows through the lymphatic system and eventually drains into veins. This system helps to get rid of waste products from the body.

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

Lymph nodes are important in cancer care because any cancer cells that have broken away from a cancerous tumour can be carried by the tissue fluid to the nearest lymph nodes. If your doctors finds a cancerous tumour, they will always examine your lymph nodes to see if there are cancer cells present there. If there are no cancer cells in the nearby lymph glands, your cancer is less likely to have spread. Sometimes lymph nodes become enlarged for other reasons that are not related to cancer e.g. infection.

Your breast tissue extends to under your armpit. The armpits have many lymph nodes, also known as lymph glands. There is also a chain of lymph nodes that runs up the center of your chest, by your breast bone. This is called the internal mammary chain. The diagram shows the network of lymph nodes around the breast.   Because your breast tissue extends to your collar bone and your armpit, it is important to check these areas during your monthly breast self-examination.