Breast cancer is the most common cancer in women. It accounts for 15 percent of new cancer cases every year. Four out of five breast cancer cases are diagnosed in women over the age of 50. The reason for this, according to a recent study, is that breast tissue changes as women age in ways that raise the risk of cancer.

Scientists at the Universities of Cambridge and British Columbia analyzed normal breast tissue from 527 women between the ages of 15 and 86. The team mapped more than three million cells using a technique called imaging flow cytometry to study the expression of 40 proteins in each tissue sample.

The researchers were able to track changes in breast tissue over time by combining these images with information about the immune cells present in breast tissue, hormone receptors on the surface of cells, and the tissue structure itself.

Breast tissue changes as women age, especially after menopause.

When cells divide and replicate, they accumulate mutations that can drive cancer. “But why can the body get rid of these mutated cells when we are younger, but struggle to do so later in life?” said Pulkit Gupta, co-first author and a researcher with the Cancer Research Institute UK at the University of Cambridge, in a statement.

Women in their twenties also experience changes in their breast tissue, he added, but these are more likely related to pregnancy and childbirth, and they are not as pronounced. The researchers found that as women age breast tissue changes, with the most dramatic changes occurring during menopause.

All the cells in breast tissue decrease in number and divide less often as women age, but the researchers reported that breast tissue in younger women has more B and active T cells, which help identify and kill cancer cells. In aging breast tissue, these immune cells are replaced by macrophages and CD8+ T cells that are associated with inflammation and offer less protection against cancer.

Why the population of immune cells in breast tissue changes is not yet clear. “We think it may be because breast milk contains a high concentration of immunoglobulins to help boost infants' immunity, and these are produced by B cells,” Raza Ali, co-senior author on the study and a researcher at the Cancer Research Institute, said in a statement.

Breast tissue in younger women has more B and active T cells, both of which help identify and kill cancer cells. In older women, these immune cells are replaced by cells that are associated with inflammation and offer less protection against cancer.

The number of fat cells also increases while the number of blood vessels decreases.

The cells in breast tissue interact with each other less often in older women than in younger women. As women age, immune cells and stromal cells, the cells that give the tissue its structure, grow further away from the epithelial cells lining the milk ducts and milk-producing lobules. These changes in breast tissue may make it easier for precancerous cells to grow and spread in breast tissue.

“Age-dependent changes in estrogen activity occur strongly in milk-producing cells in the breast. Now we can see the changes in all cell types that come with age,” co-author Samuel Aparicio, a professor of pathology and laboratory medicine at the University of British Columbia, explained.

To reduce their risk of developing breast cancer, the American Cancer Society advises postmenopausal women to avoid hormone therapy and consider nonhormonal options. The organization also recommends that women breastfeed their babies for several months since breastfeeding may reduce their cancer risk.

The study is published in Nature Aging.