Swedish researchers have found that women aged 50 and over with denser breasts – having high percentage of dense tissue – had nearly double the risk of their cancer recurring, either in the same breast or in the surrounding lymph nodes, than women with less dense breasts.
Dr Louise Eriksson and her colleagues from the Karolinska Institutet (Stockholm, Sweden) warn that doctors should take breast density into account when making decisions about treatment and follow-up for these women.
When a woman has a mammogram, the resulting scan gives an image of the breast that shows areas of white and black. The white areas represent the dense tissue, made up of the epithelium and stroma. The black areas are made up of fatty tissue, which is not dense. The percentage density (PD) of the breast is calculated by dividing the dense area by the area of the whole breast (dense and non-dense tissue included).
Breast density varies from woman to woman, and it also decreases with age.
“Density can vary greatly, even between postmenopausal women. In the group of women I studied, those with the lowest percentage density had breasts that were less than one percent dense, whereas those with highest PD had 75-80 percent dense breasts,” Dr Eriksson explained.
“The mean average PD was 18 percent. However, density does decrease with age. Studies have shown a decrease by approximately two percent per year. The largest decrease is seen at menopause when PD decreases by approximately 10 percent,” she stated.
The researchers studied the mammograms and outcomes for 1,774 post-menopausal women who were aged 50-74 and who were part of a larger study of all women with breast cancer diagnosed between 1993-1995 in Sweden.
“We found that if you have a PD at diagnosis of 25 percent or more, you have an almost two-fold increased risk of local recurrence in the breast and surrounding lymph nodes than women with a PD of less than 25 percent,” said Dr Eriksson, who is a PhD student at the Karolinska, as well as a physician at the Stockholm South General Hospital.
“However, density does not increase the risk of distant metastasis and has no effect on survival. We also see that although mammographic density is one of the strongest risk factors for breast cancer it doesn’t seem to influence tumour development in any specific way; for instance, it isn’t more associated with oestrogen receptor positive tumours than oestrogen receptor negative tumours, but seems to act as a general stimulator of tumour development,” she noted.
Cancer researchers do not know why breast density is a risk factor for breast cancer.
“It could simply be due to higher density being associated with more cells, which means that more cells are at risk of developing cancer,” Dr Eriksson said.
“Another hypothesis is that it is the relationship between mammographic density and the stroma (which has been shown to be the main compound of mammographic density) that is central to the increase in risk.
“There is no leading hypothesis on why an increased amount of stroma would increase breast cancer risk, but it is known that the interaction between the epithelial cells and the stroma is crucial to the development of breast cancer.
“Based on the results from our study, we propose that mammographic density creates a beneficial environment for epithelial cells to transform into cancer cells; much like fertile soil giving a planted seed the needed nutrients to grow and develop,” she added.
The results were presented at the eighth European Breast Cancer Conference (EBCC-8) in Vienna.