Study: Dense breast tissue doubles risk of breast cancer recurrence
Women with a high percentage of dense breast tissue are at a greater risk of breast cancer recurrence and density should be taken into account during screening and when making treatment decisions, according to research presented March 21 at the 8th European Breast Cancer Conference (EBCC-8) in Vienna.

Marie-Louise Eriksson, PhD candidate at Karolinska Institutet in Stockholm, and colleagues found that women with a percentage breast density (PD) of 25 percent or more 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.

"Density can vary greatly, even between postmenopausal women,” said Eriksson. “In the group of women I studied, those with the lowest percentage density had breasts that were less than 1 percent dense, whereas those with highest PD had 75 to 80 percent dense breasts. The mean average PD was 18 percent.” Density decreases with age, added Eriksson, with studies showing a decrease of approximately 2 percent per year.

Results were based on a study of mammograms and outcomes for 1,774 post-menopausal women who were aged 50 to 74 and who were part of a larger study of all women with breast cancer diagnosed between 1993 and 1995 in Sweden.

Researchers are unsure why breast density is a risk factor for breast cancer. Eriksson suggested two possibilities, the first being that density means more cells, which means more cells at risk of developing cancer. The second hypothesis is that there’s a relationship between mammographic density and the stroma, because it is known that interaction between the stroma and epithelial cells is crucial to the development of cancer, she explained.

While density may be “fertile soil” for recurrence, according to the study abstract, density does not increase the risk of distant metastasis or have an 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 tumor development in any specific way; for instance, it isn't more associated with estrogen receptor positive tumors than estrogen receptor negative tumors, but seems to act as a general stimulator of tumor development," said Eriksson.

Knowledge of breast density should be taken into account both during screening and after diagnosis, said Eriksson. Density decreases the sensitivity of mammograms and women with dense breasts may need to have follow-up routines adjusted during adjuvant treatment to quickly detect recurrence.

The study is also notable for being one of the largest to date to study mammographic density, tumor characteristics and prognosis, as it included nearly half of all Swedish breast cancer diagnoses between 1993 and 1995.