(KRON) -- For decades, scientists have been scratching their heads as to why women with dense breasts have a higher risk of developing breast cancer, and a new UCSF study may have found part of the answer. Doctors previously discovered that denser breasts make it harder to interpret mammograms beca use it can obscure a tumor, but that did not explain the higher risk of developing it biologically. The study found that cells in dense breast tissue attract immune cells which can release chemicals that damage neighboring breast cells' DNA, increasing cancer risk. "We found that dense breast tissue is mechanically stiffer, and that these physical changes can trigger inflammation that leads to DNA damage in nearby breast cells," said Mary Kate Hayward, the paper's first author and a postdoctoral researcher at UCSF. Specifically, the study examined breast tumor samples from UCSF patients, measuring their stiffness and collagen levels. They found that collagen, the most abundant protein in the body, created more stiffness in the tumors. The stiffest part of the tumors had the telltale signs of cancer, such as DNA mutations, fibroids, and most importantly, macrophages. Macrophages can destroy damaged cells and contribute to the development of cancer if not working properly. San Francisco-based wearables company Oura proposes multi-billion-dollar IPO The key finding was that breast cells can sense the stiffness of tumors and then activate a protein signal called STAT3, which recruits those macrophages into the area. The study found that the breast cells activate a signal to attract an immune cell into a stiff tissue. That immune cell releases a chemical called ROS, which then creates another chemical that can travel from the breast tissue to the breast cell and thus damage the DNA. If those DNA mutations accumulate, then cancer cells can develop. This undated fluorescence-colored microscope image made available by the National Institutes of Health in September 2016 shows a culture of human breast cancer cells. (Ewa Krawczyk/National Cancer Institute via AP) Hayward said that women with dense breasts shouldn't necessarily be concerned, but aware of the increased risk. "Having dense breasts does not mean that you will develop breast cancer," said Hayward, adding that about half of women at screening age have dense breasts, "the broader message we want to encourage is for women to know their individual breast cancer risk, and breast density is one part of that picture." Hayward said that breast density is largely genetic but is also influenced by factors such as age and hormones. Unvaccinated passenger with measles traveled through Alameda County: CDPH While researchers are still researching the connection between breast tissue and lifetime breast cancer risk, this research is a big step forward. "Once we understand the mechanisms behind breast density as a risk factor, we can start asking whether there are ways to interrupt these processes and ultimately reduce the associated risk in women who may benefit most. That’s what we're excited to do next," said Hayward. The FDA now requires health care providers to inform women about their breast density after a mammogram. Hayward said knowing your overall risk early can help you and your healthcare provider develop the most appropriate breast screening strategy over your lifetime, including when to screen, how frequently, and whether different forms of imaging may be appropriate. ...read more read less