New research shows that breast cancer patients who undergo radiotherapy are at an increased risk of subsequently developing a lung tumor, underscoring the importance of limiting radiation exposure to a minimum during the life-saving therapy.

"We found that for each Gray [unit of radiation] delivered to the lung as part of radiotherapy for a breast tumor, the relative risk of developing a subsequent primary lung cancer increased,” Dr. Trine Grantzau, a researcher at Aarhus University Hospital in Denmark and co-author of the new study, explained in a press release. “This increased risk was similar to the reported increased risk of heart disease after radiotherapy for breast cancer.”

The study, presented at the European Society for Radiotherapy and Oncology in Vienna, surveyed 23,627 Danish women who had been treated with post-operative radiotherapy for early breast cancer between 1982 and 2007. About 0.5 percent, or 151 women, would go on to develop a primary lung tumor. The researchers then evaluated the amount of radiation these women had received in relation to the amount received by a control group who didn’t develop a new cancer.

Grantzau and colleagues found that the patients who developed lung cancer received on average 50 percent more radiation than the control group. Though small, the risk appeared to increase by 8.5 percent per radiation unit delivered to the tumor site. Seventy percent of the subsequent cancers were diagnosed five or more years after the initial round of radiation therapy.

Breast Cancer and Radiotherapy

According to the National Institutes of Health (NIH), breast cancer is currently the second most deadly cancer among women, affecting over 200,000 and killing 40,000 each year. It is estimated that one in every eight women will develop the disease at some point in her life.

As advances in cancer research lead to more effective treatments, more patients will survive longer — however, this also means a higher incidence of long-term side effects of therapy, the researchers said.

"Our findings suggest that any reduction in the dose of radiation to the lung would result in a reduction in the risk of radiation-induced subsequent lung cancers,” Grantzau told reporters. “We need to have an increased awareness of treatment-induced second cancers and take steps to reduce those risks by using radiotherapy techniques that spare normal tissue as much as possible."