Women who have radiotherapy for breast cancer have a small but significantly increased risk of subsequently developing a primary lung tumor, and now research has shown that this risk increases with the amount of radiation absorbed by the tissue. This research was presented at the meeting of the European Society for Radiotherapy and Oncology (ESTRO33) in Vienna, Austria, held April 4-8, 2014.

“We found that for each Gray delivered to the lung as part of radiotherapy for a breast tumor, the relative risk of developing a subsequent primary lung cancer increased. This increased risk was similar to the reported increased risk of heart disease after radiotherapy for breast cancer,” said Trine Grantzau, MD, of Aarhus University Hospital in Denmark.

“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. With the advances in breast cancer treatment and the introduction of breast cancer screening, a growing number of women are becoming long-term survivors, and so 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.”

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Grantzau and her colleagues investigated the incidence of second primary lung cancers (i.e., a new lung cancer and not a secondary tumor that has spread from the original breast cancer) in a group of 23,627 women in Denmark who had been treated with postoperative radiotherapy for early breast cancer between 1982 and 2007. Among this large group of women, 151 (0.6%) were diagnosed with a new lung cancer (the group of cases) and they were matched with 443 women who had not developed lung cancer (the control group).

Grantzau explained that this study examined if a dose-response correlation existed for second primary lung cancer after breast cancer irradiation, estimated excess relative risk per delivered Gray to the lung, and the effect of radiation and smoking. The researchers estimated the amount of radiation that was delivered to the part of the lung where the subsequent tumor developed and determined the patients’ smoking status.

The median age of the women when they were diagnosed with breast cancer was 54 (range, 34-74 years) and the median age when a second primary lung cancer was diagnosed was 68 (range, 46-90 years). They found that 70% of the lung cancers were diagnosed 5 or more years after radiotherapy for breast cancer, ranging from 5 to 26 years. The majority (91%) of the lung cancer cases were smokers, whereas 40% of the controls were smokers.

Although the absolute risk of developing a second lung cancer is small, the researchers showed that among women who had survived breast cancer for at least 5 years, the relative risk of subsequently developing a lung cancer increased by 8.5% per delivered Gy to the lung.

“These results show that the risk of second lung cancer after radiotherapy in early breast cancer patients is associated with the delivered dose to the lung,” said Grantzau.