Women who undergo radiotherapy treatment for breast cancer have an increased risk of going on to develop lung cancer, a new study has found.

According to Danish scientists, the subsequent risk of developing lung cancer is small but significant. Furthermore, the risk increases the more radiation is absorbed during treatment.

"We found that for each Gray [1] delivered to the lung as part of radiotherapy for a breast tumour, the relative risk of developing a subsequent primary lung cancer increased," the scientists said.

Gray is the measurement of radiation used in radiotherapy.

The team from Aarhus University Hospital looked at the incidence of second primary lung cancers, i.e. a second type of cancer that is new, not a secondary tumour that has spread from the original breast tumour. The study involved over 23,000 women who had been treated for breast cancer with radiotherapy between 1982 and 2007. Over 150 of these went on to develop a new lung cancer.

A previous study involving the same women and 22,000 women who had not undergone radiotherapy found that one in every 200 women treated with radiotherapy was at risk of developing a second lung cancer.

"In the current study, we wanted to see if there was a dose-response correlation for second primary lung cancer after breast cancer irradiation," the scientists said.

They found that while the absolute risk of going on to develop lung cancer was small, this risk increased by 8.5% per delivered Gray to the lung. In other words, the more radiotherapy the women received during treatment for breast cancer, the higher their risk of going on to develop lung cancer.

"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. 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.

"The challenge for radiation oncologists is to reduce the delivered dose of radiotherapy in a way that minimises the dose to the normal tissue to avoid radiation-induced malignancies, without compromising its efficacy in the cancerous breast tissue," the scientists commented.

However, they also emphasised that doctors should be ‘continually advising' their breast cancer patients to quit smoking in order to reduce their overall risk of lung cancer, adding that the risk of developing tobacco-related lung cancer is much higher than the risk of developing radiation-induced lung cancer.

These findings were presented at the 33rd conference of the European Society for Radiotherapy and Oncology in Vienna.