Breast cancer patients undergo radiotherapy to treat their disease, but it may put them at a higher risk of developing second primary lung cancer (SPLC).

These were the findings of a new study published in the Journal of Thoracic Oncology.

Previous research has shown that second primary cancer, especially esophageal cancer, can develop after a patient undergoes radiotherapy for cancer treatment.
Radiation exposure is known to be a risk factor for lung cancer, but until now it was unclear whether radiotherapy to treat breast cancer elevates a patient's risk of SPLC.

Researchers in Taiwan compiled data from the National Health Insurance Research Database on 986,713 patients in an East Asian country with medical events from 2000 to 2010.

A total of 7,408 women aged 18 or older with newly diagnosed breast cancer and no other types of cancer before 2000 were recruited for the study. Among them, 5,696 received radiotherapy and 1,713 didn't.

The researchers then evaluated their age, comorbidities, insurance premium, urbanization level, location, date of medical event and hospital care level.

They used the Kaplan-Meier curve analysis and log-rank test to determine the patients' cumulative risk for SPLC. The Cox regression analysis was then used to calculate the hazard ratios.

In the follow-up period, 128 patients in the radiotherapy group and four patients in the non-radiation group were diagnosed with lung cancer. In addition, the adjusted hazard ratio was 10.08 times higher in the radiotherapy group.

"The results of this study showed that radiation for breast cancer, which is an important treatment option, was correlated with a significant increase in the incidence of SPLC, particularly among older women or those with advanced-stage breast cancer," the researchers concluded.

The Wellcome Trust Sanger Institute cracked the code on how ionizing radiation can cause cancer. Research published in September found two mutational signatures for radiation damage that were independent of cancer type.

One of the signatures involves small amounts of DNA bases being cut out. For the other signature, the DNA is cut in two places, the middle piece spins around and then joins in the opposite direction.

The Taiwanese researchers recommend that clinicians carefully monitor patients who receive radiation to treat breast cancer in order to detect potential SPLC.

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