Breast cancer patients who smoke have a much higher risk of developing lung cancer or having a heart attack than those who don’t smoke or who quit before radiotherapy, according to a study.

The risk that non-smokers who receive radiotherapy will die of lung cancer or a heart attack is only 0.6%. It increases to 5% in long-term smokers, however, the research showed.

The findings suggest that the risk of radiotherapy for long-term smokers may outweigh the benefit of lower breast cancer mortality.

But if patients stop smoking before radiotherapy, they can substantially reduce their risk, the researchers said.

The study, “Estimating the Risks of Breast Cancer Radiotherapy: Evidence From Modern Radiation Doses to the Lungs and Heart and From Previous Randomized Trials,” was published in the Journal of Clinical Oncology.

Radiotherapy is an important treatment for breast cancer, reducing patients’ likelihood of dying. But because it targets the chest region, patients may be at increased risk of developing diseases stemming from damage to the lungs or heart.

The Early Breast Cancer Trialists’ Collaborative Group looked at the records of 40,781 breast cancer patients who were included in 75 randomized trials of radiotherapy. Their objective was to investigate the doses and risks of radiotherapy to the lungs and heart.

The trials began before 2000. They randomized women with early-stage breast cancer or ductal carcinoma in situ (DCIS) to receive either radiotherapy, no radiotherapy or extra surgery.

Breast cancer radiotherapy has evolved since the trials to do a better job of sparing the lungs and heart. So the research team also examined recent literature to account for advances in radiotherapy.

“For non-smokers, the absolute risk of death from the side effects of modern radiotherapy is only about 0.5 percent, which is much less than the benefit. But for smokers, the risk is about 5 percent, which is comparable with the benefit,” lead author Dr. Carolyn Taylor, a radiation oncologist at the University of Oxford, said in a press release.

But the team found that quitting smoking before radiotherapy decreased the risk of dying of lung cancer by 3.1 percent, making the treatment beneficial.