Radiotherapy for breast cancer is much more likely to trigger fatal lung cancer or heart attacks in smokers, a study has found.
Women who smoke are 10 times more likely to die from the conditions than non-smokers given the same treatment, the research shows.
For non-smokers, the long-term risk of dying from lung cancer or a heart attack as a result of undergoing breast cancer radiotherapy is just 0.5%.
But for smokers, the risk level increases to 5%, investigators discovered.
Dr Carolyn Taylor, radiation oncologist and lead author from the University of Oxford, said: "For non-smokers, the absolute risk of death from the side effects of modern radiotherapy is only about 0.5%, which is much less than the benefit.
"But for smokers, the risk is about 5%, which is comparable with the benefit.
"Stopping smoking at the time of radiotherapy will avoid most of the lung cancer and heart disease risk from radiotherapy, and has many other benefits."
These findings are from the Early Breast Cancer Trialists' Collaborative Group study which looked at the risks of radiotherapy in 40,781 women worldwide.
Radiotherapy is an important treatment for breast cancer which can reduce the chances of dying from the disease.
However, experts warned that for some long-term smokers the risks may outweigh the benefits.
Dr Julie Sharp, from the charity Cancer Research UK, which funded the study, said: "This research highlights that breast cancer patients who smoke need to be offered help and support in order to try and quit to minimise any risks from their treatment.
"It's important to remember that modern day radiotherapy techniques have been refined and improved to make sure it is targeted and effective while reducing the risk of side effects."
The research is reported in the Journal of Clinical Oncology.