Breast cancer patients who continue to smoke after having radiotherapy are at much higher risk of their treatment causing them lung cancer in the future, research has found.

Two in three of those diagnosed with early breast cancer in the UK are given radiotherapy. It is a long-established and highly effective treatment but does also carry potential side-effects.

Radiotherapy cuts the risk of someone’s breast cancer recurring after treatment and also them dying from the disease. However, its dangers include a higher risk of dying from heart disease or either lung cancer or oesophageal (gullet) cancer.

Breast cancer is the UK’s most common cancer, with 56,400 new cases diagnosed every year, almost all of them women. It kills 11,500 people a year – 32 every day.

The new findings show that breast cancer patients who do not smoke have less than a 1% chance of radiotherapy leading directly to them developing lung cancer in the years afterwards. But for those who do smoke when they are treated and do not quit afterwards, their risk is between 2% and 6%.

The results are from research in the UK led by Carolyn Taylor, a professor of oncology at the University of Oxford, which was funded by Cancer Research UK and published in the journal Clinical Oncology.

“Radiotherapy is very good at treating cancer. In breast cancer it reduces the chance of the cancer coming back in the future,” Taylor said.

“But it’s also important to consider the risks. One of the long-term risks of radiotherapy is that it can cause second cancers many years down the line. This is a rare side-effect, but it’s important to consider.”

Her study is the first of its kind undertaken in the UK. “The reason radiotherapy can cause a second cancer is because when we give a dose to the breast we cannot avoid giving some radiation doses to the surrounding tissues, such as the lungs,” she added. “This radiation can damage cells in the lungs.

“Radiotherapy damages the DNA in cells. Damage to the DNA in cancer cells can kill them. This reduces the chance of the cancer coming back. But it can also damage the DNA in some normal cells and this damage to normal cells can rarely cause a cancer many years in the future.”

But the conclusions of the study, which analysed 14 previous studies of doses of radiation given to thousands of women with breast cancer in the UK, should be seen as “reassuring” for most women who have radiotherapy, Taylor added.

That is because most women with breast cancer who may have the treatment are non-smokers, because only 14% of women in the UK smoke.” For them their risk of lung cancer from the radiation is below 1%. So, for them, our study is good news”.

However, for long-term smokers who do not kick the habit, “their risks are higher. UK breast cancer radiotherapy would increase their risk of lung cancer by between 2% and 6%.”

Deborah Arnott, the chief executive of Action on Smoking and Health, said that smokers undergoing radiotherapy “must be warned of the risks of continuing to smoke and given the support and encouragement they need to quit.

“Continuing to smoke, and this is true for any cancer treated with radiotherapy, not just breast cancer, makes the treatment itself more unpleasant and less effective, with an increased risk of a subsequent recurrence of cancer.”

Cancer Research UK recently recognised smoking as a cause of breast cancer for the first time, alongside known risks such as diet and genetics. Labour promised in its general election manifesto to make stop smoking services available to all hospital patients, to encourage more people to quit.