The added radiation from mammograms and other types of tests with chest radiation might be especially harmful to these women, and an MRI is probably a safer method of screening women under 30 who are at high risk because of gene mutations, the authors conclude.

The study can’t prove a link between the radiation and breast cancer, but is one of the biggest ever to look at the issue. The research was published Thursday in the journal BMJ.

“This will raise questions and caution flags about how we treat women with (gene) mutations,” said Len Lichtenfeld, deputy chief medical officer of the American Cancer Society. He and the society had no role in the research.

Mammograms are most often used in women over 40, unless they are at high risk, like carrying a mutation of the BRCA1 or BRCA2 gene. Having such a mutation increases the risk of developing cancer five-fold.

About one in 400 women has the gene abnormalities, which are more common in Eastern European Jewish populations. Unlike mammograms, an MRI, or magnetic resonance imaging scan, does not involve radiation.

The breast cancer screening tests have been proven to save lives and are clearly beneficial for women aged 50 and over who have an average risk of breast cancer. Experts are divided about their value in women younger than 50.

Some studies have suggested that women with the genetic mutations could be more sensitive to radiation because the genes are involved in fixing DNA problems. If those genes are damaged by radiation, they may not be able to repair DNA properly, raising the cancer risk.

In several European countries, including Britain, the Netherlands and Spain, doctors already advise women with BRCA mutations to get MRIs instead of mammograms before age 30.

In the U.S. there is no specific advice from a leading task force of government advisers, but the American Cancer Society recommends yearly mammograms and MRIs from age 30 for women with BRCA gene mutations.

In the BMJ study, European researchers followed nearly 2,000 women over 18 with one of the gene mutations in Britain, France and the Netherlands. Participants reported their previous chest X-rays and mammograms, including the age of their first screening and the number of procedures.

About 850 women were later diagnosed with breast cancer. Roughly half of them had X-rays while one third had at least one mammogram, at an average age of 29.

The researchers did not have a breakdown of how many women were exposed to chest radiation before age 30, but estimated that for every 100 women aged 30 with a gene mutation, nine will develop breast cancer by age 40.