Screening women for breast cancer from their 40s rather than their 50s could save lives without adding to the diagnosis of harmless cancers, a UK study has found.
The research was based on 160,000 women from England, Scotland and Wales, followed up for around 23 years.
Lowering the screening age could save one life per 1,000 women checked, the scientists say.
But experts caution there are many other considerations, including cost.
Cancer Research UK says it is still “not clear if reducing the breast screening age would give any additional benefit compared to the UK’s existing screening programme”.
The charity says the priority should be getting cancer services “back on track” for women aged 50-70, after disruption caused by the pandemic.
During lockdown, cancer screening programmes which detect early signs of bowel, breast and cervical cancer were paused in Wales, Scotland and Northern Ireland, although not officially stopped in England.
Experts have warned of huge backlogs for screening, treatment and tests.
Currently in the UK, women between the ages of 50 and 70 are invited to be screened for breast cancer every three years.
Women below 50 are not routinely offered this because their risk of breast cancer is generally very low and their breast tissue is more dense, making it difficult to read the results of mammography tests used to spot cancers.
This can lead to over-diagnosis - detecting very early cell changes which may not turn into problematic cancers - and the potential for exposing women to unnecessary treatment.
Writing in the Lancet Oncology, the scientists say they found a reduction in breast cancer deaths from screening women in their 40s every year over the first 10 years they were tracked.
In the group of 53,883 women in their 40s who were screened, there were 83 deaths, compared to 219 deaths in the 106,953 women of the same age who were not checked.
The reduction in deaths came from detection of grade 1 and 2 cancers, which can progress more quickly in younger women.
After 10 years, any evidence of extra lives being saved tailed off, the researchers said.
They also found a “modest over-diagnosis in this age group” which was similar to that found in the over-50s.
In the study, 18% of women who went for screening in their 40s had at least one false positive result.
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Prof Stephen Duffy, lead researcher, from Queen Mary University of London, said: “This is a very long-term follow-up of a study which confirms that screening in women under 50 can save lives,
“In the fullness of time, it is worth thinking about lowering the age of screening.”
However, he said the financial cost of this should be taken into account, and more research was needed into the impact of modern screening equipment on diagnoses.
Sophia Lowes, health information manager at Cancer Research UK, said the charity had concerns about the study results.
“Many women received false positive results and some women would have been over-diagnosed with cancers that would never have gone on to cause them harm,” she said.
“While research into improving our screening programmes remains vital, screening programmes are already under huge strain due to the pandemic, and the priority right now should be getting services back on track for women aged 50-70.”
The charity calculates that six times more women in their 40s, compared to those aged 50-70, would need to be screened to save one life.
Ms Lowes said it was important that women - no matter how old they are - should still tell their doctor if they noticed anything unusual about their breasts.