Almost one in three breast cancers in UK are diagnosed between regular screenings, new research shows.
The cancers are mostly picked up because the woman starts developing symptoms, such as a lump in her breast.
It is the first time UK data has been analysed on breast cancers that occur between screenings.
These “interval cancers” include tumours that grow between screening appointments, as well as those that are missed or are undetectable during mammograms.
Experts behind the research, published in the British Journal of Cancer, said reducing the length of time between mammograms could lead to more cancers being detected but may not have an impact on death rates.
Women in the UK aged 50 and over are invited for NHS breast screening every three years.
Around 1.5 million women are screened annually and the programme is in the process of being extended to all women aged 47 to 73.
The new study was carried out by experts at the Institute of Cancer Research.
They examined data from more than 7.3 million women aged between 50 and 64 who had a routine screen between April 1997 and March 2003 at one of 92 screening centres in England, Wales and Northern Ireland.
The results showed that 2.91 per 1,000 women who were given the all-clear at their screening were diagnosed with breast cancer before their next mammogram was due. Over the six-year period of the study, around 44,000 cancers were detected through routine screening and another 21,000 interval cancers were diagnosed. This means almost one-third of all diagnosed breast cancers were picked up between screenings.
Senior author Sue Moss said: “The rate at which women developed breast cancer between screenings was higher than expected, and is likely to reflect the fact that breast cancer rates are rising in general.
“Reducing the length of time between screenings would result in a lower proportion of interval cancers, but not necessarily a significant reduction in mortality.”
Overall, the rate of all screen-detected cancers increased from 5.39 to 6.80 per 1,000 women over this six-year period, with an average of 6.04 per 1,000.
Fewer than five per cent of the interval cancers were non-invasive, but this figure was 21.4 per cent for those detected during screening.
This could suggest that many interval cancers are fast-growing.
Rachel Greig, senior policy officer at Breakthrough Breast Cancer, said: “These figures provide telling new evidence about the number of breast cancers picked up between screening appointments.
“They show why women need to remain vigilant for the signs of breast cancer between screening appointments, because the disease can develop at any time. This is important because the earlier breast cancer is diagnosed the better the chance of successful treatment.
“We encourage all women to be breast aware, touch, look and check for any changes, and check anything unusual with their doctor.”