Interval cancers can include cancers that develop and become symptomatic in the time between screening appointments, as well as those missed or undetectable by mammography that are subsequently detected symptomatically.
An analysis led by The Institute of Cancer Research (ICR) found 2.91 per 1,000 women who had a negative screen were diagnosed with breast cancer before their next screen was due.
The study examined data from more than 7.3 million women aged 50 to 64 who had a routine screen between April 1997 and March 2003 at one of 92 screening centres in England, Wales and Northern Ireland.
Over the six year period of the study, around 44,000 cancers were detected through routine screening and around 21,000 interval cancers were diagnosed. Interval cancers therefore accounted for almost one third of all diagnosed breast cancers over this period.
“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,” says senior author Dr Sue Moss from the ICR. “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 screened over this six year period, with an average of 6.04 per 1,000.
Interestingly, fewer than five per cent of interval cancers were non-invasive, whereas non-invasive cancers accounted for 21.40 per cent of all screen-detected cancers.
Around 0.55 women per 1,000 screened were diagnosed with breast cancer within a year of a negative screen. The researchers expected the rates of interval cancers would increase with time since last screen, however the rate between 12 and 24 months (1.13 per 1,000) was similar to the rate between 24 and 36 months (1.22 per 1,000). They suggested this could be due to breast units re-screening women at less than 36 months, or women with symptoms waiting for their next screening invitation to seek medical attention.
The UK NHS Breast Screening Programme (NHSBSP) was established in 1988 and invites women aged between 50 and 70 for a mammogram every three years. The programme aims to reduce deaths from breast cancer by detecting disease before women experience symptoms, so it can be treated earlier.
Of the 1.5 million women eligible for screening each year, attendance was 68 per cent among women issued their first invitation and about 86 per cent for women who had attended a previous screen.
The study data were collected by the Regional Cancer Registries and the analysis conducted by the Cancer Screening Evaluation Unit at the ICR, which is funded by the Policy Research Programme of the Department of Health.
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