In 2001 a Cochrane meta-analysis of randomised trials reported that mammography screening reduces breast cancer related mortality by 15% and that one in three cancers are overdiagnosed (1)
No randomised trials have been performed since then, except an update of a Canadian trial that found no effect of mammography screening on breast cancer mortality at a cost of 22% overdiagnosis of cancer (2)
In an observatonal study Autier et al analysing data from the Dutch screening programme (3)
- found at most a 5% reduction in breast cancer related mortality and overdiagnosis in one out of three cancers detected
- overdiagnosis has steadily increased over time with the extension of screening to women aged 70-75 and with the introduction of digital mammography
- after deduction of clinical lead time cancers, 33% of cancers found in women invited to screening in 2010-12 and 59% of screen detected cancers would be overdiagnosed
However a more recent review is much more positive about breast cancer screening (4):
- states for women 40-74 years of age who actually participate in screening every 1-2 years, breast cancer mortality is reduced by 40%
- with appropriate corrections, overdiagnosis accounts for 10% or fewer breast cancers
- 'overdiagnosis' is the diagnosis, as a result of screening, of a cancer (either invasive or in situ) that would never have been identified clinically or caused a problem in the individual's lifetime
- to obtain an accurate estimate for overdiagnosis, it is important that the screened and unscreened populations studied have similar risk factors for breast cancerand that adjustments be made for any confounders
- lead-time bias - the time between detection of the disease as a result of screening and the time at which the diagnosis would normally have been made when the patient presented with symptoms- must be accounted for. Because of lead time, an excess incidence of breast cancer is expected when screening starts. After the end of screening, a reduction in the incidence of breast cancer should occur because of the earlier diagnosis of cancers during screening
- if no overdiagnosis occurs, then the initial increase in breast cancer in screened women should be fully compensated by a similar decline in breast cancer in older women who no longer screen, called the 'compensatory drop.' An interval of at least 5 years of follow-up is required to observe that drop. If follow-up is insufficient, then the compensatory drop will overestimate any overdiagnosis. If no adjustment is made for the compensatory drop, then estimates of overdiagnosis are much higher, on the order of 57% for in situ and invasive cancers (5)
- ductal carcinoma in situ (DCIS)
- before the widespread use of screening mammography in the United States, 6 cases of DCIS were detected annually per 100,000 women; after the introduction of screening, 37 cases of DCIS were detected per 100,000 women
- significance of detecting DCIS - a UK retrospective study (6):
- a significant negative association was observed for screen-detected DCIS and the rate of invasive interval cancers; for every 3 screen-detected cases of DCIS, 1 fewer invasive interval cancer occurred in the subsequent 3 years. The study concluded that detection and treatment of DCIS was worthwhile for the prevention of future invasive disease
- false positives occur in about 10% of screened women, 80% of which are resolved with additional imaging, and 10%, with breast biopsy
- an important limitation of screening is the false negatives (15%-20%). The technologic advances of digital breast tomosynthesis, breast ultrasonography, and magnetic resonance imaging counter the false negatives of screening mammography, particularly in women with dense breast tissue
NHS Breast Screening Programme defines an "acceptable level" of screening as 70% (5)
- the detection rate was 8.1 per 1,000 women screened (2017-18)
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