evidence and outcomes of breast cancer screening

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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

 

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Last edited 08/2019 and last reviewed 10/2020

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