Breast screening

Introduction

Population-based cancer screening is a complex public health undertaking that is most effective when performed in the context of a programme with defined standards that targets a specific population at risk in a given geographical area with a significant cancer burden, with all those attending being offered the same level of screening, diagnosis and treatment services.

There are 26 countries that offer breast screening programmes, of which 21 offer 2-yearly screening, one annually, one 1–2-yearly and one 3-yearly.

All screening programmes utilise mammography.

The purpose of breast screening is to reduce mortality from breast cancer, as well as the morbidity associated with advanced stages of the disease, through early detection in asymptomatic women. In other words, the aim of breast screening is to identify cancers before they become palpable. In 2018–2019 just over 40% of all the invasive breast cancers diagnosed by screening in the UK (7888 women) were 15 mm or less in size and the majority of these would not have been palpable.

The NHS Breast Cancer Screening Programme in the UK began inviting women to be screened in 1988. This followed the recommendations made by Professor Sir Patrick Forrest in his report on breast screening in 1986 and was based on the Swedish Two-County Trial. This trial used a single mediolateral oblique mammographic view of the breast that reported a 40% reduction in mortality amongst women screened. The NHS Breast Screening Programme (NHSBSP) was set up to perform single-view mammography of each breast and invited women aged 50–64 years of age every 3 years. Women attending for the first time are regarded as being in the prevalent round and women who had previously attended are referred to as being in the incident round.

The IARC (International Agency for Research on Cancer) working group concluded that the evidence for the ‘efficacy of screening by mammography as the sole means of screening in reducing mortality from breast cancer’ was sufficient for women 50–69 years of age, limited for women 40–49 years of age and inadequate for women younger than 40 or older than 69 years of age based on the findings from randomised, controlled trials.

Single- versus double-view mammography

The United Kingdom Coordinating Committee on Cancer Research (UKCCR) sponsored a randomised controlled trial (RCT) of single- versus double-view screening mammography. The trial reported in 1995 and showed that the second view increased the detection of breast cancer by 24% and reduced the recall rate by 15%. In 1995 the Department of Health stated that all screening units should perform two-view mammography in the prevalent round. In 2000, The Cancer Plan announced that two-view mammography was to be used in all women (prevalent and incident rounds) by 2003 ( Fig. 6.1 ).

Controversies of breast screening

Since the time that the screening programme was established, there has been debate over the magnitude of its benefit and harm, and the balance between them. The expected major benefit is reduction in mortality from breast cancer. The major harm is overdiagnosis. Overdiagnosis refers to the detection of cancers at screening that would not have become clinically apparent in the woman’s lifetime in the absence of screening and so may result in unnecessary treatment.

The Marmot Review in 2012 provided an independent scientific review and assessment of the evidence of benefits and harms of breast screening in the context of the UK breast screening programme. The review concluded that the best evidence for the relative benefit of screening on mortality reduction came from 11 RCTs of breast screening. A meta-analysis of these trials with 13 years of follow-up estimated a 20% reduction in breast cancer mortality in women invited for screening. This corresponds to one breast cancer death averted for every 235 women invited to screening for 20 years, and one death averted for every 180 women who attend screening. As such the breast screening programme in the United Kingdom is estimated to prevent about 1300 breast cancer deaths a year. The panel also estimated that for 10 000 UK women invited to screening from age 50 for 20 years, about 681 cancers will be found, of which 129 will represent overdiagnosis. In other words, for each breast cancer death prevented, about three overdiagnosed cases will be identified and treated. It is important that doctors and patients are aware of this concept.

The value of breast screening programmes has been debated since their inception. The main focus of debate has been around the reduction in mortality attributable to screening and the numbers of women overdiagnosed due to the disagreements regarding the validity of the available RCTs of breast screening and interpretation of observational data on breast cancer incidence and mortality. This debate is ongoing.

NHS Breast Screening Programme (NHSBSP)