Breast Screening with Mammography

Breast cancer is the most common cancer in the UK, with women having a 1 in 8 lifetime risk of developing the disease^[1]. Many trials have looked at whether screening has an impact on outcome in terms of mortality and morbidity. Several countries have set up screening programmes to call women for routine screening. Debate over the benefit, however, continues.

The programme in the UK was initiated in 1988 following the Forrest Report^[2]. There are 80 breast screening centres across England, 4 in Northern Ireland, 6 in Scotland, and 10 in Wales, many providing mobile units to provide local appointments^{[3, 4, 5, 6]}.

The breast screening process

Screening in the UK is a nationally coordinated programme, with national standards monitored through a quality assurance network.

Mammography is the tool used for breast screening. This allows small tumours to be detected before they are palpable.

• Mammography is a special type of low-dose X-ray imaging of the breast used to create detailed soft tissue images.
• Most mammography now uses digital images stored on computer. A digital mammography system usually requires a lower radiation dose than film screen mammography for the same image quality. Digital detector converts the X-ray photons to an electronic signal, which is further processed and displayed as a greyscale image. The digital system provides greater contrast resolution and thus better visualisation of skin, peripheral breast tissue, and breast density. Besides this, it allows for changes in zoom, contrast and brightness, which increase the ability to detect subtle abnormalities.
• Since 2003, two images of each breast have been taken, craniocaudial and mediolateral oblique. This increases the detection rate of even smaller abnormalities by up to 43%.
• Mammography is used for screening in asymptomatic women for early detection of breast cancer; it is also used diagnostically in symptomatic women.

Recall of women with abnormalities is usually to specialised assessment units. These allow further investigation, with one or more of the following:

• Clinical examination.
• Special view mammography.
• Ultrasound examination (useful in younger patients and to identify cysts particularly).
• Fine-needle aspiration and cytology.

Around 80% of women who are recalled for assessment following an abnormal mammogram do not have breast cancer^[7]. Around 1-2 in 23 women having a mammogram are called back; some of these are for technical reasons^[8]. As an example, in England in 2014-2015, 7.8% of women attending for first-time screening were recalled for further assessment, whilst 3% of women who had had a previous screen in the past 5 years were recalled^[3]. Of those recalled for further assessment, 46.2% went on to have a fine-needle aspiration, and 2% required open biopsy.

Other imaging and screening techniques are not currently used in the UK programme.

• The role of MRI for breast cancer screening is still evolving. Currently, MRI screening, in combination with mammography, is usually reserved for the screening of high-risk patients.
• Supporters of adjunct ultrasonography to the screening regimen for breast cancer argue that it is a safe and inexpensive approach to reduce the false negative rates of the screening process. However, a Cochrane review found there to be no methodologically sound evidence available justifying the routine use of ultrasonography as an adjunct screening tool in women at average risk for breast cancer^[9].

Target population^{[6, 10, 11]}

All women between the ages of 50 and 70 years are invited to attend for screening every three years. It is a rolling programme, which means that not all women will be invited when they reach 50 years, but all will be invited before their 53rd birthday. Women over the target age may request mammography by contacting their local screening centre. In some areas of the UK, women aged 47-49 years and 71-73 years receive invitations for screening. This is part of a trial which began in 2009 looking at whether to extend the breast screening age range. It is expected to take until at least the mid-2020s for conclusions to be reached.

Women who have a family history of breast cancer can access advice about their risks and further screening if required, through their GP. National Institute for Health and Care Excellence (NICE) guidelines of 2013 may be consulted for appropriate referral pathways for genetic testing and surveillance for those at higher risk^[13]:

• Those who have had breast cancer.
• Those who have a strong family history of breast cancer.
• Those who have a relevant genetic history.

Otherwise, primary care clinicians should be aware of women who may be at higher risk of breast cancer - for example, no history of breast-feeding, having no children, having children at late ages (especially over the age of 30 years), long-term hormone replacement therapy (HRT) use, obesity (for postmenopausal women only) and high consumption of alcohol - and advise them opportunistically of breast awareness.

For women aged 40-49 years there is only limited evidence that screening leads to a reduction in mortality. This may be due to the difficulties in interpreting mammograms in pre-menopausal women, due to glandular breast tissue, and also to the lower incidence of breast cancer in this age group.

Screening for breast cancer: benefit vs harm

The breast cancer screening programme in the UK was set up to allow for the early detection and treatment of breast cancer in women. As evidence emerges from years of screening across several countries, debate continues about whether the benefit outweighs the negative effects. Most recent reviews show benefits come at a significant price in terms of harm.

• An independent review in the UK in 2012 concluded^[14]:
• Screening as currently performed in the UK gives a 20% relative risk reduction in mortality.
• For every 10,000 women aged 50 years beginning the screening programme for the next 20 years, 43 deaths from breast cancer would be prevented, and 129 cases would be overdiagnosed. This equates to one death prevented for every three cases overdiagnosed.
• A 2013 Cochrane review concluded^[15]:
• Screening reduces mortality by around 15%, and overdiagnosis and over-treatment is around 30%.
• For every 2,000 women invited for screening over 10 years, one death from breast cancer will be prevented, and 10 healthy women will be treated unnecessarily. More than 200 women will experience psychological distress caused by false positive findings.
• Breast cancer mortality in itself is an unreliable outcome measure, as it is biased in favour of screening.
• The Canadian Task Force in 2011 found that mammography is associated with significant reductions in the relative risk of death from breast cancer in those aged 50-69 years. The benefits of mammography for women aged 60-69 years (number needed to screen (NNS) 432) are greater than for women aged 50-59 years (NNS 910). Screening about 720 women aged 50-69 years once every 2-3 years for about 11 years would prevent one death from breast cancer, but it would also result in about 204 women having a false positive result on a mammogram and 26 women having an unnecessary biopsy of their breast^[16]. However, a report on 25 years of breast screening in Canada in 2014 suggested for women aged 40-59 years having annual mammography, there was NO significant reduction in mortality^[17].
• An international working group reviewed the evidence in November 2014 and concluded that there is a net benefit from inviting women aged 50-69 years for screening, with^[18]:
• A 23% reduction in risk of death from breast cancer.
• Summary estimates of over-diagnosis around 6.5%.
• Evidence of short-term psychological harm resulting from false positive mammograms. (There is evidence that women in the UK who have had false positive breast screening results may experience psychological distress which may persist for up to three years, and reduce their likelihood of returning for their next screening test^[7].)
• A risk of radiation-induced breast cancer of 1 to 10 per 100,000 women screened.
• No evidence found that showed teaching breast self-examination reduces mortality from breast cancer.
• No evidence found that clinical breast examination alone reduces mortality, although when combined with mammography it may increase detection rates.

Women need to be informed of the risk of over-diagnosis but a UK-based study suggested that this did not have much impact on their decision to have screening^[19]. Studies elsewhere have shown that when women are fully informed by a decision aid leaflet, participation in screening drops^[20].

Ductal carcinoma in situ (DCIS) occurs more frequently in screening-detected cancer than in symptomatic cancer, and may contribute to overdiagnosis^[14]. It is not yet possible to ascertain which cases of DCIS will go on to become invasive or to relapse or to affect mortality, so all cancer detected is treated.

Uptake and detection rates

Uptake and detection rates are similar across England, Scotland, Wales and Northern Ireland, although there are regional variations. In the group of women aged 50-70 years:

• In England in 2014-2015, uptake was 71.3% with regional variation, the highest uptake being in the East Midlands^[3]. 1.75 million women were screened in all. 8.6 cancers were detected per 1,000 women screened.
• In Northern Ireland in 2012-2013, average uptake was 74.2%, although regional variation was noted. 6.8 cancers were detected per 1,000 women screened^[21].
• In Scotland uptake was 72.5% over the 2012-15 three-year period^[22]. Detection rate of invasive cancer in women aged 53-70 with a previous screen within the past 5 years was 6.5 per 1,000 women.
• In Wales uptake was 72.1% in 2014-2015. 10.6 cancers were detected per 1,000 women screened^[6].