NHS Evidence Health Information Resources formerly National Library for Health

Breast cancer deaths have been nearly halved by NHS breast cancer screening, reported four newspapers (9 January 2008). The newspaper reports were based on a case-control study. The newspaper reports were generally accurate but only one raised the possibility that the study could have over-estimated the benefit of screening.

On 9 January 2008 four newspapers (1-4) reported that breast cancer screening almost halves the risk of women dying from the disease.
The newspaper articles were based on a case-control study conducted in East Anglia which compared the breast cancer screening histories of 284 women who had died from the disease to 568 women who had not (5). There was a 48% reduction in risk of death for women who had least one screening compared to those who never attended screening. A more conservative analysis estimated a 35% reduction in risk of breast cancer death.
Three of the papers accurately reported the data from the study though they report only the analysis that found the highest reduction in risk whereas in the study paper the authors make a more conservative overall conclusion (1-3). One paper incorrectly reported that the study found that breast cancer deaths had been reduced by 48% over the past 20 years (3). Only one study reported the risk of the case-control study design over-estimating the benefits of screening compared to randomised controlled trials (4).

Evaluation of the evidence base for a regional NHS breast cancer screening programme

Where does the evidence come from?

The research was conducted by Dr PC Allgood and colleagues from Wolfson Institute of Preventive Medicine, Addenbrooke's Hospital and Strangeways Research Laboratory, UK.

What were the authors' objectives?

To estimate the effect of the National Health Service Breast Screening Programme (NHSBSP) in the East Anglia region of the UK on death from breast cancer.

What was the nature of the evidence?

A case-control study was conducted of 852 women to compare the screening history of women who had died from breast cancer to women who had not died from the disease. Cases of breast cancer deaths, of women aged 50 to 70 years old at diagnosis, from 1995 onwards, were randomly selected from the East Anglia cancer registry database. For each case two controls were selected from NHS databases, for the same area. The controls were alive at the time of death of their matched case and were matched to the case by date of birth. The analysis was adjusted to attempt to take into account the possibility that women who do not take up the offer of screening may have poorer health and therefore more likely to die from breast cancer independent of screening (self-selection bias).

How did participants differ on their levels of exposure to the factor of interest?

The primary factor of interest was whether the women had ever attended breast cancer screening. All participants were invited at least once to be screened. 89% of controls had attended at least one screening compared to 73% of cases of breast cancer death. The mean number of screens was 1.7 for controls compared to 1.4 for cases.

What were the findings?

Without accounting for self-selection bias, attendance at one or more screens was associated with a 65% reduction in risk of death due to breast cancer compared to non-attendance. This was unchanged when the data were adjusted by socio-economic status using an index of multiple deprivation. When the analysis was adjusted for self-selection bias, invitation to screening was associated with a 35% reduction, and actually being screened was associated with a 48% reduction in breast cancer death.

What were the authors' conclusions?

This study provides strong evidence that NHS breast screening programme is achieving at least a 30% reduction in breast cancer mortality in women attending screening, consistent with that found in randomised controlled trials.

How reliable are the conclusions?

This well-conducted study does provide evidence that the breast screening programme in this region is associated with a reduction in breast cancer deaths. However, the estimated extent of the reduction may not be reliable due to limitations of the case-control study design which is susceptible to bias and may overestimate the effect of a screening programme. Although self-selection bias was corrected for, the method used is based on data from other countries, so there is some uncertainty that the correction used is appropriate.

Systematic reviews

Information staff at CRD searched for systematic reviews relevant to this topic. Systematic reviews are valuable sources of evidence as they locate, appraise and synthesize all available evidence on a particular topic.

There was one related systematic review identified on the Cochrane Database of Systematic Reviews (CDSR) (6) and five on the Database of Abstracts of Reviews of Effects (DARE) (7-11).

References and resources

1. Screening 'halves' risk of breast cancer death. Daily Telegraph, 09 January 2008, p11.

2. Death rate falls due to screening. Daily Mirror, 09 January 2008, p6.

3. How NHS screening beats breast cancer. Daily Express, 09 January 2008, p32.

4. Screening halves breast cancer deaths says study. The Guardian, 09 January 2008, p1-2.

5. Allgood PC, Warwick J, Warren RML, Day NE, Duffy SW. A case-control study of the impact of the East Anglian breast screening programme on breast cancer mortality. British Journal of Cancer 2008;98(1):206-09.

6. Gøtzsche PC, Nielsen M. Screening for breast cancer with mammography. Cochrane Database of Systematic Reviews 2006, Issue 4. Art. No.: CD001877. DOI: 10.1002/14651858.CD001877.pub2.

7. Harstall C. Mammography screening: mortality rate reduction and screening interval. Edmonton, AB, Canada: Alberta Heritage Foundation for Medical Research; 2000. [DARE Abstract]

8. Ringash J, Canadian Task Force on Preventive Health Care. Preventive health care, 2001 update: screening mammography among women aged 40 - 49 years at average risk of breast cancer. Canadian Medical Association Journal 2001;164(4):469-76. [DARE Abstract]

9. Humphrey LL, Helfand M, Chan BK, Woolf SH. Breast cancer screening: a summary of the evidence for the U.S. Preventive Services Task Force. Annals of Internal Medicine 2002;137(5 Part 1):347-60. [DARE Abstract]

10. Kerlikowske K, Grady D, Rubin SM, Sandrock C, Ernster VL. Efficacy of screening mammography: a meta-analysis. JAMA 1995;273(2):149-54. [DARE Abstract]