Intended for healthcare professionals

Rapid response to:

Papers

Outcomes of screening to prevent cancer: analysis of cumulative incidence of cervical abnormality and modelling of cases and deaths prevented

BMJ 2003; 326 doi: https://doi.org/10.1136/bmj.326.7395.901 (Published 26 April 2003) Cite this as: BMJ 2003;326:901

Rapid Response:

Think of screening as insurance

Dr. Raffle and colleagues[1] provide interesting new data on the
outcome of cervical screening. It is particularly useful to be able to
tell women that over 20 years of five-yearly screening, approximately 16%
will have an abnormal smear, 8% will have a biopsy and 4% will be treated
for high-grade disease. The authors also estimate the number of cancers
and deaths that might be prevented over 30 years in such a cohort. It is
unclear exactly how they obtained their estimates, but numbers are
surprisingly low. When estimating the number of premature deaths avoided
in screened women, they apply the factor 60% obtained from a population in
which approximately 1 in 5 eligible women are not screened regularly. In
screened women the figure should be closer to 75%, which is more in
keeping with the results from case-control studies[2,3].

Fitting an age-cohort model to mortality data from England and Wales
for 1950 to 1987 and extrapolating to 2011, we estimate the cumulative
number of deaths in an unscreened cohort to be approximately 50% greater
than the authors. Assuming that 75% of the deaths after 1996 would be
prevented in a screened cohort, the number of premature deaths avoided is
2.4 times greater than in the paper. Over the next 30 years, the effect of
screening in women born in the early 1960s will be much greater -
approximately 2% of those screened will be prevented from getting cervical
cancer.

Describing the benefits of screening in terms of the number needed to
be screened in order to prevent one death, equates screening with
treatment. Screening is not treatment. It is perhaps better to think of it
as insurance. The issue is not how many need to be insured in order for
one person to avoid bankruptcy. It is not even simply a question of
whether the cost of insurance premium is more or less than the expected
pay out (it will always be more). Insurance is put in place in order to
avoid catastrophic consequences of an unlikely event. Women need to be
aware of the common negative consequences of regular screening, but they
should perhaps think of it as a costly and imperfect insurance policy that
may save them from the horrors of invasive cervical cancer.

1. Raffle AE, Alden B, Quinn M, Babb PJ, Brett MT. Outcomes of
screening to prevent cancer: analysis of cumulative incidence of cervical
abnormality and modelling of cases and deaths prevented. BMJ.
2003;326:901.

2. IARC Working Group on evaluation of cervical cancer screening
programmes. Screening for squamous cervical cancer: duration of low risk
after negative results of cervical cytology and its implication for
screening policies. BMJ. 1986;293:659-64.

3. Sasieni PD, Cuzick J, Lynch-Farmery E. Estimating the efficacy of
screening by auditing smear histories of women with and without cervical
cancer. The National Co-ordinating Network for Cervical Screening Working
Group. Br J Cancer. 1996;73:1001-5.

Competing interests:  
Funded by Cancer Research UK. Additional funding from the NHS Cervical Screening Programme

Competing interests: No competing interests

30 April 2003
Peter D Sasieni
Professor of Biostatistics and Cancer Epidemiology
Wolfson Institute of Preventive Medicine, Charterhouse Square, London EC1M 6BQ