Ethical issues and cancer screening

Efficacy The extent to which a specific intervention, procedure, regimen, or service produces a beneficial result under ideal conditions. Ideally the determination of efficacy is based on the results of a randomised controlled trial. Last JM (ed). A Dictionary of Epidemiology. Third edition New York: Oxford University Press, 1995.

Effectiveness The extent to which a specific intervention, procedure, regimen, or service, when deployed in the field in routine circumstances, does what it is intended to do for a specified population. Last JM (ed). A Dictionary of Epidemiology. Third edition New York: Oxford University Press, 1995.

Assessing the Efficacy of Screening Lead-time bias Length bias Selection bias Overdiagnosis bias

Screened X X death Not screened X X death Disease begins Screen diagnosisSigns/symptoms

Screened X X death Not screened X X death Disease begins Screen diagnosisSigns/symptoms Survival time

__________ ____________________________ _______________ __________ ___________ ______ ________________________________ ________ _____________ ________ ____ _______ _________ ____________________ _____ _______ ___________ ___________ _________________________ _____________ Horizontal lines represent the pre-symptomatic screen-detectable phase in various individuals

screen screen __________ ____________________________ _______________ __________ ___________ ______ ________________________________ ________ _____________ ________ ____ _______ _________ ____________________ _____ _______ ___________ ___________ _________________________ _____________

Cumulative breast cancer mortality (Deaths per 1,000 women entered after 7 years follow-up) TrialNon-attenders Controls HIP Stockholm S2C Malmo5.63.1

Design of RCT Study participants Intervention group Control group Outcome (breast cancer mortality)

Randomised controlled trial Population-based Appropriate outcome measure Intention to treat analysis

Population based screening differs from other medical interventions because it is offered to asymptomatic people with the understanding that they will benefit

Benefits and risks of screening for cancer Benefits:Risks: Improved prognosis for some Longer morbidity for those people diagnosed by screening.whose prognosis is unaltered. Less radical treatment whichOver-treatment of questionable cures some people with early cancer.abnormalities. Reassurance for those withFalse reassurance for those with negative test results.false negative results. Anxiety and sometimes morbidity for people with false positive results. Adapted from Chamberlain JM. J Epid Com Hlth 1984; 38:

Sensitivity and Specificity of FOBT Nottingham RCT: Sensitivity53.6% Specificity98% PPV10% Hardcastle JD, Chamberlain JO, Robinson MHE, et al. Randomised controlled trial of faecal-occult blood screening for colorectal cancer. Lancet 1996; 348:

Predicted outcome of 1,000 FOBTs True positives2 False negatives2 False positives18 True negatives978 Number requiring colonoscopy20 Number diagnosed with CRC2 Number diagnosed with an adenoma > 10mm6 Number with no abnormality detected12