Cancer screening: Evidence and practice in Europe 2008
Introduction
Screening can be defined as the application of a simple test to identify early, asymptomatic disease. The primary purpose of cancer screening is to reduce cancer mortality. In addition, screening has other important consequences, including the use of economic resources (health expenditure) and potential quality of life effects, whether positive or negative.
Since cancer is always a potentially lethal disease, the primary goal of cancer screening (and subsequent treatment) is extending lives. The ultimate effectiveness of cancer screening is measured in terms of mortality reduction and this aim motivates initiation, management and evaluation of cancer screening as a public health policy.
Cancer screening requires a detectable pre-clinical phase of the target disease, during which the disease can be treated so that its progression to overt disease is stopped.1 The duration of the detectable pre-clinical phase is referred to as the ‘sojourn time’.2 Its duration varies, depending on the natural course of the disease, the screening test and diagnostic assessment.
Ideally, a screening programme should reduce the burden of disease in terms of death and morbidity and/or improve the quality of life. The prognosis of screen-detected cancer cases should be better than for those detected clinically, due to earlier detection and treatment. Yet, screening always has also adverse effects. Screen-detected cases often include indolent lesions, some of which would not progress even if untreated.3, 4, 5 Such lesions may fulfil the histological criteria for malignancy, but nevertheless behave clinically in an indolent fashion. Any screening programme will detect such abnormalities; therefore one of the adverse effects of screening is over-diagnosis and over-treatment, i.e. detection and management of disease that would not have been diagnosed during the lifetime of the subject in the absence of screening.
The natural history of disease and treatment outcomes define the limits of screening. If a disease is fully curable after detection at a symptomatic phase, there is no need for screening. Screening should not be applied to diseases for which no effective treatment is available.
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Evaluating the effectiveness of screening
A screening programme should have high sensitivity and specificity. Sensitivity is the capacity to detect cases in the pre-clinical detectable phase amongst those screened.6 Sensitivity sets the limits for achievable effectiveness, since, reduction in disease burden is achieved only through screen-detected disease. Specificity is the ability to correctly identify subjects without the disease. Specificity is important because it is essential to minimise the number of ‘false positives’ or healthy
Organising a screening programme
Screening is a chain of activities that starts with defining the target population and extends to the treatment and follow-up of the screen-detected patients. A screening programme consists of several interlinked activities. Various cancer screening programmes may consist of different components (Table 1).
Screening can be opportunistic (spontaneous and unorganised) or organised (mass screening and screening programmes). The major differences lie in the level of organisation and planning and the
Effectiveness of cervical cancer screening
The objective of cervical cancer screening is to reduce both cervical cancer incidence and mortality. A successful screening programme detects early, pre-invasive lesions during the pre-clinical detectable phase and is able to reduce deaths by preventing the occurrence of invasive cancer. Diagnostic assessment requires colposcopy examination, with the evaluation of morphological features of the cervix as well as histological assessment.
The value of the Papanicolaou (Pap) smear in reducing the
Screening for breast cancer
In breast cancer screening, the primary target lesion is early invasive cancer, but ductal carcinoma in situ is also detected with a frequency that is up to a fifth of that for invasive cancer.
Mammography involves radiological imaging of the breast with one or two views, read by one or two radiologists. A screen-positive finding is a lesion that is suspicious for breast cancer. Two views are likely to increase the sensitivity by approximately 20%, with the greatest incremental benefit for the
Effectiveness of colorectal cancer screening
Several screening methods are available for colorectal cancer screening, including faecal occult blood testing, sigmoidoscopy, colonoscopy and double-contrast barium enema.
Faecal occult blood (FOB) testing is based on the detection of haemoglobin in stools using guaiac-impregnated patches, where an oxidative reaction results in a colour change that is detectable on inspection. The most commonly used test, Hemoccult II®, is not specific for human blood and so may yield false positives in people
Screening for prostate cancer
Screening for prostate cancer is mainly based on serum prostate-specific antigen (PSA), which is a serine protease (enzyme) secreted by the prostate gland. It is usually found in low concentrations in serum, with levels increased by prostate diseases such as benign prostatic hyperplasia, prostatitis or prostate cancer.
Two large randomised trials are being carried out, one in Europe and the other in the USA. The European Randomised trial of Screening for Prostate Cancer (ERSPC, //www.erspc.org/
Lung cancer
The target lesion for lung cancer screening is early, resectable (stage 1) carcinoma. Conclusive diagnosis of early lung cancer is based on biopsy, usually obtained by bronchoscopy for central tumours and excision biopsy for peripheral tumours.
The available screening protocols for lung cancer include screening with chest X-rays, with or without sputum cytology, and spiral low-dose CT. Several trials have been conducted to assess the mortality effect of chest X-rays and sputum cytology compared
Conclusion
In summary, establishing the benefits of screening requires evidence on mortality effects from large randomised trials. A summary of the current evidence for cancer screening is provided in Table 5. Screening tests are available for many primary cancer sites, but either their effectiveness has not been evaluated adequately or a lack of effectiveness has been demonstrated. Even when efficacy trials (typically conducted in specialist centres with volunteer subjects) have been successful, mass
Conflict of interest statement
None declared.
Acknowledgements
The article is partly based on ’Cancer Screening’ chapter in Encyclopedia of Public Health (Elsevier 2008) by Matti Hakama and Anssi Auvinen. We thank prof. Jan-Willem Coebergh for comments and suggestions.
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