Cancer Screening and Early Detection
Cancer screening and early detection covers the use of tests in people without symptoms to find cancer, or its precursors, at an earlier and more treatable stage than would occur if the disease declared itself clinically. It is a form of secondary prevention: it does not stop cancer from arising but aims to reduce death and serious illness by shifting diagnosis earlier in the natural history of disease.
Definition
Cancer screening is the systematic application of a test or examination to asymptomatic individuals to identify those more likely to have a cancer or a pre-cancerous lesion, who are then referred for diagnostic confirmation; early detection is the resulting shift of diagnosis to an earlier disease stage.
Scope
This area orients the reader to organized cancer screening as a public-health and primary-care activity. It groups the major organ-specific programmes (cervical, breast and colorectal screening), the cross-cutting principles by which any screening test and programme are judged, and the role of shared decision-making when benefits and harms are closely balanced. It is a reference overview of how screening evidence is generated and appraised, not a source of individual screening advice.
Sub-topics
Core questions
- Does detecting a given cancer earlier through screening actually reduce mortality or serious morbidity, rather than only advancing the date of diagnosis?
- How are the benefits of screening weighed against harms such as false positives, overdiagnosis and overtreatment?
- Which populations, ages and intervals make an organized screening programme worthwhile?
Key concepts
- Secondary prevention
- Asymptomatic detection
- Natural history and detectable preclinical phase
- Sensitivity and specificity
- Overdiagnosis and overtreatment
- Lead-time and length-time bias
- Organized versus opportunistic screening
- Mortality reduction as the primary endpoint
Mechanisms
Screening exploits the existence of a detectable preclinical phase, a window during which a cancer or its precursor can be found by a test before it would cause symptoms. Finding disease in this window allows treatment of earlier-stage cancer, which is generally more effective, or removal of precursor lesions before they become invasive, as with cervical and colorectal precursors. The intended endpoint of an organized programme is a reduction in cause-specific mortality, demonstrated where possible in randomized trials; because earlier diagnosis can appear beneficial through lead-time and length-time bias even when survival is unchanged, mortality rather than survival is the decisive measure (Wilson & Jungner, 1968).
Clinical relevance
Organized screening programmes for cervical, breast and colorectal cancer are among the most widely implemented preventive activities in primary care, and major bodies issue periodically updated recommendations on whom to screen and how often (USPSTF, 2018; USPSTF, 2021). This area describes the evidentiary basis and trade-offs of those programmes for educational reference; it does not constitute screening recommendations for any individual.
Epidemiology
Cervical, breast and colorectal cancers are leading contributors to cancer incidence and mortality worldwide, which is why each has been the subject of large screening trials and population programmes. The measured effect of screening varies by cancer and programme: removal of precursors has substantially reduced cervical and colorectal cancer incidence in well-organized settings, while mammographic breast screening reduces breast-cancer mortality at the cost of a debated burden of overdiagnosis (Marmot, 2012).
History
Systematic thinking about screening was crystallized by Wilson and Jungner's 1968 World Health Organization monograph, whose principles for appraising a screening programme remain a reference framework. Through the later twentieth century, organ-specific programmes were built and tested: cytology-based cervical screening, randomized trials of mammography, and trials of faecal occult blood testing and endoscopy for colorectal cancer. More recent decades have added molecular tests such as HPV testing and have sharpened attention to the harms of overdiagnosis.
Debates
- How should the benefits of screening be balanced against overdiagnosis?
- Because some screen-detected cancers would never have caused harm in a person's lifetime, screening can lead to diagnosis and treatment that confer no benefit; quantifying this overdiagnosis and weighing it against mortality reduction is a central and unresolved tension, most prominently for mammographic breast screening.
Key figures
- J. M. G. Wilson
- Gunnar Jungner
- Michael Marmot
Related topics
Seminal works
- wilson-jungner-1968
- marmot-2012
Frequently asked questions
- How is cancer screening different from cancer prevention?
- Primary prevention tries to stop cancer from developing, for example through vaccination or reducing exposures; screening is secondary prevention, which does not prevent the cancer but aims to find it, or its precursors, earlier so that treatment is more effective.
- Why is mortality, not survival, used to judge a screening programme?
- Earlier diagnosis automatically lengthens the time from diagnosis to death (lead-time bias) and tends to over-select slow-growing cancers (length-time bias), so survival can look better even when screening saves no lives; a reduction in cause-specific mortality is the measure that is not distorted by these biases.