title: Principles, effectiveness and caveats in screening for cancer.
abstract: BACKGROUND
  Cancer screening has the potential to prevent or reduce incidence and mortality of the target disease, but may also be harmful and have unwanted side-effects.
  
  
  METHODS
  This review explains the basic principles of cancer screening, common pitfalls in evaluation of effectiveness and harms of screening, and summarizes the evidence for effects and harms of the most commonly used cancer screening tools.
  
  
  RESULTS
  Cancer screening has either been established or is considered for breast, lung, prostate, cervical and colorectal cancer. In contrast, screening for gastrointestinal malignancies outside the large bowel is not generally accepted, available or implemented. Oesophageal and gastric carcinoma, and hepatocellular carcinoma, may be subject to screening in certain risk populations, but currently not for population screening based on available technology. Screening for colorectal cancer and cervical cancer by endoscopy and cytology respectively can decrease incidence of the target disease, whereas screening tools for lung, prostate and breast cancer detect early-stage invasive disease and thus do not decrease disease incidence. Overdiagnosis (detection of cancers that will not have become clinically apparent in the absence of screening) is a challenge in lung, prostate and breast cancer screening. The improvement of quality of clinical practice following the introduction of cancer screening programmes is an appreciated 'side-effect', but it is important to disentangle the effect of screening on cancer incidence and mortality from that of quality improvement of clinical services. As new, powerful screening tests emerge-particularly in molecular and genetic fields, but also in radiology and other clinical diagnostics-the basic requirements for screening evaluation and implementation must be borne in mind.
  
  
  CONCLUSIONS
  Cancer screening has been established for several cancer forms in Europe. The potential for incidence and mortality reduction is good, but harms do exist that need to be addressed, and communicated to the public.
authors.affiliation: Department of Health Management and Health Economy, Institute of Health and Society, University of Oslo, Norway. michael.bretthauer@medisin.uio.no
authors.forename: M
authors.initials: M
authors.lastname: Bretthauer
authors.forename: M
authors.initials: M
authors.lastname: Kalager
mesh_terms: D000075202:Contraindications; D055088:Early Detection of Cancer / Q000379:methods*; D006801:Humans; D009369:Neoplasms / Q000145:classification / Q000175:diagnosis* / Q000517:prevention & control*