Screening requires special effort to ensure the quality of the services because only some of the people invited to participate in cancer screening will have the disease or early conditions that may lead to cancer. Many of these people will benefit from undergoing screening because treatment that is started before symptoms occur can be more effective and cause fewer side-effects. However, some cancers will be detected that still cannot be cured, even with the best treatment, and some cases will be detected and treated that might not have been found without screening (overdiagnosis). Also, even the safe tests recommended for screening are not completely free of any risk. For example, most people with abnormal screening test results will require additional testing to find out whether cancer or a precancer is actually present. Additional testing may cause worry, and in a few people tissue samples may need to be taken, which involves the risks of a medical procedure.

Early cancers found in screening can also be more difficult to treat for doctors experienced with more advanced cancer; special training and skills may therefore be required. Although the average risk of harm from participating in screening for any individual is slight, the overall risks and the cost to society add up due to the large number of people involved.

To achieve the greatest benefit of screening with the least harm to all, the services provided must be of the best possible quality. The guiding principles and detailed protocols, standards and recommendations in the European Guidelines for quality assurance in cancer screening published by the European Commission help programmes to achieve this high quality. Organized screening programmes provide the best conditions to follow the European quality assurance guidelines. Effective quality assurance ensures that the balance between the risk and the achieved benefit of cancer screening remains acceptable.