5 Screening - What, Who and When
What - Generally there are not multiple different screening technologies to choose from for screening for a single cancer although exceptions exist. More often, the question usually centres on how a procedure will be done (e.g. what views will be used in mammographic screening, how will cervical smears be taken and prepared). Decisions about these questions are based on accuracy, ease, cost, etc.
Who - Eligibility for cancer screening is based on age, sex and history/symptoms of the disease. History and symptoms are used to exclude subjects because their risk of disease is much greater and more definitive follow-up procedures are indicated. Age and sex are used since they determine the likelihood of cancer and are often related to the performance of the test in order that those being screened have a reasonable probability that they will benefit from being screened.
When - The screening test should be repeated at intervals determined by the natural history of the cancer. Cancer develops throughout life and the screen should be used frequently enough to provide a reasonable expectation that the test will be applied at least once while the cancer is in a pre-clinical phase. The specification of the optimum frequency for screening represents a judgment regarding the benefits of more frequent screening (e.g. increased disease detection, better outcomes) versus the costs (e.g. greater numbers of false positive tests, increased medical costs).
The progressive nature of cancer would suggest that screening longer (starting earlier in life and continuing later) and more frequently will result in better disease specific outcomes when an effective screen is available. It is also likely that more screens will result in greater numbers who are adversely affected by screening (e.g. larger numbers of false positives, increased diagnostic tests) and those who are adversely affected can come to form the largest group in the population. The determination of screening recommendations will therefore always represent a balance between the individual and societal benefits and costs.