Determinants of cancer screening frequency: the example of screening for cervical cancer.

BACKGROUND: Cancer screening frequency should be based on the rate of progression of the disease and the sensitivity of the screening test. A common misconception is that a person's risk of getting the disease determines how often they should be screened. METHODS: We describe algebraically the theoretical interaction of disease progression rate and screening test sensitivity determining the portion of invasive cancers prevented by screening. After discussing the assumptions and limitations of the model, we apply this model to the example of screening for cervical cancer. Actual data from large screening programs assembled by the International Agency for Research on Cancer (IARC) are used to test the assumptions of the model. RESULTS: A simple formula can express the relation between disease progression rate, sensitivity of the screening test, screening frequency, and screening error. Disease prevalence does not figure in this equation. The IARC data suggest that, at least for cervical cancer, as screening frequency increases, incremental sensitivity of the test decreases or remaining undetected cases progress more rapidly so that anticipated benefits from more frequent screening are not realized. CONCLUSIONS: Rate of disease progression and sensitivity of the screening test are the proper determinants of cancer screening frequency. Because these factors can vary depending on screening frequency, however, the optimal screening interval for a particular cancer must be determined by clinical trials.