refers to follow-up screening for new evidence of cancer in patients who have already been diagnosed with and treated for cancer or premalignant disease.

This chapter reviews the principles used for determination of the effectiveness of cancer screening and applies those principles in an examination of current scientific evidence regarding the benefits and harms of screening for four types of cancer (cancers of the colon and rectum, breast, prostate, and cervix). Chapter 6 examines strategies for optimization of the delivery of recommended cancer screening tests from the perspective of the health care system, providers, and, most importantly, the patient. Chapter 7 presents a case study that reviews the history and prospects for screening for lung cancer, illustrating the difficulties of adopting new technologies in the face of uncertain science.

PRINCIPLES FOR ASSESSMENT OF THE EFFECTIVENESS OF SCREENING FOR CANCER

The principal considerations in judging the effectiveness of cancer screening are (1) the burden of suffering, the frequency of cancer, and the severity of its health effects; (2) the accuracy and reliability of the screening test in detecting cancer and minimizing inaccurate test results; (3) the effectiveness of early detection, including the incremental benefit of detecting and treating cancer at an earlier stage; (4) the harms of screening, both from the testing process and from the incremental harms from evaluation and treatments that follow; and (5) costs. These considerations form the tradeoffs used to weigh the benefits and harms of screening. All of the preceding analytical steps are necessary to address the pivotal question of whether patients and populations experience better outcomes with screening than without it.

Burden of Suffering

The first consideration in assessing the effectiveness of cancer screening is the frequency with which cancer occurs in the population and its attendant health effects. The prevalence rate determines the pretest probability of disease or the average likelihood that a person in the screened population will have cancer. The lower this value is, the larger the number of tests that must be performed to detect one case of cancer (i.e., it will have lower yield) and, for statistical reasons discussed below, the greater the chances that a positive test result will be erroneous (a false-positive result).

Mortality rates and other measures of the probability of adverse health effects from cancer influence the absolute benefit of screening (see the discussion of absolute benefit versus relative benefit below). For example, if a screening test reduces the risk of dying from cancer by 20 percent (relative



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