As shown in Figure 12.1, cancer begins as a small number of cells. If it were possible to detect every cancer at this point, and accurately distinguish the potentially fatal ones from the nonfatal, then our treatments would have a high rate of success. As the cancer grows, however (moving to the right in the figure), the potentially fatal cancers reach a point at which they are less effectively treated. This critical point varies between cancers and within cancers with the same name. It also varies between treatments. An important advance in treatment may mean that cancers can be effectively treated at a later stage in their development (i.e., farther toward the right of the figure).
The relationship of the critical point to the point at which a screening test can detect a cancer helps determine the potential benefits of screening. If the critical point is between points A and B in the figure (i.e., before the screening test can detect the cancer), then screening with the present test will not reduce the burden of suffering of the cancer. If the critical point is between points B and C (i.e., within the detectable preclinical phase of the cancer), then screening may well be helpful in reducing mortality and/or morbidity. If the critical point is to the right of point C in the figure, then the treatment is effective for even advanced cancers, and earlier detection is not needed.
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