Prostate and Bladder Cancer Screening

Screening is the process by which asymptomatic people are tested to determine whether they are likely to have a particular disease. Those who are deemed likely by the screening test are evaluated further to determine whether they actually have the disease. Those diagnosed are usually treated for the screened disease. The goal, of course, is to detect and treat the disease earlier than if it were detected only after symptoms had occurred. The screening process is considered successful if these individuals not only are diagnosed earlier (ie, at earlier stages), but also have decreased cause-specific morbidity and mortality rates. For a screening program to be of value, candidate diseases must meet certain criteria (Table 1). The disease must be a serious health concern. It must be relatively common and occur in a population that can be defined by easily ascertainable demographic and risk factors. There must be some apparent benefit (primarily for outcome, but also for diseaseand treatment-related morbidity) for early treatment compared with later treatment. Additionally, the disease must have a preclinical phase of “reasonable” duration (eg, many months to years) during which the disease is detectable but undiagnosed because it has not yet caused symptoms. The interval between when the disease is detected in screened individuals and when it would have been detected if the diagnosis were delayed until symptoms provoked an evaluation is the “lead time.” Because screening detects the condition before it would normally be diagnosed, time-interval survival data will favor the outcome of a screened population compared with an historic control unscreened population simply because of the lead time in diagnosis that screening has created, even if screened and unscreened patients eventually die at the same age from the disease. This is called “lead time bias.” Length bias sampling is the tendency for screening tests to detect more slowly growing cancers because they are in the asymptomatic population longer than more rapidly growing ones, which quickly become symptomatic and no longer need screening to become detected. Length time bias can also give an impression of increasing survival in people with screening-detected cancers. These biases are important arguments against accepting the shift to earlier stages as proof that screening improves disease outcome (Fig. 1).