++
One of the most important functions of medical care is to prevent disease or discover it early enough that treatment might be more effective. All risk factors for cancer have not yet been defined. However, a substantial number of factors that elevate risk are within a person’s control. Some of these factors are listed in Table 203-1. Every physician visit is an opportunity to teach and reinforce the elements of a healthy lifestyle. Cancer screening in the asymptomatic population at average risk is a complex issue. To be of value, screening must detect disease at a stage that is more readily curable than disease that is treated after symptoms appear. For cervix cancer and colon cancer, screening has been shown to save lives. For other tumors, benefit is less clear. Screening can cause harm; complications may ensue from the screening test or the tests done to validate a positive screening test or from treatments for the underlying disease. Furthermore, quality of life can be adversely affected by false-positive tests. Evaluation of screening tools can be biased and needs to rely on prospective randomized studies. Lead-time bias occurs when the natural history of disease is unaffected by the diagnosis, but the pt is diagnosed earlier in the course of disease than normal; thus, the pt spends more of his/her life span knowing the diagnosis. Length bias occurs when slow-growing cancers that might never have come to medical attention are detected during screening. Overdiagnosis is a form of length bias in which a cancer is detected when it is not growing and is not an influence on length of survival. Selection bias is the term for the fact that people who volunteer for screening trials may be different from the general population. Volunteers might have family history concerns that actually elevate their risk, or they may be generally more health-conscious, which can affect outcome.
++++
The various groups that evaluate and recommend screening practice guidelines have used varying criteria to make their recommendations (Table 203-2). The absence of data on survival for a number of diseases has led to a lack of consensus. In particular, four areas are worth noting.
++