Cancer screening in theory and in practice.

Improvements in technology have led to a number of tests that can be used to suggest that a patient has a cancer. Advances in cancer biology and medical imaging have led to a number of cancer screening tests. Cancer screening is commonly advocated, but its complexity is often lost in guidelines that have sound-bite quality. It is commonly viewed as of no harm, when in fact there are harms associated with every known screening test. Indeed, many screening experts believe a screening test should only be used when the potential for benefit clearly outweighs the potential for harm. Cancer screening principles are classically within the realm of the epidemiologist. As more screening tests are developed, these principles have become more relevant to the practicing clinician. What is known and what is unknown about screening is distinctly different from what is believed by the public and many practicing clinicians. Many tests have both screening and diagnostic uses, and it is only the context in which these are used that determines whether they are screening or diagnostic. A screening test is done on asymptomatic individuals who receive the test principally because they are of the age or sex at risk for the cancer. A diagnostic test is done on an individual because of clinical suspicion of disease.