Screening is done to detect cancer before it has started to cause symptoms i.e., for the early diagnosis of cancer. Screening is used to detect particular cancers, either in the general population (for example, breast cancer screening), or in people with a higher risk of developing cancer (for example, bowel cancer screening in older people).
Screening trials are used to determine the reliability of new tests to detect particular types of cancer, or to try to find out if there is a benefit to people in diagnosing cancer earlier.
Prevention trials are used to determine whether a particular treatment can prevent cancer from developing. They are carried out with people who do not have cancer, and can be done with the general population, or a group of people at higher risk of developing cancer.