Abstract
Cancer screening involves early detection coupled with early intervention. For cancer screening to reduce cancer mortality, the cancer must be detected sufficiently early and the early intervention must be effective. For cancer screening to minimize harms, the cancers detected early should, to the extent possible, exclude cancers that would not cause medical problems in a person’s lifetime (a phenomenon known as overdiagnosis). Detection of overdiagnosed cancers leads to unnecessary work-ups, biopsies, and treatment. The gold standard for evaluating benefits and harms of cancer screening is a randomized controlled trial (1).