Screening
Testing people who have no symptoms to catch cancer, or its precursors, early enough to cure. Proven for breast, cervical, colorectal and (in smokers) lung cancer; every test also finds some cancers that would never have caused harm.
A good screening test must find cancers at a stage where treatment changes the outcome, be acceptable and affordable for millions of healthy people, and cause more benefit than harm from false positives, unnecessary biopsies and overdiagnosis; it is judged ultimately by whether it lowers deaths in randomised trials. Mammography, cervical smears and HPV testing, colonoscopy and stool tests, and low-dose CT for heavy smokers meet this bar; PSA testing for prostate cancer is a marginal case where shared decision-making is advised. Multi-cancer early detection blood tests such as Galleri aim to screen for dozens of cancers at once and are being tested in large trials, with the risks of overdiagnosis and false reassurance under close scrutiny.