Screening for cancer: concepts and controversies

Early detection of cancer is a core task in family medicine, and patients have come to expect screening tests, sometimes out of proportion to what evidence can justify. To understand the controversies surrounding screening and to provide sound advice to patients, family physicians should be familiar with the fundamental concepts of screening. Failure to account for the effects of lead-time, length-time, and overdiagnosis biases can lead to overestimation of screening benefits. For this reason, the best method for evaluating the benefit of screening tests is a randomized controlled trial showing decreased disease-specific or all-cause mortality. The number needed to screen can be used to measure the magnitude of benefit of screening tests. Accepted screening tests often require screening several hundred to more than 1,000 asymptomatic patients to prevent one death from the disease. The U.S. Preventive Services Task Force and American Academy of Family Physicians recommend screening for colorectal cancer in adults 50 to 75 years of age, and recommend against prostate-specific antigen testing to screen for prostate cancer. Annual low-dose computed tomography screening for lung cancer reduces mortality in persons 55 to 80 years of age with at least a 30-pack-year history who are otherwise healthy smokers or who have quit smoking within the past 15 years; however, it is associated with a high false-positive rate, uncertain harms from radiation exposure, and overdiagnosis. Therefore, it should be performed only in conjunction with smoking cessation interventions.