Claudia I. Henschke 05 oct 2008--Screening is the pursuit of the early diagnosis of cancer before symptoms occur. The purpose of early diagnosis is to provide early treatment, which potentially prevents death from the cancer. The usefulness of screening depends on how early the cancer can be diagnosed and how many deaths can be prevented by early treatment as compared with later symptom-prompted diagnosis and treatment. The goal of the Early Lung Cancer Action Project investigators was to develop an efficient methodology that would provide an ever-accumulating, continually updated body of evidence for evaluation of emerging new technologies for screening for cancer. This methodology recognizes that screening is a sequential process that starts with the pursuit of the early diagnosis of cancer followed by early treatment. It also recognizes that diagnostic research is fundamentally different from treatment research. To fully understand the current discussions on the evidence for lung cancer screening, key definitions are provided, including the differentiation between the first, baseline round of screening and all subsequent rounds of repeat screening and baseline and repeat cancers and their distribution by cell type. These definitions are critical in analyzing the results of various screening reports as they are not used by all. To provide optimal screening, a regimen for the diagnostic workup must be specified starting with the definition of the initial test, its positive result, and the workup for a positive result leading to a diagnosis of cancer. Assessment of diagnostic performance does not require a control group, but does require confirmation of the diagnosis. For assessment of the effectiveness of early treatment, a comparison group is needed. The comparison group may be formed by randomly assigning people with screen-diagnosed lung cancer to immediate or delayed treatment, as has been done for prostate cancer. This provides a direct assessment of any potential overdiagnosis of the cancer resulting from screening. Alternatively, a quasiexperimental control group can be used consisting of participants diagnosed with the cancer who have refused or delayed their treatment even though they are candidates for it.