All participants were expected to receive three annual screening exams (T0, T1, T2) for lung cancer. Participants were randomized into two study arms at the time of enrollment, with each arm receiving a different type of screening exam. Half of NLST participants were randomized to receive low-dose helical (spiral) CT exams, and half to receive chest radiograph (CXR) exams.
Radiologists were required to review the images in two ways: looking at the image without reference to historical images (isolation read), and then looking at the image again with reference to historical images (comparison read). An exam was considered positive (i.e., suspicious for lung cancer) if the radiologist observed a non-calcified nodule or mass with diameter ≥ 4 mm (always positive) or if other abnormalities appeared suspicious for lung cancer (in the radiologist's judgment). For the T1 and T2 comparison reads, the radiologist was required to record whether a positive exam displayed any significant change relative to prior images. The radiologist had discretion at T2 to classify as negative an exam that met the criteria for a positive exam but displayed no significant change relative to the previous two exams. The comparison read yielded the official screening result reported to participants and used to determine follow-up recommendations.
After reviewing the screening images, radiologists provided recommendations for follow-up procedures.