Imaging procedures including chest X-ray and low-dose computed tomography may be effective in lung cancer early detection. Yet it is unknown whether low-dose computed tomography combined with computer aided detection （CAD) is more effective than LDCT in screening of early lung cancer.

The randomized clinical trial is to investigate and compare the effectiveness of CAD-guided low-dose computed tomography and low-dose computed tomography in lung cancer screening for community individuals in Shanghai, China. Thus, an imaging protocol which can detect early lung cancer in asymptomatic high risk patients will be proposed.

Inclusion Criteria:

• Age of 55 to 74 years; smoking history of at least 30 pack-years; and if former smoker, have quit within 15 years
• Age 50 or older ,20 or more pack-year history of smoking, and one additional risk factor (occupational exposure, residential radon exposure, cancer history, family history of lung cancer, history of lung disease)
• Age 35 and one additional risk factor
• Ability to accept LDCT examination and sign informed consent form

Exclusion Criteria:

• Individuals with any symptoms suggestive of lung cancer （e.g., vigorous coughing, chest pain, weight loss, blood-stained sputum）
• Individuals with acute pulmonary abnormalities or those in poor health conditions or those have metallic implants or devices in the chest or back will be excluded due to the potential impact on the results as well as difficulty of acquiring the images.
• Pregnant women