SU-D-217BCD-04: How Do We Know How Low Can We Go in Lung Cancer Screening CT?

PURPOSE To demonstrate a technique for determining the acceptable radiation dose level for lung cancer screening CT. METHODS 20 patients who underwent a clinically indicated routine chest CT scan with 120 kV and 180 quality reference mAs were enrolled in this IRB approved study. Automatic exposure control was used to adjust for patient size. Scanner output, as expressed in CTDIvol, varied accordingly (mean=16.8 mGy, range=7.6 to 26.6 mGy). A previously validated noise insertion software tool developed in our lab was used to simulate scans acquired with reduced dose levels (12.5%, 25%, 50% of the original dose level). Images reconstructed from the same raw data but at the original and simulated, reduced dose levels were randomized and blindly reviewed by three thoracic radiologists. Lung and soft tissue regions were graded for two different imaging tasks: 1) routine diagnostic CT exam of the chest and 2) screening exam of the chest for lung cancer. A 5 point scale was used to rate each exam for each task (1=too noisy, significantly impacted confidence to 5=excellent quality, excellent confidence). Exams were considered acceptable when at least two of the three radiologists gave a score of 3 or higher. The percentage of patients meeting this definition of acceptable was calculated for each imaging task, dose level, and tissue type (lung or soft tissue). RESULTS The percentages of exams considered acceptable for the purpose of lung cancer screening were 90%, 100%, 100%, 100% (lung tissue) and 85%, 100%, 100%, 100% (soft tissue) at 12.5%, 25%, 50% and 100% of original dose level. For the purpose of routine diagnostic chest CT, the percentage of acceptable exams were 75%, 95%, 100%, 100% (lung tissue) and 35%, 85%, 90%, 100% (soft tissue) at 12.5%, 25%, 50% and 100% of original dose level. CONCLUSIONS At 25% of the dose used for routine diagnostic chest CT at our institution, acceptable image quality for the purpose of CT screening for lung cancer was achieved in 100% of the evaluated cases. This factor of 4 in dose reduction relative to routine chest exams is similar to what was used in the NLST trial.