(Wednesday, 8/1/2018) 10:00 AM - 10:30 AM
Room: Exhibit Hall | Forum 1
Purpose: To evaluate size-specific dose estimates (SSDE) as a function of patient size for adults examined with low-dose lung cancer screening CT.
Methods: With institutional-review-board approval, data was retrospectively collected for 388 adult patients examined with lung cancer screening CT during a one-year period. Patients were examined on four CT scanners from two different manufacturers. Automatic exposure control (AEC) was used to adjust scanner output for each patient according to the measured CT attenuation. The volumetric CT dose index (CTDIvol) was used with size-based conversion factors from the American Association of Physicists in Medicine Report 220 to determine SSDE. Regression models were used to assess dependence of CTDIvol, dose-length-product (DLP), and SSDE on patient weight, body mass index (BMI), effective diameter (Deff), and water-equivalent diameter (Dw). Results were compared with standards established by the US Centers for Medicaid and Medicare Services (CMS) that the CTDIvol for a standard-sized patient be 3.0 mGy or less, with adjustments for patients of different sizes.
Results: Patients ranged from severely underweight to severely obese. CTDIvol increased quadratically with all patient size metrics, having significant correlations (P<0.001) with weight (R2=0.83), BMI (R2=0.80), Deff (R2=0.87) and Dw (R2=0.89). DLP and SSDE also had significant correlations with all size metrics (R2>0.80, P<0.001). All exams produced CTDIvol values compliant with CMS requirements (Average=2.3 mGy, Range=0.9-5.4 mGy). CTDIvol values greater than 3.0 mGy were only observed for 28 overweight or obese patients. One vendor’s AEC had a stronger dependence on patient size than the other.
Conclusion: For all examinations, dose indices increased quadratically with patient size, with the strongest correlation with Dw. SSDE offered a better estimate of average patient dose than CTDIvol. Performing lung cancer screening with AEC methods allowed CTDIvol to be reduced for smaller patients and increased for larger patients, producing values compliant with CMS requirements.
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