Methods of CT Dose Estimation in Whole-Body 18F-FDG PET/CT

We evaluated the effective dose (ED) of the CT component of wholebody PET/CT using software dedicated to CT dose estimation and from dose-length product (DLP) values to establish practical methods of ED estimation. Methods: Eighty adult patients who underwent F-18-FDG whole-body PET/CT were divided into groups A and B, each consisting of 20 men and 20 women. In group A, ED of the CT component was calculated using CT-Expo for 6 anatomic regions separately, and whole-body ED was obtained by summing the regional EDs (CT-Expo method). DLP was calculated for each of the 6 regions and multiplied by a corresponding conversion factor described in International Commission on Radiological Protection publication 102 to obtain the ED for each region (regional DLP method). Whole-body ED was also calculated as the product of a whole-body DLP value provided by the scanner automatically and a conversion factor (simple DLP method). Moreover, the ED/DLP values were calculated using whole-body ED estimated by the CT-Expo method and the scannerderived DLP, to optimize the conversion factor. In group B, the optimized conversion factor was applied for the estimation of ED by the simple DLP method. Results: In group A, the regional DLP method allowed an accurate estimation of mean whole-body ED as a result of counterbalance of mild overestimation in men and mild underestimation in women, regarding the CT-Expo method as a standard. The simple DLP method using a conversion factor for the trunk (0.015 mSv/mGy/cm) caused overestimation. On the basis of the ED/DLP values in group A, a modified conversion factor of 0.013 mSv/mGy/cm and sex-specific conversion factors of 0.012 and 0.014 mSv/mGy/cm for men and women, respectively, were determined. In group B, the use of the modified conversion factor improved accuracy, and the use of sex-specific conversion factors eliminated sex-dependent residual errors. Conclusion: ED of the CT component of whole-body PET/CT can be assessed by multiplying the scannerderived DLP by a conversion factor optimized for whole-body PET/CT.