Improving Size Specific Dose Estimates (TG 204): Extension to Tube Current Modulated CT Scans with Regional and Local CTDIvol Values
M Khatonabadi1*, D Zhang2, C Cagnon1, J DeMarco1, M McNitt-Gray1, (1) UCLA School of Medicine, Los Angeles, CA, (2) Formerly UCLA/ Currently Toshiba America Medical Systems, INC., Tustin, CATU-G-217BCD-7 Tuesday 4:30:00 PM - 6:00:00 PM Room: 217BCD
Purpose: The recently published TG 204 report established conversion factors based on measurements and Monte Carlo simulations, which adjust scanner-reported CTDIvol for patient size, called size specific dose estimates (SSDE). However, those estimates were based on fixed tube current scans, while most performed clinical protocols make use of Tube Current Modulation (TCM). Therefore, the purpose of this study was to assess the performance of these conversion factors compared to Monte Carlo based simulated organ dose from fixed and TCM scans and to introduce region and organ specific CTDIvol to extend the calculated SSDEs to TCM scans.
Methods: Using Monte Carlo based methods; liver doses were estimated from both constant tube current and TCM abdomen/pelvis CT exams. The effective diameter was calculated for each model and used to look up conversion factors which were then applied to scanner-reported CTDIvol (CTDIvol,TCM)and CTDIvol based on constant tube current exam (CTDIvol,cTC) to calculate SSDEs. Calculated SSDEs were compared to liver dose estimates from constant tube current and TCM simulations. Additionally, regional and liver-specific CTDIvol (CTDIvol,Abdomen and CTDIvol,Liver) were calculated and used to calculate TCM adjusted SSDEs.
Results: Liver doses obtained from fixed tube current simulations were compared to SSDEs calculated using CTDIvol,cTC, and doses obtained from TCM simulations were compared to scanner-reported CTDIvol,TCM. For these two datasets RMSE values of 3.0 and 3.3 mGy were calculated, respectively. RMSE value decreased to 1.5 and 1.3 mGy for SSDEs calculated based CTDIvol,Abdomen and CTDIvol,Liver, respectively.
Conclusion: Conversion factors determined in TG 204 are based on fixed tube current measurements and Monte Carlo simulations and need further adjustments if they are being used to assess TCM exams. We have shown that region and organ-specific CTDIvol can be used to adjust for TCM and to improve the performance of calculated SSDEs to estimate dose from TCM exams.