Patient Individual Equivalent Dose Calculations for CT Examinations

The aim of this study is the comparison of pa- tient specific dose-estimation in computed tomography (CT) of a dedicated Monte Carlo simulation tool with precalculated data of anthropromorphic phantoms. The developed tool GMctdospp, which allows the calculation of dose to defined structures, was employed to determine equivalent dose to lungs of six randomly chosen CT thorax- scans. The results for the estimation of lung dose were com- pared with the CTExpo program. To allow a better compara- bility the Monte Carlo data was calibrated to the CTDIair values of CTExpo for a Siemens VolumeZoom scanner with 4x2.5 mm collimation and 15 mm table shift at 90mAs. The Monte Carlo simulations included filter geometries and energy spectra provided by the vendor. Equivalent doses to the lung of the single CT-scans varied between 5.2 mSv and 7.9 mSv. The simulation times needed for a statistical uncertainty of less than 1% was approximately 10 minutes. The value obtained from the CTExpo tool and thus from an generalized anthropromorphic phantom was 6.4 mSv for this type of scan. Hence, the lung dose is influenced by the individuality of the patient by ~20%.