Validation of a calculation algorithm for organ doses in the existing CT dosimetry tools using paediatric anthropomorphic phantom measurements

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Background: Rapid and user-friendly software tools for individual estimates of organ doses in CT are of the utmost importance to support the professionals in the choice of the appropriate imaging modalities. Most CT-dosimetry tools, like CT Expo [1], CT Dosimetry [2], as well as a new dosimetry tool [3], rely on the assumption that organ doses estimated for one CT scanner can be converted to other CT scanner using the ratio of the Computed Tomography Dose Index (CTDI) between two CT scanners. We tested this assumption for paediatric patients using physical phantom measurements for various protocols and CT scanners. Measurements were compared with dosimetry tools. Materials and methods: Dose to 22 radiosensitive organs were measured using thermoluminescence dosimeters inserted within an anthropomorphic 5-year old phantom. Dose measurements were performed for wholebody scans on 4 scanner models from the 4major manufacturers (Siemens, GE, Philips and Toshiba); both axial and helical modes were tested with small collimations (below 12 mm). Measured doses were compared with the results of Monte-Carlo simulations [3]. Results:Whereas the measured organ doses showed important variability across the different scanner models, doses normalised to CTDI showed significantly less variation. Higher tube voltage also showed less variation. The simulated organ doses reasonably compared with the measurements. For example, at 120 kV in axial mode with 10 mm collimation, the mean coefficient of variation between the scanner models was below 10%; the relative difference between measured and simulated organ doses was within ±15% for most organs. Discussion: For the tested collimations (below 12 mm), both in axial and helical modes, the CTDI fairly accounts for the difference in CT design across different models. Further measurements are being carried out to test the algorithm against protocols more representative of the clinical practice. This study proved that, for small collimations and paediatric patients, organ doses simulated from one CT scanner can be converted to other CT scanners using CTDI-ratio. Those results are of interest for fast dose assessment and retrospective dosimetry studies. Acknowledgments: These measurements were performed within the framework of the EPI-CT study, supported by the European Community's Seventh Framework Programme (FP7/2007e2013) under grant agreement number 269912-EPI-CT: Epidemiological study to quantify risks for paediatric computerized tomography and to optimise doses. [4].