Evaluation of various approaches for assessing dose indicators and patient organ doses resulting from radiotherapy cone-beam CT

Autor: Francesca Romana Giglioli, Veronica Rossetti, Christian Fiandra, Riccardo Ragona, Roberto Ropolo, Osvaldo Rampado
Rok vydání: 2016
Předmět:
Zdroj: Medical Physics. 43:2515-2526
ISSN: 0094-2405
DOI: 10.1118/1.4947129
Popis: Purpose: The aim of this study was to evaluate various approaches for assessing patient organdoses resulting from radiotherapy cone-beam CT(CBCT), by the use of thermoluminescent dosimeter(TLD) measurements in anthropomorphic phantoms, a Monte Carlo based dose calculation software, and different dose indicators as presently defined. Methods: Dose evaluations were performed on a CBCT Elekta XVI (Elekta, Crawley, UK) for different protocols and anatomical regions. The first part of the study focuses on using pcxmcsoftware (pcxmc 2.0, STUK, Helsinki, Finland) for calculating organdoses, adapting the input parameters to simulate the exposure geometry, and beamdose distribution in an appropriate way. The calculated doses were compared to readouts of TLDs placed in an anthropomorphic Rando phantom. After this validation, the software was used for analyzing organdose variability associated with patients’ differences in size and gender. At the same time, various dose indicators were evaluated: kerma area product (KAP), cumulative air-kerma at the isocenter (Kair), cone-beam dose index, and central cumulative dose. The latter was evaluated in a single phantom and in a stack of three adjacent computed tomography dose index phantoms. Based on the different dose indicators, a set of coefficients was calculated to estimate organdoses for a range of patient morphologies, using their equivalent diameters. Results: Maximum organdoses were about 1 mGy for head and neck and 25 mGy for chest and pelvis protocols. The differences between pcxmc and TLDsdoses were generally below 10% for organs within the field of view and approximately 15% for organs at the boundaries of the radiationbeam. When considering patient size and gender variability, differences in organdoses up to 40% were observed especially in the pelvic region; for the organs in the thorax, the maximum differences ranged between 20% and 30%. Phantom dose indexes provided better correlation with organdoses than Kair and KAP, with average ratios ranging between 0.9 and 1.1 and variations for different organs and protocols below 20%. The triple phantom setup allowed us to take into account scatter dose contributions, but nonetheless, the correlation with the evaluated organdoses was not improved with this method. Conclusions: The simulation of rotational geometry and of asymmetric beam distribution by means of pcxmc 2.0 enabled us to determine patient organdoses depending on weight, height and gender. Alternatively, the measurement of an in phantom dose indicator combined with proper correction coefficients can be a useful tool for a first dose estimation of in-field organs. The data and coefficients provided in this study can be applied to any patient undergoing a scan by an Elekta XVI equipment.
Databáze: OpenAIRE
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