Intercomparison of 6 phantoms for ESD evaluation in 11 standard x-ray examinations

Abstract

The most useful ways of evaluating the entrance skin dose, an important parameter to assess the dose received by a patient in a single radiographic exposure, are either direct measurements on phantoms by ionization chamber or calculations by mathematical model. We intercompared six phantoms (three anthropomorphic, two physical and one mathematical) in eleven standard clinical examinations (AP Abdomen, PA Chest, AP Chest, LAT Chest, AP Lumbar Spine, LAT Lumbar Spine, LAT Lumbo-Sacral Joint, AP Pelvis, PA Skull, LAT Skull, AP Urinary tract) in order both to have conversion factors to use for entrance skin doses measured on different phantoms and to validate the mathematical model used. Firstly, an intercomparison was done between the three anthropomorphic (Alderson Rando, Chest RSD-77SPL, 3M Skull) and the two physical (Uniform, AAPM 31) phantoms, and for every examination "relative entrance skin dose factors" were obtained. Secondly, these five phantoms were intercompared with the mathematical one: the overall accuracy of the model was better than 14%. Total mathematical model uncertainties were estimated to be on the order of +12% and total ionization chamber uncertainties were estimated to be on the order of +3%. To reduce the most significative source of uncertainty, the overall accuracy of the model was recalculated using new backscatter factors: the overall accuracy of the model improved, resulting better than 12%. For every examination an anthropomorphic phantom was considered as gold standard relatively to the physical ones: in this way it was possible to analyze the variations in phantoms design and characteristics. Finally, the mathematical model has been validated by more than 400 measurements taken on different phantoms and radiological equipments: therefore it can be used satisfactorily in entrance skin dose evaluations because it allows the optimization of the available resources, it is based on direct measurements and it represents an easy dynamic tool.