Dose Determination at kV X-ray Qualities Using Different Protocols
Abstract: Absorbed dose was determined under reference conditions at four x-ray qualities; 30, 80, 120 and 200 kV using five different dosimetry protocols and codes of practice; IAEA TRS-398, AAPM, IPEMB, NCS and DIN. The dose determined by the IAEA protocol was chosen as reference. The dosimetry protocols use different formalisms for determining the absolute dose in a reference point. The IAEA and DIN protocols are based on a dose-to-water calibration factor, ND,w. The other protocols (including DIN) are air kerma based, with calibration factor NK. Some standard laboratories can provide the user with both types of calibration factors. The reference conditions for the different codes are either “in water” or “in air”. In the latter case the dose has to be recalculated into dose to water. The use of plane parallel ionization chambers are generally recommended for low energy photon beams, but at an interval from about 80 kV to 160 kV both plane parallel and cylindrical chambers may be used depending on protocol. The measurements for dose determination were carried out three times at different occasions to minimize uncertainties. The results were generally very stable with a total mean variation of 0.5 %, and in any single measurement series never exceeding 1 %. A mean value from the three measurements was used to calculate the dose. For low energy x-rays the correlation in absolute dose per MU using the different protocols was good. For 30 kV the general deviation was only fractions of a percent. For 80 kV the deviation was around 1 %. At medium energy, however, the different formalisms resulted in larger deviations and could be explained by different reference conditions for the different protocols, for example use or no use of phantom and type of ionization chamber. The DIN protocol was the one that best correlated with the IAEA protocol. DIN uses both formalisms. AAPM dose determinations at medium energy x-rays in phantom and NCS were also well correlated, probably due to similar reference conditions. To be able to better compare doses calculated using the different protocols, more uniformity between the protocols is needed.
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