Comparison between absorbed dose to water standards established by water calorimetry at the LNE-LNHB and by application of international air-kerma based protocols for kilovoltage medium energy x-rays
Résumé
Until now, the only way to determine absorbed dose to water for medium energy X-ray beams was to calculate it from references in terms of air kerma by application of international dosimetric protocols. New standards in terms of absorbed dose to water had just been established at LNELNHB, the French primary standard laboratory for ionizing radiation, thanks to water calorimetry, for medium energy X-rays in the reference conditions described by protocols (at 2 cm depth in water). The aim of this study is to compare these new references in terms of absorbed dose to water measured by calorimetry in six medium energy X-ray beams to the dose values calculated from the application of four international protocols based on references in terms of air kerma: IAEA TRS277, AAPM TG-61, IPEMB, NCS-10. The acceleration potentials of these beams are 80 kV; 120 kV; 150 kV; 180 kV; 250 kV and 300 kV with half value layers between 3.01 mm of aluminum and 3.40 mm of copper. A difference lower than 2.1 % was reported between the two methods. Standard uncertainty of water calorimetry being below 0.8 %, the one associated to the values from protocols being around 2.5 %, results are consistent considering the uncertainties. Calibration factors in terms of absorbed dose to water, established by application of calorimetry and air kerma dosimetric protocols were also compared. It appears that calibration factors established thanks to AAPM TG-61 protocol are in best agreement with the calibration factor established by water calorimetry
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