Comparison between experimental and calculated ratios DW/Ka in the field of X-radiation with generating potentials from 70 kV to 280 kV

The measurand used in dosimetry for the therapy with X-radiation is the water absorbed dose, D_W. As a primary standard in the range of mean-energy X-radiation has so far not been available, dose measurement is performed via the ionometric determination of the measurand air kerma, K_a, and with calculated conversion factors with the aid of which conversion into the water absorbed dose is performed. As a matter of principle, a water calorimeter is also suitable as a primary standard in the field of X-radiation and has been used for 6 different X-ray qualities for direct DW determination. A provisional assessment of a comparison of the results of the calorimetric determination of D_W and of the ionometric determination of K_a with the ratio of the two measurands calculated with the aid of Monte Carlo methods shows an agreement within approx. ± 1.5%.

Measurand in dosimetry of X-radiation is the water absorbed dose, D_W. As a primary standard in the range of mean-energy X-radiation has so far not been available, dose measurement is at present performed at PTB via the ionometric determination of the measurand air kerma, K_a, and with the aid of calculated conversion factors for conversion into the water absorbed dose. As a matter of principle, a water calorimeter is suited as primary standard also in the field of X-radiation and has been used for 6 different X-ray qualities for direct D_W determination. In addition, the results of the calorimetric determination of D_W and of the ionometric determination of K_a have been compared with the ratio of the two measurands calculated by Monte Carlo methods.

With the aid of PTB‘s transportable water calorimeter, experiments for the direct determination of the water absorbed dose were performed for 6 different X-radiation qualities with generating potentials from 70 kV to 280 kV. For this purpose, the calorimeter was installed - depending on the irradiation device - in front of the X-ray equipment at a focus-to-measuring location distance of 55 cm or 88 cm. The diameter of the radiation field at the measuring location amounted to approx. 15 cm, at a water depth of 5 cm. Due to the low dose rate between approx. 0.15 Gy/min at 280 kV and 0.35 Gy/min at 140 kV, calorimetric measurements were carried out for each radiation quality over a period of at least one week. The duration of the irradiation per single measurement amounted to 120 s.

The calorimetric determination of the water absorbed dose requires, among other things, exact knowledge of the heat transport processes in the calorimeter. These are determined by irradiation of the detector materials and by dose distribution of the radiation. Simulation of the heat transport with the aid of the finite element method showed that in particular the effects caused by the irradiation of the temperature sensors of the detector, including the Pt lead wires, are of great importance. This furnishes corrections of the measured temperature increase of approx. 2 % - 3 %. Additional corrections result from the perturbation of the radiation field by the detector of the calorimeter. The perturbation effect was determined both by experiment with the aid of a dummy detector and theoretically with the aid of the Monte Carlo method. The corrections for the perturbation effect vary between 0.5 % at 280 kV and 8.2 % at 70 kV X-radiation. The provisional assessment of the uncertainty budget of the calorimetric determination of the water absorbed dose in the range of X-radiation with generating potentials between 70 kV and 280 kV leads to a standard uncertainty of approx. 1 %.

After the calorimetric experiments, the air kerma were determined free in air for each radiation quality, at an identical focus-to-measuring location distance with the aid of a calibrated ionization chamber. In addition, the ratio between air kerma and water absorbed dose at the measuring location in the calorimeter can in principle be calculated with the aid of the Monte Carlo method provided that the radiation transport in the different materials of the water calorimeter (isolation, water phantom, water, detector) can be simulated. In the case of the insulation layers of the calorimeters, the attenuation of the X-radiation was, however, determined by experiment, as the lack of knowledge of the exact composition of these materials complicates a simulation. The comparison between the ratio of the water absorbed dose and the air kerma determined by experiment with the calculated ratios showed an agreement within approx. ± 1.5 % across the energy range of the X-radiation observed here.