Abstract
Introduction: Three cylindrical graphite ionization chambers have been constructed, one of which, DW3, participated in the BIPM.RI(I)-K4 key comparisons for absorbed dose to water in a gamma radiation field of the 60Co radionuclide, achieving a positive result. This prompted us to attempt to use the experimental DW2 chamber (twin to DW3 chamber) for dosimetry of high-energy photon beams used clinically with the aim of reducing the value of the total standard uncertainty of the measured radiation dose.
Material and methods: The performance of the DW2 chamber was first tested in gamma ray fields of 137Cs and 60Co. Subsequently, the chamber was calibrated in a 60Co beam relative to the GUM primary standard (DW3). Then, the kQ factors for 6, 10, and 15 MV photon beams produced by Versa HD, Elekta accelerators were determined using Monte Carlo methods. Next, for the same beams, the correction factors for saturation (ks) and the polarity (kpol) were experimentally determined. Finally, the DW2 chamber was used to measure the absorbed dose to water under reference conditions. The obtained results were compared to those of a commercial Farmer-type ionization chamber, model 30013 by PTW. A metrological conformity test was used for evaluation.
Results: The relative differences between the two chambers (DW2 vs Farmer chamber) in measured absorbed dose to water for 6, 10, and 15 MV photon beams were −0.1 %, 0.2 %, and 0.7 %, respectively. The relative standard uncertainties of the dose measurement under reference conditions using the DW2 chamber were approximately 0.6 % which are lower than those for the commercial chamber (Farmer) of 0.89 %.
Conclusions: The obtained results confirm that the DW2 ionization chamber is an accurate tool for measuring absorbed dose to water under reference conditions for therapeutic photon beams. Lower measurement uncertainty will translate into higher precision in monitoring and calibration of the accelerator’s output.