Preliminary study on dose conversion parameters for absorbed dose to water in kV X-ray beams

Although accurate X-ray dosimetry standards are crucial for certain medical and industrial applications, insufficient research attention has been given to the standards concerning the absorbed dose to water for kV X-rays. To determine the absorbed dose to water based on measurement standards for the air kerma, it is necessary to evaluate the dose conversion factor from the air kerma to the absorbed dose to water. This factor can be derived from the dose conversion parameter, obtained through theoretical calculations such as Monte Carlo (MC) simulations, and the response ratio of the ion chamber in water and air, obtained through measurements. In this study, the dose conversion parameters were preliminarily evaluated for kilovolt X-ray beams using MC simulations. The modeling of the X-ray tube at the Korea Research Institute of Standards and Science (KRISS) was optimized to reproduce the beam qualities of the reference kilovolt X-ray beams recommended by the Consultative Committee for Ionizing Radiation (CCRI) using the Electron Gamma Shower MC code from the National Research Council Canada. The spectral distributions of the reproduced X-ray beams were used to calculate the dose conversion parameters in MC simulations. To obtain the dose conversion parameter, the air kerma and absorbed dose to water at the reference positions were calculated, and responses of an ion chamber in the air and in a water phantom were calculated, as well. The evaluated dose conversion parameters were found to be in the range of 1.0355-1.0961 for the KRISS reference X-ray beams of 100-250 kV. These results were agreed within 1% of those of other research groups. To understand the sensitivity of the obtained conversion parameters, case studies were performed under different conditions, such as different X-ray beam qualities and geometric specifications of the ion chamber. A maximum difference of 0.56% was found in the case studies.