Determination of absorbed dose to water from a miniature kilovoltage x-ray source using a parallel-plate ionization chamber

摘要

Electronic brachytherapy sources are widely accepted as alternatives to radionuclide-based systems. Yet, formal dosimetry standards for these devices to independently complement the dose protocol provided by the manufacturer are lacking. This article presents a formalism for calculating and independently verifying the absorbed dose to water from a kV x-ray source (The INTRABEAM System) measured in a water phantom with an ionization chamber calibrated in terms of air-kerma. This formalism uses a Monte Carlo (MC) calculated chamber conversion factor, C-Q, to convert air-kerma in a reference beam to absorbed dose to water in the measurement beam. In this work C-Q was determined for a PTW 34013 parallel-plate ionization chamber. Our results show that C-Q was sensitive to the chamber plate separation tolerance, with differences of up to 15%. C-Q was also found to have a depth dependence which varied with chamber plate separation (0 to 10% variation for the smallest and largest cavity height, over 3 to 30 mm depth). However for all chamber dimensions investigated, C-Q was found to be significantly larger than the manufacturer reported value, suggesting that the manufacturer recommended method of dose calculation could be underestimating the dose to water.