Performance evaluation of lead-monoxide dosimeter with parylene coating for quality assurance of brachytherapy devices

Abstract

The source position of irradiation is identified using a method that uses rulers and films for quality assurance (QA) in brachytherapy. However, this method involves a high probability of errors, because the scales are checked using the naked eye, and QA is indirectly performed using photographs. Lead monoxide (PbO) is widely used as a semiconductor dosimeter, because it is a photoconductor that generates electrons in response to electromagnetic waves. Moreover, PbO has excellent sensitivity to reflected rays, owing to its high atomic number (Z Pb: 82, Z O: 8) and density (ρPbO: 9.53 g/cm3). We applied PbO to a dosimeter for QA in a brachytherapy device and attempted to increase the signal stability with a parylene coating for performance improvement. Subsequently, a comparative analysis was performed with a PbO dosimeter that was not coated with parylene to determine whether the fabricated dosimeter is applicable as a dosimeter for QA of the brachytherapy device, by analyzing the reproducibility, linearity, percentage interval distance (PID), and angular dependence in the 192Ir source used for brachytherapy. The RSD of the non-parylene PbO dosimeter was 0.85%, and the RSD of the parylene PbO dosimeter was 0.40% in the reproducibility results. In the linearity evaluation results, the R 2 value of the non-parylene PbO dosimeter was 0.9996, and that of the parylene PbO dosimeter was 0.9997 In the PID evaluation results, the difference in the intensity distribution measured according to the distance due to the dose was attenuated at the coated parylene in the case of the parylene PbO dosimeter. Therefore, adjustments using correction coefficients are required for suitable performance. In the angular dependence evaluation results, the parylene PbO dosimeter had 3.44% less angular dependence than the non-parylene dosimeter at an angle of 45°. The parylene-coated PbO dosimeter showed better performance than the non-parylene-coated PbO dosimeter in terms of the reproducibility, linearity, and angular dependence. Therefore, it is considered that the parylene-coated PbO dosimeter can be implemented for QA of brachytherapy devices.