Response characterization of gan for radio-luminescence dosimetry using homogenous and heterogeneous phantom

Abstract

Purpose and objective It was demonstrated that the radio-luminescence of GaN coupled with an optical fiber can be used in implantable real-time dosimetric probe for in vivo dosimetry [1]. The objective of this study was to evaluate the performance of GaN system for in vivo dosimeter for external radiotherapy according ESTRO and AAPM recommendations. Material and methods This study was realized in radiotherapy clinical conditions with a 6MV photon beam. PMMA homogenous phantom and equivalent lung heterogeneous phantoms were used. With these irradiation conditions, the dose was first evaluated using two different algorithms implemented in the Eclipse® treatment planning system. The dose was then measured with a reference ionization chamber and the GaN based dosimetric system. The calibration and correction factors for the GaN dosimetric were defined according the methods described in [2]. We studied the influence on the response of GaN of the following parameters: SSD, field size, beam incidence, dose rate and cumulative dose. We studied the several different metrological parameters such as the response linearity, reproducibility and repeatability of measurements. The percent depth dose for field sizes of 4 × 4, 15 × 15 cm2 and the dose lateral profiles for field sizes of 3 × 3, 5 × 5 and 10 × 10 cm2 were measured. Dosimetric analysis was based c index with accuracy requirement of 2% for dose and 2 mm for distance [3]. A statistical analysis was implemented using Wilcoxon signed rank test. Results and discussion: No statistically significant difference have been observed between measured dose with the ionization chamber and the GaN dosimetric system (p-value >5%) for the tested parameters. The system exhibited a linear dosimetric response with good reproducibility and repeatability for the measurements. For external radiotherapy tolerance limit of 2 mm for distance and of 2% for dose is verified on the central beam axis. Accuracy better than 5 mm for distance and 5% for dose is achieved on the field edges. Conclusion GaN showed a good accuracy with ionization chamber for clinic application using homogenous and heterogeneous phantom. The correction factors were conformed to ESTRO and AAPM recommendations.