Accurate calibration of a polymer gel dosimeter with a plastic scintillation detector.

Purpose: Three dimensional dose polymer gel dosimetry measurements provide unique information on sophisticated dose distributions. In this study, the authors propose a novel method to improve the accuracy of polymer gel dosimeters by inserting a plastic scintillation detector (PSD) to provide a dose reference.

Methods: PSD dosimeters were calibrated using chromatic deconvolution and then inserted into polyacrylanide gel (PAG) dosimeters. The gel and the PSDs were immersed into water and irradiated with 6 MV wedge filtered beams to obtain a wide range of dose variation. Calibration vials containing the same gel were also irradiated to generate a standard calibration curve. The distribution of magnetic nuclear transverse relaxation rate (R2) values of the gel was determined with a multislice multiecho MRI sequence at 1.5 T. Another calibration curve was obtained by assigning the R2 values in the gel surrounding the scintillators to the dose determined by the PSDs. A reference calibration point from a PSD located in a low dose gradient area served to correct the standard calibration method yielding three novel calibration methods. The results were compared with EBT2 GAFCHROMIC film measurements acquired in the same condition and with the Pinnacle3 treatment planning dose calculations,

Results: The mean absolute error of the standard calibration method ranged from 6.1 to 12.4%. The corresponding gamma index (3%/3 mm distance to agreement) criterion was satisfied for only 56% of the pixels in the middle slice of the gel compared to Pinnacle3 dose calculations and to EBT2 film measurements in the center part of the field. Calibration methods using a PSD reduced the mean absolute error to less than 4%; this value was under 2.6% for one of the three methods. In that case, 98% of the pixels satisfied the gamma index criterion.

Conclusions: The accuracy of PAG dosimeters may be highly improved using one reference dose point measurement using a plastic scintillation detector. The best calibration procedure corrected the slope of the calibration curve derived from the calibration vials to match the R2 value around a PSD calibration, while keeping the R2 value at 0 Gy constant.