Optimized Polymer Gel Dosimetry for Kilovoltage X-Ray Irradiations and 3D Isocenter Verification
Abstract
Purpose
To optimize a N-Isopropylacrylamide (NIPAM)-based polymer gel dosimeter for kilovoltage x-ray irradiations, and to demonstrate its application for 3D isocenter verification.
Methods
A sensitivity study tested six gel formulations with varying N,N’-Methylenebis(acrylamide) (BIS) and NIPAM concentrations (3-8% and 5-15%, respectively) to improve sensitivity to kilovoltage x-rays. BIS increases the dosimeter’s sensitivity to radiation, and sufficient NIPAM concentration is required for solubility before crystallization. The optimized formula selected for optical clarity, dose response, and cost was calibrated with a 225kVp beam on a dual-robot radiotherapy system (KOALA). The gel was irradiated in a 50mL vial 24-hours after manufacturing using ~1cm beams and imaged 24-hours post irradiation. Calibrated EBT4 films were used for dose cross-calibration. The resulting change in CT number, ΔNCT, was measured from the averaged image of 10 scans acquired on a GE Revolution CT scanner within a 2x2mm2 ROI along the beam’s center over a 5-10mm depth range. Values were averaged and background-subtracted for each beam, then compared to Monte Carlo calculated doses within the ROI scaled by film measurements. The optimized formula was further evaluated with a non-coplanar star shot performed on KOALA and a LINAC. Six beam trajectories defined the isocenter walkout diameter for each machine.
Results
A formula with 4% BIS and 7% NIPAM was found optimal for high dose sensitivity before crystallization. A tanh fit was obtained over a dose range of 0-11.5Gy with a mid-range sensitivity of 2.4HU/Gy. The 3D non-coplanar star shot analysis determined the isocenter for both KOALA and LINAC, with isocenter walkout diameters of 7mm and 1.3mm, respectively. The result demonstrates a significant misalignment of the KOALA treatment robot beyond the 2mm clinical threshold.
Conclusion
The optimized polymer gel dosimeter was calibrated for kilovoltage x-ray irradiations and was used to verify the 3D isocenter of experimental and clinical radiotherapy machines.