Introduction of a deformable x-ray CT polymer gel dosimetry system

This study introduces the first 3D deformable dosimetry system based on x-ray computed tomography (CT) polymer gel dosimetry and establishes the setup reproducibility, deformation characteristics and dose response of the system. A N-isopropylacrylamide (NIPAM)-based gel formulation optimized for x-ray CT gel dosimetry was used, with a latex balloon serving as the deformable container and low-density polyethylene and polyvinyl alcohol providing additional oxygen barrier. Deformable gels were irradiated with a 6 MV calibration pattern to determine dosimetric response and a dosimetrically uniform plan to determine the spatial uniformity of the response. Wax beads were added to each gel as fiducial markers to track the deformation and setup of the gel dosimeters. From positions of the beads on CT images the setup reproducibility and the limits and reproducibility of gel deformation were determined. Comparison of gel measurements with Monte Carlo dose calculations found excellent dosimetric accuracy, comparable to that of an established non-deformable dosimetry system, with a mean dose discrepancy of 1.5% in the low-dose gradient region and a gamma pass rate of 97.9% using a 3%/3 mm criterion. The deformable dosimeter also showed good overall spatial dose uniformity throughout the dosimeter with some discrepancies within 20 mm of the edge of the container. Tracking of the beads within the dosimeter found that sub-millimetre setup accuracy is achievable with this system. The dosimeter was able to deform and relax when externally compressed by up to 30 mm without sustaining any permanent damage. Internal deformations in 3D produced average marker movements of up to 12 mm along the direction of compression. These deformations were also shown to be reproducible over 100 consecutive deformations. This work has established several important characteristics of a new deformable dosimetry system which shows promise for future clinical applications, including the validation of deformable dose accumulation algorithms.