A Comprehensive Evaluation of NIPAM Polymer Gel Dosimeters on Three Orthogonal Planes and Temporal Stability Analysis

Abstract

Polymer gel dosimeters have been proven useful for dose evaluation in radiotherapy treatments. Previous studies have demonstrated that using a polymer gel dosimeter requires a 24 h reaction time to stabilize and further evaluate the measured dose distribution in two-dimensional dosimetry. In this study, the short-term stability within 24 h and feasibility of N-isopropylacrylamide (NIPAM) polymer gel dosimeters for use in three-dimensional dosimetry were evaluated using magnetic resonance imaging (MRI). NIPAM gels were used to measure the dose volume in a clinical case of intensity-modulated radiation therapy (IMRT). For dose readouts, MR images of irradiated NIPAM gel phantoms were acquired at 2, 5, 12, and 24 h after dose delivery. The mean standard errors of dose conversion from using dose calibration curves (DRC) were calculated. The measured dose volumes at the four time points were compared with those calculated using a treatment planning system (TPS). The mean standard errors of the dose conversion from using the DRCs were lower than 1 Gy. Mean pass rates of 2, 5, 12, and 24 h axial dose maps calculated using gamma evaluation with 3% dose difference and 3 mm distance-to-agreement criteria were 83.5% ± 0.9%, 85.9% ± 0.6%, 98.7% ± 0.3%, and 98.5% ± 0.9%, respectively. Compared with the dose volume histogram of the TPS, the absolute mean relative volume differences of the 2, 5, 12, and 24 h measured dose volumes were lower than 1% for the irradiated region with an absorbed dose higher than 2.8 Gy. It was concluded that a 12 h reaction time was sufficient to acquire accurate dose volume using the NIPAM gels with MR readouts.

The Impact of Temporal Changes in Irradiated nMAG Polymer Gels on Their Applicability in Small Field Dosimetry in Radiotherapy

“…It is important to note that this process is extremely fast when the solution is cooled down below 35 °C, but the evolution is much slower in later phases. This process can take up to 30 daysor longer and may cause shrinkage of the gel by losing water, which leads to the density changes in irradiated gels contributing to the evolution/instability in the dose responses [ 10 , 39 , 43 , 44 , 45 ]. Post-irradiation polymerization reactions progress up to 10 h after the irradiation [ 19 , 42 ], however, they do not fully stop due to the presence of remaining long-term radicals.…”

“…The successful application of imaging modalities (MRI and OCT) for the evaluation of polymer gel dosimeters applicability in the small field dosimetry are provided in studies [ 38 , 39 ], where it was shown that the normoxic NIPAM polymer gel dosimeters can be applied for the assessment of small field dose deliveries using both read out techniques: cone beam optical computed tomography and MRI. Both methods indicated an acceptable gamma passing rate (<1) since a voxel agreement of 95% for the target level was achieved between the treatment plan and gel-measured dose distributions, applying 3% dose and 3 mm distance-to-agreement gamma function criteria.…”

“…Despite an impressive number of papers that focused on the investigation of dose gels and their applicability for dosimetry purposes [ 18 ], there was not much attention paid to the investigation of the temporal behavior of gel dosimeters after irradiation, just roughly assuming that gels must be read out not any earlier than 12 h after irradiation. However, gelation and polymerization reactions in polymer gels can continue for many hours after irradiation, causing temporal evolution/instability in dose responses [ 12 , 39 , 40 , 41 ], thus influencing the adequacy of the results of the dose profile measurements.…”

Hydrogels for Three-Dimensional Ionizing-Radiation Dosimetry

“…The calibration tubes filled with NIPAM gel exposed to given doses (reported in Figure A ) are located around the centran phantom. Reproduced from [ 64 ].…”