An NMR relaxometry and gravimetric study of gelatin-free aqueous polyacrylamide dosimeters

Abstract

In conformal radiation therapy, a high dose of radiation is given to a target volume to increase the probability of cure, and care is taken to minimize the dose to surrounding healthy tissue. The techniques used to achieve this are very complicated and the precise verification of the resulting three-dimensional (3D) dose distribution is required. Polyacrylamide gelatin (PAG) dosimeters with magnetic resonance imaging and optical computed tomography scanning provide the required 3D dosimetry with high spatial resolution. Many basic studies have characterized these chemical dosimeters that polymerize under irradiation. However, the investigation of the fundamental properties of the radiation-induced polymerization in PAG dosimeters is complicated by the presence of the background gelatin matrix. In this work, a gelatin-free model system for the study of the basic radiation-induced polymerization in PAG dosimeters has been developed. Experiments were performed on gelatin-free dosimeters, named aqueous polyacrylamide (APA) dosimeters, containing equal amounts of acrylamide and N,N'-methylene-bisacrylamide. The APA dosimeters were prepared with four different total monomer concentrations (2, 4, 6 and 8% by weight). Nuclear magnetic resonance (NMR) spin-spin and spin-lattice proton relaxation measurements at 20 MHz, and gravimetric analyses performed on all four dosimeters, show a continuous degree of polymerization over the dose range of 0-25 Gy. The developed NMR model explains the relationship observed between the relaxation data and the amount of crosslinked polymer formed at each dose. This model can be extended with gelatin relaxation data to provide a fundamental understanding of radiation-induced polymerization in the conventional PAG dosimeters.

Mathematical Modeling of PAG‐ and NIPAM‐Based Polymer Gel Dosimeters Contaminated by Oxygen and Inhibitor

“…During this analysis, other parameters, for which estimates are available in the literature, remain at their original values shown in Table 5. The improved parameter estimates are determined using gravimetric data from gelatin-free dosimeters [24] and calorimetric data [14] from typical PAG dosimeters containing gelatin. One calorimetry experiment was conducted on an oxygen-contaminated dosimeter, so the data set contains some information about oxygen-inhibition kinetics (see Figure 4).…”

“…Figure 5. Comparison of simulated (---) and experimental (symbols) gravimetric data [24] obtained for gelatin-free 50%C PAG dosimeters irradiated at 1 Gy Á min…”

“…Trial-and-error simulations were used to select physically reasonable values of unknown parameters, resulting in a good qualitative match with experimental data available in the literature. Subsequently, new data [24] became available from dosimetry experiments involving recipes without gelatin, using a variety of radiation dose rates and total monomer concentrations. Unfortunately, Fuxman's parameter values do not give good predictions of these gelatin-free gravimetric data, indicating that better parameter values are required so that the models can be used to simulate new recipes reliably.…”

“…[25][26][27][28] In the current paper, the reaction mechanism in the spatially-uniform dosimeter model [21] is extended to account for the effects of oxygen and MEHQ contamination. Estimability analysis is then used to identify the poorlyknown model parameters that can be estimated from the available gravimetric [24] data and calorimetric data with and without oxygen contamination. [14] Updated parameter values are obtained using the weighted-least squares estimation algorithm in Predici, with the remaining parameters held at their literature values or the initial values of Fuxman et al The extended model is then used to simulate the behavior of PAG dosimeters that have been contaminated with oxygen and MEHQ.…”

A study on the role of gelatin in methacrylic-acid-based gel dosimeters

“…De Deene et al (2006) reported that no NMR dose response was observed in MAG systems without gelatin. They suggested a polymerization mechanism with chemical grafting of polymethacrylic acid onto gelatin in the MAG system, which is different from that in the PAG system where a significant NMR response is observed without gelatin (Babic and Schreiner, 2006). Fuxman et al (2003Fuxman et al ( , 2005 explained the reduction in monomer consumption rates with increasing gelatin concentration in PAG system in terms of chain transfer to gelatin, leading to the formation of gelatin radicals that are slow to react with monomer.…”