This study reports on efforts to increase the dose sensitivity of polymer gel dosimeters used in 3D radiation dosimetry. The potential of several different cosolvents is investigated, with the aim of increasing the solubility of N,N'-methylene-bisacrylamide crosslinker in polymer gel dosimeters. Glycerol and isopropanol increase the limit for the crosslinker solubility from approximately 3% to 5% and 10% by weight, respectively. This enables the manufacture of polymer gel dosimeters with much higher levels of crosslinking than was previously possible. New dosimeter recipes containing up to 5 wt% N,N'-methylene-bisacrylamide were subjected to spatially uniform radiation and were studied using nuclear magnetic resonance (NMR), as well as x-ray and optical CT techniques. The resulting dosimeters exhibit dose sensitivities that are up to 2.7 times higher than measured for a typical dosimeters with 3% N,N'-methylene-bisacrylamide without the addition of cosolvent. Two additional cosolvents (n-propanol and sec-butanol) were deemed unsuitable for practical dosimeters due to incompatibility with gelatin, cloudiness prior to irradiation, and immiscibility with water when large quantities of cosolvent were used. The dosimeters with high N,N'-methylene-bisacrylamide content that used isopropanol or glycerol as cosolvents had high optical clarity prior to irradiation, but did not produce suitable optical CT results for non-uniformly irradiated gels due to polymer development outside of the high dose regions of the pencil beams and significant light scatter. Further experiments are required to determine whether cosolvents can be used to manufacture gels with sufficiently high dose sensitivity for readout using x-ray computed tomography.

A mathematical model for crosslinking copolymerization of acrylamide (or NIPAM) and N,N'‐methylenebisacrylamide is extended to account for contamination by oxygen and the inhibitor MEHQ. This model improves basic understanding of interactions among oxygen, MEHQ and polymerization reactions in gel dosimeters that are used to verify radiation doses delivered by cancer treatment equipment. Improved parameter estimates result in a good match between model predictions and data. The model predicts that a larger absorbed dose will be required to overcome the oxygen inhibition with increasing oxygen contamination, in agreement with experimental data. The model also predicts that MEHQ, in the absence of oxygen, has almost no influence on dosimeter response.magnified image

The potential of ten different crosslinkers was investigated, with the aim of improving the performance of polymer gel dosimeters used for detecting radiation dose distributions generated by cancer radiation therapy equipment. Unfortunately, none of the candidate crosslinkers was shown to be more effective than N,N 0 -methylene-bisacrylamide, the standard crosslinker used in polymer gel dosimetry applications. Two co-solvents, glycerol and isopropanol, were used to increase the solubility of N,N 0 -methylene-bisacrylamide crosslinker in polymer gel dosimeter recipes. Using isopropanol, the crosslinker solubility increased from approximately from 3 to 10% by weight, enabling the manufacture of polymer gel dosimeters with much higher levels of crosslinking than was previously possible. The new dosimeter recipes can be imaged effectively using nuclear magnetic resonance and optical techniques, and may be suitable for read-out using x-ray CT (Computed Tomography).

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