Dose integration characteristics in normoxic polymer gel dosimetry investigated using sequential beam irradiation

Dose integration properties were investigated for normoxic polymer gels based on methacrylic acid (nMAG) and acrylamide/ N, N'-methylenebisacrylamide (nPAG). The effect of sequential irradiation was studied for different fractionation schemes and varying amounts of methacrylic acid for the nMAG gels. Magnetic resonance imaging (MRI) was used for read out of the absorbed dose response. The investigated gels exhibited a dependence on the fractionation scheme. The response when the total dose was divided into fractions of 0.5 Gy was compared with the response when the total dose was delivered in a single fraction. The slope of the R2 versus the absorbed dose response decreased when the absorbed dose per fraction was increased. Also, for higher amounts of methacrylic acid in the nMAG system the difference in the response increased. For gels containing 2, 4, 6 and 8% methacrylic acid, the R2 versus the absorbed dose response increased by 35, 37, 63 and 93%, respectively. Furthermore, the effect of the fractionation was larger when a higher total absorbed dose was given. The effect was less pronounced for the investigated nPAG, containing 3% acrylamide and 3% N, N'-methylenebisacrylamide, than for the nMAG systems. Consequently, this study indicates that the nPAG system has preferable beam integration characteristics compared with the nMAG system.