The impact of self-oxidation on the dosimetric performance of ferrous-sulfate/xylenol-orange gels

Since their introduction in the 1980’s, most of the research on ferrous-sulfate gel dosimeters has aimed at minimizing the ferric ion diffusion phenomena that blur the 3D distribution of the signal. However, equally important are the spontaneous oxidation effects, which alter the sensitivity of the gels over time. In some gels, this effect is very pronounced and must be accounted for even when production, calibration and use of the gels are separated by just a few days. In other cases, the effect is much milder, but should still be properly accounted for when gels are used over a long period a time, such as in the proposed monitoring of adaptive radiotherapy treatments. Our work examined and modeled the spontaneous oxidation processes occurring in ferroussulfate/ xylenol-orange gels for 3D dosimetry. The model accounts for the time interval between production of the gels and their use, as well as for the fractionation schemes adopted for the irradiations. Based on a single kinetic constant and on the initial amounts of Fe2+ and Fe3+, our model predicts as a function of time the amount of Fe3+ present in the gels and coordinated with xylenol-orange.