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 iondiffusion 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 whenproduction, calibration and use of the gels are separated by just a few days. In other cases, the effect is much milder, but shouldstill be properly accounted for when gels are used over a long period a time, such as in the proposed monitoring of adaptiveradiotherapy 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 theiruse, as well as for the fractionation schemes adopted for the irradiations. Based on a single kinetic constant and on the initialamounts of Fe2+ and Fe3+, our model predicts as a function of time the amount of Fe3+ present in the gels and coordinated withxylenol-orange.