Controlling sensitivity and stability of ferrous–xylenol orange–gelatin 3D gel dosimeters by doping with phenanthroline-type ligands and glyoxal

Abstract

The ferrous-xylenol orange-gelatin (FXG) dosimeter is widely used for three-dimensional ionizing radiation field mapping through optical scanning. Upon irradiation, the ferrous iron (Fe(2+)) is oxidized to ferric iron (Fe(3+)), which forms an intensely coloured complex with xylenol orange (XO). XO also acts as a diffusion-limiting additive; however, its presence may cause rapid auto-oxidation of Fe(2+) during storage and low stability of the dose response. In this work, phenanthroline-type ligands were added to FXG system in a bid to bind the ferrous iron in a stable complex and minimize the rate of the auto-oxidation, whereas glyoxal was used as a chemical cross-linker, aiming to minimize the ferric iron diffusion. It was found that addition of either 1,10-phenanthroline or 5-nitro-1,10-phenanthroline can improve the auto-oxidation behaviour of the gels. However, the initial background absorbance was slightly increased, and the sensitivity of the dosimeters was decreased. Doping with glyoxal led to a moderate decrease of the diffusion only in those gels that also contained a phenanthroline-type ligand, and did not affect the initial dose response. Glyoxal also afforded an extended period of stable background absorbance level after an initial period of bleaching of the gel. Following re-irradiation, most glyoxal-containing dosimeters showed an excellent linearity of the dose response, albeit at a decreased sensitivity. We recommend further testing of FXG dosimeters, doped with phenanthroline-type ligands and glyoxal as a means for controlling the dose response and improving the long-term storage properties of the gels and the potential for dose fractionation.

Multifactorial study and kinetics of signal development in ferrous–methylthymol blue–gelatin gel dosimeters

“…Diffusion coefficients were determined as previously described. 11 The rates of auto-oxidation were taken at 40°C for 60 min, at room temperature for 120 min, and at 4°C over the course of a week. This temperature range covers the preparation, application, and storage conditions of the gels.…”

“…The concentrations of gelatin (5% w/v), FAS (0.1 mM), and MTB (0.1 mM) were kept constant. 11,22 A total of 18 formulations were prepared, using three SA concentrations (30,35, and 40 mM), three Nn concentrations (0, 0.15, and 0.3 mM), and two Gx concentrations (0 and 2 mM) in a three-factor, mixed-level design of experiment. The concentrations of the tested compounds were selected based on prior knowledge of a similar system 11 and are coded as factors x 1 to x 3 and scaled between À1 and 1 for SA and Nn, and 0 and 1 for Gx (Table I) Immediately after preparation, a sample of each composition was heated at 40°C for 1 h to accelerate the auto-oxidation reaction and evaluate its rate, while four other samples were left at room temperature for the same duration.…”

“…The addition of glyoxal had a negligible effect on the observed characteristics; however, its presence led to a significant decrease in the diffusion coefficient, as in our previous research. 11 The composition containing 35 mM SA, 0.1 mM MTB, 0.3 mM Nn, and 5.0 mM Gx, hereafter designated FMNnGxG, was chosen for the follow-up experiments. The first set of experiments with the new composition concerned the analysis of the dose sensitivity in the range of 0-10 Gy and the evaluation of any effects that the beam energy or the dose rate may have on the dose sensitivity (Fig.…”

“…19 We have previously reported a strategy to limit the rate of auto-oxidation in Frickebased gels using 5-nitro-1,10-phenanthroline (Nn) as a ferrousspecific ligand and glyoxal (Gx) as a cross-linking agent. 11 Our strategy was later translated into a commercial product (Tru-View gel; Modus QA, London, Canada), 20 and has been used for 3D dose verification of model-based dose calculation algorithms for 192 Ir brachytherapy. 21 One of the main disadvantages of the ferrous-XO-Nn-Gx-gelatin dosimeter is the high background absorbance, due to the formation of an intensely colored complex between Fe 2+ and Nn, whose absorbance gradually decreases between 520 and 650 nm.…”

“…21 One of the main disadvantages of the ferrous-XO-Nn-Gx-gelatin dosimeter is the high background absorbance, due to the formation of an intensely colored complex between Fe 2+ and Nn, whose absorbance gradually decreases between 520 and 650 nm. 11 More recently, we have reported the possible substitution of xylenol orange with methylthymol blue (MTB) in Fricke gels. 22 MTB and XO are structurally related metal indicators; however, the maximum absorbance of Fe 3+ -MTB complex is at a higher wavelength relative to Fe 3+ -XO (620 vs 585 nm).…”