Studies of Fricke-PVA-GTA xylenol orange hydrogels for 3D measurements in radiotherapy dosimetry

The Fricke Xylenol gels (FXGs) composition has been modified over the years in order to improve their dosimetric characteristic for spatial dose assessment in radiotherapy. FXGs dosimeter exploits the Fe2+ to Fe3+ oxidation produced by ionizing radiation and the [Fe3+-OX] complex formation, giving optical absorbance (OA) peak centered around 585 nm. Some drawbacks, in particular that related to the diffusion of Fe3+ ions in the gel matrix, have limited the clinical use of FXGs and still represent important challenges for the scientific community working in the field of gel dosimetry. In this work, FXG based on poly-vinyl alcohol (PVA) as gelling agent and glutaraldehyde (GTA) as cross-linker [1], were developed with the aim to overcome the diffusion drawbacks affecting the FXG prepared with traditional natural gel matrices like gelatin or agarose [2]. Furthermore, driven by recent works available in the literature [3-5], we investigated the dosimetric response of PVA-GTA-FXG against different additives like sulfuric acid [3-4] and saccharide [5]. FXGs with PVA and GTA concentrations of 10% w/w and 1% w/w respectively, were prepared using Fricke solutions with different amounts of sulfuric acid in the interval 18-100 mM and with different amounts of glucose and sucrose in the interval 0-1.5 g/L. FXGs in spectrophotometry cuvettes were uniformly irradiated with a 137Cs source. The OA properties and dose-response in the interval 0-25 Gy were investigated. The shapes of the OA spectra of the gel dosimeters in the wavelength interval 350-750 nm showed only a slight dependence on the saccharide presence. Therefore, the addition of saccharide did not modify significantly the dosimetric properties of Fricke gels in term of OA spectra and dose-response curves. All investigated samples exhibited a linear dose response in the studied interval. By contrast, the increase in the concentration of sulfuric acid gave appreciable effects. In fact, as in traditional FXGs, sulfuric acid concentration proved to significant ly affect the OA spectra. As the concentration of sulfuric acid increases, a decrease in the sensitivity of the dosimeters and an increase in the linearity range were observed. Finally, the results of a stability analysis over time of the optical response of PVA-GTA-FXG samples prepared with and without saccharide will be presented and discussed. The results proved the adequacy of the studied gels for utilization in dosimetry for radiotherapy.