Diffusion and sensitivity characteristics of a chemically cross-linked PVA-Fricke gel dosimeter

The most recent radio-therapeutic technics adopt treatment plans that use volumetric distribution of dose with complex shapes and sharp gradients. The agreement between the plan and the dose that is truly delivered has to be verified. Thus, there is the need for a dosimetric system that is truly three dimensional, sensitive to radiation in each point and tissue equivalent. In this context, a great interest has been given to radio-chromic gel dosimeters. In order to solve the shortcomings of gels based on natural polymer, some authors have proposed dosimeters based on poly vinyl alcohol (PVA) made by freezing-thawing cycles (Chu et al.; Hill et al.). Even though these gels have low diffusion rate, they are also difficult to prepare and they have low sensitivity. Furthermore, depending on the gelation temperature they can only be read by MRI. We here present a PVA gel that is chemically crosslinked, and that exhibits high sensitivity and low diffusion. These gel can be read with optical techniques. The proposed gel is 10% w/v in PVA and was crosslinked adding 6.62 mM glutaraldehyde (GTA). A 3% w/v gelatine gel has been prepared for comparison. Both gels contain the chemical dosimeter, whose composition is: 25 mM sulphuric acid, 0.5 mM iron ammonium sulphate, 0.165 mM Xylenol Orange. The gel can be read optically, then absorbance measurements were collected to assess its sensitivity and diffusion coefficient. The dose response of our dosimeter is linear between 0 and 30 Gy and its sensitivity is equal to 0.073 Gy-1. The diffusion coefficient of the PVA-based gel is 0.23 mm2/h. These characteristics has been compared to gelatine and to the data available for so called PVA hydrogels and cryogels. Our dosimeter made by PVA crosslinked with GTA has shown a sensitivity that is comparable to gelatine, and greater than PVA hydrogels. The diffusion coefficient is 0.23 mm2/h, against 0.56 and 0.14 of gelatine and cryogels.