DIFFUSION AND SENSITIVITY CHARACTERISTICS OF A CHEMICALLY CROSS-LINKED PVA-FRICKE GEL DOSIMETER

Abstract

Introduction: Current radiotherapy techniques implement treatment plans based on volumetric distributions of dose with complex shapes and sharp gradients. The agreement between these plans and the dose that is truly delivered is very challenging to verify. 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 framework, great interest is encountered by radio-chromic gel dosimeters. In order to address the limitations of gels based on natural matrices, some investigators have proposed dosimeters based on poly vinyl alcohol (PVA) made by freezing-thawing cycles. Even though these gels have a 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 out by MRI. We hereby present a PVA gel that is chemically crosslinked, and that offers high sensitivity and low diffusion. These gels can be read out with optical techniques. Materials and Methods: The proposed gel contains 10% w/v of PVA and is crosslinked by adding glutaraldehyde (GTA). A common formulation gelatin gel was also prepared and studied for comparison. Both gels contain Xylenol Orange and the “Fricke” chemical dosimeter: 25 mM sulphuric acid and 0.5 mM iron ammonium sulphate. Absorbance measurements were done to assess sensitivity and diffusion coefficient of the gel. Results: 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 PVAbased gel is 0.23 mm2/h. These characteristics were compared to gelatin and to data available for so called PVA hydrogels and cryogels. Conclusions: Our dosimeter made with PVA and crosslinked with GTA showed a sensitivity that is comparable to gelatin, and greater than PVA hydrogels. The diffusion coefficient is 0.23mm2/h, against 0.56 and 0.14 of gelatine and cryogels.