Low dose characterization of Fricke gel dosimeters by optical absorbance and MR relaxation methods

Abstract

Introduction Fricke gel dosimeters allow measurements of 3D dose distributions and can be an effective tool for dosimetry verifications in radiotherapy. Various authors have reported drawbacks of the gels affecting the accuracy in case of low dose measurements. Purpose This study aims at investigating such drawbacks and at establishing the conditions for an optimal use of the gels. Materials and methods Our dosimeters consist of gels infused with a Fricke ferrous sulphate solution combined with xylenol orange (XO). Two kinds of xylenol orange (XO) and two different gelling agents (gelatin from porcine skin and Agarose) were utilized. Gels are red out by light absorption techniques: images of absorbance in two wavelength regions (around 430 nm and 585 nm) are acquired with a CCD camera. Absorbance spectrum measurements were done with a compact spectrometry based on optical fibres. Magnetic resonance (MR) measurements of Longitudinal Relaxation Time (T1) were also performed. Results Different trends characterized the absorbance spectra of dosimeters obtained with different gelling agents or different XOs. Moreover, the trends were different for different absorbed doses and varied over a few hours after irradiation. Conclusion The variation of the absorbance spectra is consistent with various previously unexplained effects reported in literature and may explain them. The results are a valid aid for a better use of these dosimeters and warrant further studies of the changes in the Fricke gel matrices after irradiation. Disclosure Authors declare no relationship that may bias the presentation.