Dosimetry aspects of a non-diffusing genipin–gelatin gel J.B. Davies a,b,n , S.G. Bosi a,c , C. Baldock a a Institute of Medical Physics, School of Physics, University of Sydney, NSW 2006, Australia b Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia c Radiation Oncology Prince of Wales Hospital, High Street Randwick, NSW 2031, Australia HIGHLIGHTS c The calibration of any optically measured gel dosimetry system is presented. c Dosimetric evaluation of a genipin–gelatin gel dosimeter was investigated. c This genipin–gelatin gel may be used as a non-diffusing 3D dosimeter. c Measurement uncertainty was evaluated and used to determine the dose resolution. article info Article history: Received 6 July 2012 Accepted 18 September 2012 Available online 27 September 2012 Keywords: Genipin Gel dosimetry Calibration Absorbed dose Uncertainty Zero diffusion abstract Genipin–gelatin gel shows promise as a stable, three-dimensional dosimeter for use in quality assurance for radiotherapy treatments. Genipin creates cross-links in gelatin, forming a blue colour that bleaches quantitatively upon irradiation. A formulation suitable for dosimetry was investigated by varying the concentrations of genipin, gelatin and sulphuric acid and determining the dose sensitivity. An important parameter of the gel preparation that affects dose sensitivity is the temperature at which the cross-linking reaction takes place. The most suitable formulation for dose measurements in 1 cm pathlength cuvettes was found to be made from 50 mM genipin, 4% w/w gelatin and 100 mM sulphuric acid in the final gel. An evaluation of the diffusion coefficient of chromophores in this gel dosimeter demonstrated that this genipin–gelatin gel is a non-diffusing dosimeter. This dosimeter was also evaluated for stability, dose sensitivity, irradiation and measurement temperature dependence and dose rate dependence. No appreciable dependence on dose rate in the range 0.4–40 Gy min 1 was found. No appreciable dependence on measurement temperature between 15 and 23 1C was found. A slight dependence on irradiation temperature was found and this was used to determine the product of the molar linear absorption coefficient and the radiation chemical yield. Finally, the dosimeter measurement uncertainty was evaluated and this was used to determine the dose resolution. Although the focus of this work is on a genipin–gelatin gel dosimeter, the measurement and calibration techniques presented may be applied to any gel dosimetry system measured spectrophotometrically. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Dosimetry systems developed in standards laboratories utilise calorimetry, ionisation and chemical methods for determining absorbed dose to water (International Atomic Energy Agency (IAEA), 2000). One chemical method, the gel dosimeter, in which substances carrying dosimetric information are suspended in a gel matrix, allows dose distributions to be measured in three dimen- sions (3D), promising true 3D quality assurance measurements in radiotherapy treatment planning. Readout techniques that have been developed include magnetic resonance imaging (MRI), optical computed tomography (CT), x-ray CT, ultrasound, and vibrational spectroscopy, offering significant advantages in obtaining dose distributions in gel dosimeters (Baldock et al., 2010). Gel dosimetry is gaining more acceptance in radiation oncology as a means of assuring the quality of radiation treatments. Genipin, a fruit extract from Gardenia jasminoides Ellis, is a cross-linker of proteins, such as gelatin, forming blue pigments (Bigi et al., 2002; Yao et al., 2004). As the genipin cross-links the gelatin, the solution slowly changes from colourless to blue and steadily darkens over time. This blue hydrogel is radiochromic (Jordan, 2008) and may be scanned using optical CT (Jordan, 2009). In particular, it bleaches quantitatively upon irradiation Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/radphyschem Radiation Physics and Chemistry 0969-806X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2012.09.018 n Corresponding author at: Institute of Medical Physics, School of Physics, NSW 2234, Australia. Tel.: þ61 2 97179002; fax: þ61 2 97179325. E-mail address: jbd@ansto.gov.au (J.B. Davies). Radiation Physics and Chemistry 83 (2013) 19–27