Gel dosimeters, also called Fricke gel dosimeters, are manufactured from radiation sensitive chemicals that, upon irradiation with ionising radiation, undergo a fundamental change in their properties as a function of the absorbed radiation dose.
Over many years individuals have endeavoured to measure absorbed radiation dose distributions using gels. As long ago as 1950, the radiation-induced colour change in dyes was used to investigate radiation doses in gels.Day M J and Stein G 1950 Chemical effects of ionizing radiation in some gels Nature 166 146– 7 Further, in 1957 depth doses of photons and electrons in agar gels were investigated using spectrophotometry.Andrews H L, Murphy R E and LeBrun E J 1957 Gel dosimeter for depth dose measurements Rev Sci Instrum 28 329–32 Gel dosimetry today however, is founded mainly on the work of Gore et al who in 1984Gore J C, Kang Y S and Schulz R J 1984 Measurement of radiation dose distributions by nuclear magnetic resonance (NMR) imaging Phys Med Biol 29 1189–97 demonstrated that changes due to ionising radiation in Fricke dosimetry solutions,Fricke H and Morse S 1927 The chemical action of roentgen rays on dilute ferrous sulphate solutions as a measure of radiation dose Am J Roentgenol Radium Therapy Nucl Med 18 430–2 developed in the 1920s, could be measured using nuclear magnetic resonance (NMR).
Gel dosimeters generally consist of two types; Fricke and polymer gel dosimeters and are usually evaluated or 'read-out' using magnetic resonance imaging (MRI), optical computer tomography (CT), x-ray CT or ultrasound.
Since 1999 the DosGel and IC3DDose Conference Series on gel dosimetry has been held at various international venues.
Fricke gel dosimeters
Gore et al investigatedGore J C, Kang Y S and Schulz R J 1984 Measurement of radiation dose distributions by nuclear magnetic resonance (NMR) imaging Phys Med Biol 29 1189–97 the nuclear magnetic resonance (NMR) relaxation properties of irradiated Fricke or ferrous sulphate dosimetry solutionsFricke H and Morse S 1927 The chemical action of roentgen rays on dilute ferrous sulphate solutions as a measure of radiation dose Am J Roentgenol Radium Therapy Nucl Med 18 430–2 showing that radiation-induced changes, in which ferrous (Fe2+) ions are converted to ferric (Fe3+) ions, could be quantified using NMR relaxation measurements. In 1986 Appleby et alAppleby A, Christman E A and Leghrouz A 1986 Imaging of spatial radiation dose distribution in agarose gels using magnetic resonance Med Phys. 14 382-4 reported that Fricke dosimetry solutions dispersed throughout a gel matrix could be used to obtain three-dimensional (3D) spatial dose information using magnetic resonance imaging (MRI). It was subsequently shown that irradiated Fricke-type gel dosimeters did not retain a spatially stable dose distribution due to ion diffusion within the irradiated dosimeters.Schulz R J, de Guzman A F, Nguyen D B and Gore J C 1990 Dose-response curves for Fricke- infused agarose gels as obtained by nuclear magnetic resonance Phys Med Biol 35 1611-22 Fricke solutions with various gelling agents such as gelatine, agarose, sephadex and polyvinyl alcohol (PVA) were investigated along with chelating agents such as xylenol orange (XO) to reduce diffusion. Numerous authors subsequently published results of their work to inhibit the ion diffusion with limited success and which was summarised by Baldock et al in 2001.Baldock C, Harris P J, Piercy A R, Healy B 2001 Experimental determination of the diffusion coefficient in two-dimensions in ferrous sulphate gels using the finite element method Australas Phys Eng Sci Med 24 19-30 By the early 1990s the diffusion problem was considered to be a significant one in the advancement of gel dosimetry.
Polymer gel dosimeters
Polymer systems for the use of radiation dosimetry were first proposed as early as 1954, where Alexander et alAlexander P, Charlesby A and Ross M 1954 The degradation of solid polymethylmethacrylate by ionizing radiations Proceedings of the Royal Society A223 392 discussed the effects of ionising radiation on polymethylmethacrylate. Following this, Hoecker et alHoecker F E and Watkins I W 1958 Radiation polymerization dosimetry Int J Appl Rad Isotop 3 31-35 in 1958 investigated the dosimetry of radiation-induced polymerisation in liquids, and in 1961 BoniBoni A L 1961 A polyacrylamide gamma dosimeter Radiation Research 14 374-80 used polyacrylamide as a gamma dosimeter. Much later in 1991, Audet et alAudet C and Schreiner L J 1991 Radiation dosimetry by NMR relaxation time measurements of irradiated polymer solutions Proc Intl Soc Mag Reson Med (New York) reported changes in NMR transverse relaxation measurements of irradiated polyethylene oxide. In 1992, Kennan et alKennan R P, Maryanski M J, Zhong J and Gore J C 1992. Hydrodynamic effects and cross relaxation in cross linked polymer gels Proc Intl Soc Mag Reson Med (New York) reported on NMR longitudinal relaxation studies performed on an irradiated aqueous solution of N,N’-methylene-bis-acrylamide and agarose, which showed that the relaxation rates increased with absorbed dose.
In 1992 a new gel dosimetry formulation was proposed by Maryanski et al,Maryanski M J, Gore J C and Schulz R J 1992 3-D radiation dosimetry by MRI: solvent proton relaxation enhancement by radiation-controlled polymerisation and cross-linking in gels Proc Intl Soc Mag Reson Med (New York) which was based on the polymerisation of acrylamide and N,N’-methylene-bis-acrylamide (bis) monomers infused in an aqueous agarose matrix. This system was given the acronym BANANA due to the use of the chemical components (bis, acrylamide, nitrous oxide and agarose).Maryanski M J, Gore J C, Kennan R P and Schulz R J 1993 NMR relaxation enhancement in gels polymerized and cross-linked by ionizing radiation: a new approach to 3D dosimetry by MRI Magn Reson Imaging 11 253-58 This type of gel dosimeter did not have the associated diffusion problem of Fricke gels and was shown to have a relatively stable post-irradiation dose distribution. The polymerisation reaction occurred by cross-linking of the monomers induced by the free radical products of water radiolysis. In 1994 the BANANA formulation was refinedMaryanski M J, Schulz R J, Ibbott G S, Gatenby J C, Xie J, Horton D and Gore J C 1994 Magnetic resonance imaging of radiation dose distributions using a polymer-gel dosimeter Phys Med Biol 39 1437-55 by replacing agarose with gelatine and given the acronym BANG (bis, acrylamide, nitrogen and aqueous gelatine), the first of a series of new polymer gel formulations. In 1994 this formulation was patentedMaryanski M J, Gore J C and Schulz R 1994 Three-dimensional detection, dosimetry and imaging of an energy field by formation of a polymer in a gel US Patent 5321357 and became commercially available through MGS Research Inc. as BANG®. Subsequently, due to the naming of the commercial product, PAGBaldock C, Burford R P, Billingham N, Wagner G S, Patval S, Badawi R D and Keevil S F 1998 Experimental procedure for the manufacture and calibration of polyacrylamide gel (PAG) for magnetic resonance imaging (MRI) radiation dosimetry Phys Med Biol 43 695-702 became the polymer gel dosimeter acronym of choice for most authors. Numerous authors subsequently published results of work investigating different compositions and formulations of polymer gel dosimeters which were summarised by Lepage et al.Lepage M, Jayasekera M, Bäck S Å J, Baldock C 2001 Dose resolution optimization of polymer gel dosimeters using different monomers Phys Med Biol 46 2665-80
Although polymer-type dosimeters did not have the diffusion limitations of Fricke-type gel dosimeters, there was another significant limitation in their use. Due to the nature of their free radical chemistry, polymer gel dosimeters were susceptible to atmospheric oxygen inhibition of the polymerisation processes. As a result, these gel dosimeters had to be manufactured in an oxygen-free environment,Baldock C, Burford R P, Billingham N, Wagner G S, Patval S, Badawi R D and Keevil S F 1998 Experimental procedure for the manufacture and calibration of polyacrylamide gel (PAG) for magnetic resonance imaging (MRI) radiation dosimetry Phys Med Biol 43 695-702De Deene Y, De Wagter C, Van Duyse B, Derycke S, De Neve W and Achten E 1998 Three- dimensional dosimetry using polymer gel and magnetic resonance imaging applied to the verification of conformal radiation therapy in head-and-neck cancer Radiotherapy and Oncology 48 283–291 such as in a glove box pumped with nitrogen gas. Along with the use of potentially toxic chemicals,Baldock C and Watson S 1999 Risk assessment for the manufacture of radiation dosimetry polymer gels in DOSGEL 1999 Proceedings of the 1st International Workshop on Radiation Therapy Gel Dosimetry (Lexington, USA) Eds L J Schreiner and C Audet this was a significant limitation in the introduction of gel dosimetry into the clinic.
During this period a number of studies were undertaken to investigate the clinical applications of radiological tissue-equivalentKeall P, Baldock C, 1999. A theoretical study of the radiological properties and water equivalence of three types of gels used for radiation dosimetry Australas Phys Eng Sci Med 22 85-91Venning AJ, Nitschke KN, Keall PJ, Baldock C, 2005. Radiological properties of normoxic polymer gel dosimeters Med Phys 32 1047-1053Brown S, Venning A, De Deene Y, Vial P, Oliver L, Adamovics J and Baldock C 2008 Radiological properties of the PRESAGE and PAGAT polymer dosimeters Applied Radiation and Isotopes 66 (12) 1970-1974 PAG-type polymer gel dosimeters using MRI.Maryanski M J, Gore J C, Kennan R P and Schulz R J 1993 NMR relaxation enhancement in gels polymerized and cross-linked by ionizing radiation: a new approach to 3D dosimetry by MRI Magn Reson Imaging 11 253-58Maryanski M J, Schulz R J, Ibbott G S, Gatenby J C, Xie J, Horton D and Gore J C 1994 Magnetic resonance imaging of radiation dose distributions using a polymer-gel dosimeter Phys Med Biol 39 1437-55Ibbott G S, Maryanski M J, Eastman P, Holcomb S D, Zhang Y, Avison R G, Sanders M and Gore J C 1997 Three-dimensional visualization and measurement of conformal dose distributions using magnetic resonance imaging of BANG polymer gel dosimeters Int J Radiat Oncol Biol Phys 38 1097-103Oldham M, Baustert I, Lord C, Smith T A D, McJury M, Warrington A P, Leach M O and Webb S 1998a An investigation into the dosimetry of a nine-field tomotherapy irradiation using BANG-gel dosimetry Phys Med Biol 43 1113–32Low D A, Harms W B, Mutic S and Purdy J A 1998 A technique for the quantitative evaluation of dose distributions Med Phys 25 656-61 De Deene et alDe Deene Y, De Wagter C, Van Duyse B, Derycke S, Mersseman B, De Gersem W, Voet T, Achten E and De Neve W 2000 Validation of MR-based polymer gel dosimetry as a preclinical three-dimensional verification tool in conformal radiotherapy Magn Reson Med 43 116–25 undertook an investigation into the overall accuracy of an anthropomorphic polymer gel dosimetry phantom for the verification of conformal radiotherapy treatments. It was established that significant issues relating to the accuracy of this dosimetry technique were a result of oxygen inhibition in the polymer gel and MRI imaging artefacts.