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The aim of this project was to develop an absorbed dose to water primary standard for Ir-192 brachytherapy based on the Fricke dosimeter. To achieve this within the framework of the existing TG-43 protocol, a determination of the absorbed dose to water at the reference position, D(r0,θ0), was undertaken. Prior to this investigation, the radiation chemical yield of the ferric ions (G-value) at the Ir-192 equivalent photon energy (0.380 MeV) was established by interpolating between G-values obtained for Co-60 and 250 kV x-rays.An irradiation geometry was developed with a cylindrical holder to contain the Fricke solution and allow irradiations in a water phantom to be conducted using a standard Nucletron microSelectron V2 HDR Ir-192 afterloader. Once the geometry and holder were optimized, the dose obtained with the Fricke system was compared to the standard method used in North America, based on air-kerma strength.Initial investigations focused on reproducible positioning of the ring-shaped holder for the Fricke solution with respect to the Ir-192 source and obtaining an acceptable type A uncertainty in the optical density measurements required to yield the absorbed dose. Source positioning was found to be reproducible to better than 0.3 mm, and a careful cleaning and control procedure reduced the variation in optical density reading due to contamination of the Fricke solution by the PMMA holder. It was found that fewer than 10 irradiations were required to yield a type A standard uncertainty of less than 0.5%.Correction factors to take account of the non-water components of the geometry and the volume averaging effect of the Fricke solution volume were obtained from Monte Carlo calculations. A sensitivity analysis showed that the dependence on the input data used (e.g. interaction cross-sections) was small with a type B uncertainty for these corrections estimated to be 0.2%.The combined standard uncertainty in the determination of absorbed dose to water at the reference position for TG-43 (1 cm from the source on the transverse axis, in a water phantom) was estimated to be 0.8% with the dominant uncertainty coming from the determination of the G-value. A comparison with absorbed dose to water obtained using the product of air-kerma strength and the dose rate constant gave agreement within 1.5% for three different Ir-192 sources, which is within the combined standard uncertainties of the two methods.

High dose rate brachytherapy (HDR) using 192Ir sources is well accepted as an important treatment option and thus requires an accurate dosimetry standard. However, a dosimetry standard for the direct measurement of the absolute dose to water for this particular source type is currently not available. An improved standard for the absorbed dose to water based on Fricke dosimetry of HDR 192Ir brachytherapy sources is presented in this study. The main goal of this paper is to demonstrate the potential usefulness of the Fricke dosimetry technique for the standardization of the quantity absorbed dose to water for 192Ir sources. A molded, double-walled, spherical vessel for water containing the Fricke solution was constructed based on the Fricke system. The authors measured the absorbed dose to water and compared it with the doses calculated using the AAPM TG-43 report. The overall combined uncertainty associated with the measurements using Fricke dosimetry was 1.4% for k = 1, which is better than the uncertainties reported in previous studies. These results are promising; hence, the use of Fricke dosimetry to measure the absorbed dose to water as a standard for HDR 192Ir may be possible in the future.

Very thin material layers (<100 microm) partially absorb ionizing radiation of low energy. When irradiating monolayer cell cultures from above, attention must be paid to absorption by the medium. Frequently, the volume of the nutrient medium is variable, and this leads to differences in the radiation doses delivered to the cells. In the present work these conditions were investigated for x-rays of energies between 13 kV and 100 kV in comparison with 60Co gamma rays using chemical dosimetry to measure the absorption by liquid layers between 25 microm and 500 microm thick. When the dose as measured with the ionization chamber was held constant, the dose absorbed in the Fricke solution was shown to increase with decreasing thickness of the layer of liquid because of a dose gradient. The effect of the dose gradient disappeared, however, in thick liquid layers of the Fricke solution by mixing during spectrophotometry. Secondary (photoeffect and Compton) electrons produced in air or filters are responsible for this effect in plastic petri dishes where back scattering at the interface does not occur. This interpretation is suggested by the same results of an analogous experimental setup using gamma rays with a 5-mm-thick Perspex plate. This dose increase in very thin layers, however, could not be verified by irradiating monolayer cells in poured-out plastic petri dishes because the secondary electrons are already absorbed in the remaining liquid film above the cells.

Glucose, sucrose, starch, and locust bean gum have been used as additives to the ferrous-agarose-xylenol orange (FAX) gel dosimeter. The saccharide enhanced dosimeters were found to have a higher dose sensitivity over a standard FAX gel as measured by both optical density change and magnetic resonance imaging (MRI). With optical density measurement, OD-dose sensitivity increases were up to 55% for glucose, 122% for sucrose and 43% for starch, while locust bean gum did not give a consistent response. With MRI, R1-dose sensitivity increases were up to 178% with sucrose addition. The FAX gel with sucrose was studied in greatest detail. The OD-dose sensitivity dependence on cooling rate was reduced for the sucrose FAX gel over the standard FAX gel, which has significant implications for uniform dose sensitivity in large gel phantoms. The thermal oxidation rate in the sucrose FAX gel was up to 2.3 times higher than in the standard gel. The OD-dose sensitivity of oxygenated sucrose FAX gels was 4.3 times greater than standard FAX gels, while continued enhancement in OD-dose sensitivity with increased sucrose concentrations beyond 2.0 g/l was found only for the oxygenated sucrose FAX gels. Both the molar absorption coefficient of the ferric ion-xylenol orange complex at 543 nm and gel pH were not affected by the presence of sucrose, with the implication that the higher OD-dose sensitivity of gels with saccharides is due to increased chain reaction production of ferric ions.

The spatial accuracy of magnetic resonance imaging (MRI) Fricke-gel dosimetry is limited by diffusion of ferric ions. This paper describes a honeycomb structure to limit diffusion of Fe3+ ions in a three-dimensional phantom. Such a phantom containing the dosimeter gel was irradiated to a known dose distribution. Maps of dose distributions were produced from the MR images acquired at 2 and 24 hours after the dose was given. The dose distribution maps verified that the honeycomb structure precludes ion diffusion from one honeycomb cell to another, thus improving the usefulness of MRI Fricke-gel dosimetry.

The purpose of this work is to undertake a critical appraisal of the evidence in the published literature concerning the basic parameters of accuracy and precision associated with the use of Fricke and polymer gels (in conjunction with MR imaging) as radiation dosimeters in photon radiotherapy, condensing and analysing the body of published information (to the end of April 2002). A systematic review was undertaken addressing specific issues of precision and accuracy asking defined questions of the published literature. Accuracy and precision in relation to gel dosimetry were defined. Information was obtained from published, peer-reviewed journals. A defined search strategy utilizing MeSH headings and keywords, with extensive use of cross-referencing, identified 115 references dealing with gel dosimetry. Exclusion criteria were used to select only data from publications which would give unequivocal evidence. For accuracy, results had to be compared with an ionization chamber as gold standard and all gel samples had to be manufactured in the same batch. For precision, in addition to gels being from the same batch, samples must all have been irradiated at the same time and scanned simultaneously (or within a short time frame). Many results were found demonstrating 'dose mapping' examples using gels. However, there were very few publications containing firm evidence of precision and accuracy. There was no evidence which fulfilled our criteria about accuracy or precision using Fricke gels. For polymer gels only one paper was found for accuracy (4% (Low et al 1999 Med. Phys. 26 1542-51)) and precision (1.7% (Baldock et al 1998 Phys. Med. Biol. 43695-702)); however, both were carried out at only one dose level. If the exclusion criteria were relaxed to include accuracy results comparing gel to a non gold standard dosimeter (e.g. TLD), results give a median accuracy of 10% (range 8-23.5%) for polymer gel (Cosgrove et al 2000 Phys. Med. Biol. 45 1195-210, De Deene et al 1998 Radiother: Oncol. 48 283-91, Farajollahi et al 2000 Br. J. Radiol. 72 1085-92, McJury et al 1999b Phys. Med. Biol. 44 2431-44, Murphy et al 2000b Phys. Med. Biol. 45 835-45, Oldham et al 2001 Med. Phys. 28 1436-45) and 5% for Fricke gel (Chan and Ayyangar 1995b Med. Phys. 22 1171-5). Evidence also points to accuracy worsening at lower dose levels for both gels. The precision data should be viewed with caution as repeated MR measurements were not performed with the same samples. The only precision data for Fricke gels was 1.5% (Johansson Back et al 1998 Phys. Med. Biol. 43 261-76), but for zero dose. In conclusion, despite the amount of published data, sparse research has been undertaken which provides clear evidence of the accuracy and precision for both gels. That which has been published has used higher doses than would be routine in radiotherapy. The basic radiation dosimeter qualities of accuracy and precision have yet to be fully quantified for polymer and Fricke gels at clinically relevant dose levels.

Gel dosimetry based on magnetic resonance imaging (MRI) has previously been shown to provide verification of calculated dose distributions in soft tissue equivalent homogeneous phantoms. This study demonstrates how measurements of dose distribution can also be achieved in a phantom containing porous, lung-equivalent, Fricke gel. A phantom was made of Fe2+ infused low-density gel and conventional ferrous sulphate gel, filled in separate compartments in a Perspex container. Absorbed dose measurements were accomplished by MR imaging and by calibrating the proton spin-lattice relaxation rate (R1) versus absorbed dose by means of TLD measurements. This study shows that the production of lung-equivalent low-density (LD) dosimeter gel (mean CT number of -610 HU) is feasible. The MR signal detected in the LD gel dosimeter was substantially more noisy (i.e. displayed larger random fluctuations) than the signal from conventional gel, as expected. A deviation between calculated (TPS) and measured dose of about 3% (6 MV) and 4-7% (15 MV) was found in the LD region of the phantom. These results correspond well with data from other studies of dose distribution in lung-equivalent phantoms. The Fe2+ infused LD gel therefore seems suitable for measurement of absorbed dose distribution in phantoms that contain lung tissue compartments.

Spin-spin NMR relaxation rate in the ferrous sulphate gelatin dosimeter has been studied in terms of pH, gelatin concentration, the addition of benzoic acid, and sample size. It is demonstrated that R2 is more sensitive to changes in Fe3+ ion concentration than R1 when measuring at frequencies of 64 and 100 MHz. pH has an important effect on dose response curves, and oxygen depletion occurs significantly more rapidly in FeSO4 gelatin than in the liquid FeSO4, resulting in a saturation dose of approximately 80 Gy at depths greater than approximately 3 mm in phantom. The concentration of gelatin can be increased to 12% by weight, and the dosimeter will continue to exhibit a linear dose response. Sensitivity is maintained at higher gel concentrations by pH compensation. Addition of low-concentration benzoic acid to the system does not alter the dose response of the gelatin FeSO4 system. Finally, spontaneous oxidation of Fe2+ ions does not significantly alter the shape of dose response curves but does result in increases in R2 by up to 4% per day.

Ferrous sulphate gel analysed by relaxation time measurements with NMR imaging is considered a useful dosimeter for 3D determinations of absorbed dose. A protocol for the gel preparation with agarose SeaPlaque that leads to a dosimeter with very high dose sensitivity is described. The dose-response curve slope is about 0.2 s(-1) Gy(-1) and the G factor turns out to be approximately 185 ions per 100 eV of absorbed energy. A method for making the measurements and analysing the results that brings about good result reproducibility is suggested. A thorough experimental study of the dependence of the dosimeter response on the elapsed time from preparation to irradiation and from irradiation to NMR measurement has revealed good reproducibility. The above characteristic of the gel system is very interesting, because it shows the possibility of utilizing the dosimeter for absolute dose determinations with satisfactory reliability.

The radiation-response characteristics of agarose gels prepared with Fricke dosemeter solution have been studied. The response mechanism is an increase in the NMR longitudinal relaxation rate of protons caused by ferric ions. It has been observed that: (i) oxygen saturation assures consistent and maximum sensitivity; (ii) agarose concentrations in the range 1.0-2.0% have no effect upon sensitivity; (iii) the initial G value is 150 Fe3+/100 eV for gels containing 0.5 mM Fe2+ ions; (iv) increasing NMR frequencies only causes a moderate increase in sensitivity; (v) the gel dosemeters are dose rate independent in the range 4.7-24.2 Gy min-1; (vi) sensitivity is pH dependent, being zero at pH 7; (vii) freshly prepared gels are slightly more sensitive than those more than 24 h old; and (ix) the diffusion coefficient for ferric ions in a 1.0% agarose gel containing 0.0125 M H2SO4 is 1.83 x 10(-2) cm2 h-1, and this will require consideration for the NMR imaging of dose distributions.

The measurement of absorbed dose distributions using dosemeter gel and magnetic resonance imaging (MRI) in a standard geometry has been investigated. Absorbed depth-dose curves and profiles measured with this new technique show good agreement with corresponding measurements using diodes. This was proven in a 60Co beam as well as an electron beam. The dosemeter gel is made of agarose and ferrous sulphate solution. The dose response is linear (r = 0.9996) in the investigated dose interval, 0-40 Gy. The sensitivity is a factor of about six higher compared to ordinary ferrous sulphate solution, known as 'Fricke'. This is a true 3D dose measurement technique which will have a number of applications in radiation therapy, since it is possible to mould the gel to arbitrary geometries, mix different radiation qualities and integrate the absorbed dose from different kinds of fields.