Fricke Gel Research Articles

Gel dosimetry: An overview of dosimetry systems and read out methods

Gel dosimetry has emerged over the past three decades in response to a growing need in high-precision radiotherapy to assess, in three dimensions, the absorbed radiation dose, as would be administered in cancer patients. Radiation-induced reaction mechanisms are dependent on the class of gel dosimeter, with four classes emerging as primary dosimeters for use in radiation therapy dose verification: (i) Fricke gel dosimeters contain a Fricke solution consisting of ammonium iron (II) sulfate in an acidic solution of sulfuric acid. In Fricke systems an oxidation of ferrous ions results in a change in the nuclear magnetic resonance (NMR) relaxation rate, which enables reading out Fricke gel dosimeters by use of MRI. The radiation-induced oxidation in Fricke gel dosimeters can also be visualized by adding a redox indicator. (ii) Polymer gel dosimeters exploit the radiation induced polymerization reaction of vinyl monomers and are predominantly read out by quantitative MRI or X-ray CT. (iii) Radiochromic dosimeters do not demonstrate a significant radiation-induced change in NMR properties but can be scanned by use of optical scanners (optical CT). In contrast to Fricke gel dosimeters, radiochromic gel dosimeters do not rely on the oxidation of a metal ion but exhibit a color change upon radiation. (iv) Radiofluorogenic dosimeters become fluorescent when exposed to ionizing radiation and can be read out with a planar scanning light beam. Likewise, the imaging modality used to extract quantitative dose information depends on the class of dosimeter being used, and three primary imaging modalities have emerged in this context: quantitative MRI, x-ray CT, and optical CT imaging. The accuracy and precision of the dose information extracted from gel dosimetry systems depends on both the dosimetric properties of the gel dosimeters and the readout technique, and the optimal readout method depends on the gel dosimeter response. Despite remaining an active field of research and illustrations of the application of gel dosimetry for the validation of clinical dose distributions, the utilizatio of gel dosimetry as a routine clinical dosimeter has been rather limited. However, with the introduction of new radiotherapy techniques that focus on organ motion compensation, new fractionation schemes, and extreme dose rates, the need for 3D radiation dosimetry is apparent. Even with the need for 3D dosimetry being apparent, gel dosimetry faces continued challenges in areas regarding the extraction of reproducible, accurate, and precise dose information. This review paper focuses on an introduction to gel dosimeter classes; a detailed examination of the three readout techniques with emphasis on the achievable accuracy, precision, and optimization of readout parameters; an outlook on future applications in emerging new radiotherapy techniques. We note that the introduction of theragnostic hybrid MRI-Linacs that combine an MRI-scanner and a clinical linear accelerator create new opportunities for inline polymer gel dosimetry. Likewise, the use of cone beam CT on linear accelerators opens up the possibility to read out gel dosimeters on the linac. Multiple optical CT designs have shown that optical CT gel dosimetry is eminently capable of providing high quality dosimetric information from clinically relevant treatment regimes. As a result, gel dosimetry provides exciting opportunities for 3D radiation dosimetry that were not available even a few years ago. The unique feature set of a properly executed gel dosimetry workflow allows for the extraction of dosimetric information, in 3D, that is not possible with any other dosimetry system and hence gel dosimetry provides exciting opportunities for clinical and research work in the area of radiation dose measurement.

Characterization of ferrous-xylenol orange-polyvinyl alcohol gel for gamma dosimetry using spectroscopy

Abstract Fricke gel dosimeters are appropriate candidates for gamma dosimetry. Polyvinyl alcohol Fricke gel dosimeters are the most recent introduced gel dosimeters which have low ion diffusion. In this work, samples of ferrous-xylenol orange-polyvinyl alcohol gel dosimeters were prepared and characterized using optical spectroscopy. Using win XCOM program and the elemental composition of the gel, the mass attenuation coefficients for photons were evaluated. The results exhibited that the prepared gel is the nearly radiological blood-, soft tissue- and water-equivalent. The 60Co gamma cell unit was used to irradiate the gel samples. A dose range response was found linear from 10 to 30 Gy and suitable for blood irradiation dosimetry. Additionally, the gel response good repeatability was confirmed by the coefficient of variation calculations. Furthermore, chemical yield of the gel was estimated to be 34.6. The good characteristics of the prepared gel make it appropriate for dosimetry of blood irradiators.

Diffusion Correction in Fricke Hydrogel Dosimeters: A Deep Learning Approach with 2D and 3D Physics-Informed Neural Network Models.

In this work an innovative approach was developed to address a significant challenge in the field of radiation dosimetry: the accurate measurement of spatial dose distributions using Fricke gel dosimeters. Hydrogels are widely used in radiation dosimetry due to their ability to simulate the tissue-equivalent properties of human tissue, making them ideal for measuring and mapping radiation dose distributions. Among the various gel dosimeters, Fricke gels exploit the radiation-induced oxidation of ferrous ions to ferric ions and are particularly notable due to their sensitivity. The concentration of ferric ions can be measured using various techniques, including magnetic resonance imaging (MRI) or spectrophotometry. While Fricke gels offer several advantages, a significant hurdle to their widespread application is the diffusion of ferric ions within the gel matrix. This phenomenon leads to a blurring of the dose distribution over time, compromising the accuracy of dose measurements. To mitigate the issue of ferric ion diffusion, researchers have explored various strategies such as the incorporation of additives or modification of the gel composition to either reduce the mobility of ferric ions or stabilize the gel matrix. The computational method proposed leverages the power of artificial intelligence, particularly deep learning, to mitigate the effects of ferric ion diffusion that can compromise measurement precision. By employing Physics Informed Neural Networks (PINNs), the method introduces a novel way to apply physical laws directly within the learning process, optimizing the network to adhere to the principles governing ion diffusion. This is particularly advantageous for solving the partial differential equations that describe the diffusion process in 2D and 3D. By inputting the spatial distribution of ferric ions at a given time, along with boundary conditions and the diffusion coefficient, the model can backtrack to accurately reconstruct the original ion distribution. This capability is crucial for enhancing the fidelity of 3D spatial dose measurements, ensuring that the data reflect the true dose distribution without the artifacts introduced by ion migration. Here, multidimensional models able to handle 2D and 3D data were developed and tested against dose distributions numerically evolved in time from 20 to 100 h. The results in terms of various metrics show a significant agreement in both 2D and 3D dose distributions. In particular, the mean square error of the prediction spans the range 1×10-6-1×10-4, while the gamma analysis results in a 90-100% passing rate with 3%/2 mm, depending on the elapsed time, the type of distribution modeled and the dimensionality. This method could expand the applicability of Fricke gel dosimeters to a wider range of measurement tasks, from simple planar dose assessments to intricate volumetric analyses. The proposed technique holds great promise for overcoming the limitations imposed by ion diffusion in Fricke gel dosimeters.

Attenuation images of optical CT using Fricke xylenol orange gel for dose mapping in radiotherapy

The Fricke gel dosimeter, a gel-based solution containing ferrous ions, can be used for three-dimensional dosimetry by adding xylenol orange to the solution. This changes the optical properties, affecting radiation absorption. These gel dosimeters are recommended for clinical use, and 3D dosimetry uses optical computed tomography (OCT) to analyze reconstructed images. Advancements in radiotherapy focus on accurate irradiation while reducing doses to nearby tissues. This study uses modified FXO gel for 3D dosimetry, utilizing non-toxic, easy-to-handle reagents and optical CT Vista 16 equipment. The experiment aims to characterize the system's practicality and evaluate the impact of lead filters on attenuation values. The study also examines trends in attenuation values for different thicknesses of lead shields, enhancing our understanding of the relationship between attenuation and dose using optical CT. To prevent artifacts, a modified FXO gel was created by introducing a variation of XO in the preparation. The ideal concentration of XO is 0.01 mM to prevent distortions within the dose range of 1 Gy–10 Gy. A calibration system using FXO gel can be developed by comparing the maximum irradiation dose with the attenuation observed in the gel. The modified gel showed a sensitivity of 5.5·10−3 ± 2.6·10−4 cm−1/Gy and a starting dose of 4.2 Gy. Two samples were tested using different configurations of lead filters, with the central holes showing dose peaks of 4.72 Gy, 4.57 Gy, and 4.32 Gy, respectively. The results suggest the feasibility of producing modified FXO gels for examination in optical CT systems and their potential application in radiotherapy systems. Future research is being conducted in clinical irradiation to facilitate comparison with system designs.

Sorbitol to reduce Fe diffusion in a Fricke gel dosimeter and enhance its resistance to elongation

Sorbitol to reduce Fe diffusion in a Fricke gel dosimeter and enhance its resistance to elongation

Development and characterization of a Fricke gel dosimeter for precise measurement in low-dose photon fields

In the field of radiation medicine, particularly within radiotherapy applications, radiochromic chemical dosimeters are indispensable tools for dose measurement. This study focuses on the novel development of a radiochromic dosimeter tailored for the precise detection of low-dose radiation, aiming to construct a dosimeter with tissue-equivalent properties suitable for accurately measuring low to medium radiation doses. Utilizing ferrous xylenol orange gel (FXG), we developed two tissue-equivalent dosimeter formulations based on gelatin and polyvinyl alcohol (PVA). Our findings demonstrate that the gelatin-based FXG dosimeter exhibits a robust linear dose-response relationship, facilitating precise dose measurements in the range of 50 to 5000 mGy. Conversely, the PVA-based FXG dosimeter proved effective for dose measurements within a narrower range of 600 to 5000 mGy. Notably, the gelatin-based dosimeter's performance underscores its potential as a versatile tool in radiation detection, promising significant benefits for both medical and industrial applications. This research confirms the efficacy of the Fricke dosimeter gel, demonstrating its linear response across a dose range of 0.05 to 5 Gy, thus establishing a foundation for further advancements in accurate and reliable low-dose radiation monitoring.

Deep learning approach for diffusion correction in Fricke hydrogel dosimeters

In recent years studies on Fricke gel dosimeters have demonstrated their potential in performing fast measurements of 3D spatial dose distributions. These dosimeters work by oxidising ferrous (Fe2+) ions to ferric (Fe3+) ions, which can be read by magnetic resonance or optical techniques. However, the accuracy of these measurements can be affected by the diffusion of Fe3+ ions within the gel matrix, causing image blurring. To overcome this issue, the study proposes a computational method using artificial intelligence (AI) and specifically deep learning (DL). The method is based on Physics Informed Neural Networks (PINNs), which are effective in solving partial differential equations, especially in inverse problems. The PINNs are optimised using physical equations as loss functions, allowing them to predict real-life phenomena like ion diffusion in Fricke gel. The idea is to solve the backward diffusion equation using a PINN model and predict the initial condition, providing as input the spatial distribution of Fe3+ ions at measurement time T, the boundary conditions and the diffusion coefficient D of Fe3+ ions inside the medium. The method was first tested using 1D simulated data in diffusion processes, achieving a mean squared error of 1−4×10−4a.u. in step-wise and rectangular distributions. It was then tested using experimental data, achieving a mean squared error of 2×10−4OD2. The technique here proposed is very promising for overcoming the limits due to the diffusion of Fe3+ ions in Fricke gel dosimeters.

Dosimetry with the TruView gel on a 0.35 T MR-Linac: A feasibility study

The purpose of this work is to assess the feasibility of performing dosimetry with the TruView (ModusQA, London, Ontario, Canada) MTB Fricke gel by MRI reading on a low field MR-Linac (0.35 T). Radiochromic TruView gels were manufactured at IRSN. A 4 echoes time (TE) single-shot fast spin echo sequence (HASTE) was used for 2D images acquisitions. Raw MRI images were denoised (MP-PCA) and transversal relaxation rates (R2) maps were extracted using a homemade Matlab routine. Temperature influences on R2 as well as gel homogeneity were investigated. A dosimetric evaluation of the TruView gel was conducted implying four gels made from the same batch, with three of them dedicated to calibration, and the fourth one received a simple dosimetric plan. Results showed the TruView gel to be sensitive to temperature drift. A protocol was thus developed to ensure thermal stability with the use of a dedicated temperature-controlled water bath. An inhomogeneity that manifests as a R2 increase near the air/gel interface was found to cause a 4% amplitude deviation in R2 along unirradiated gels. Air sealing drastically reduced the inhomogeneity. Dose versus R2 calibration was established. TruView gel sensitivity was found to be low (0.0093 Gy−1 s−1). 1D and 2D comparison to planned dose distribution displayed overall agreement with global gamma index (5%/3 mm) evaluation with an 87 % passing rate. Main discrepancies in the low and gradient dose regions were attributable to the calibration range and diffusion effects. This study demonstrated the feasibility of using the TruView gel for dosimetric evaluation on a low field MR-Linac. The protocol developed needs to be extended to a full 3D evaluation to enable patient QA and may require modifications to the gel formulation to increase its overall sensitivity and ensure limited dependence on temperature drift.

Carbon ion mono-energetic and spread-out Bragg peak measurements using nanocomposite Fricke gel dosimeters with LET-independent response

This study investigates the LET-dependence of dose measurements performed with a nanocomposite Fricke gel (NC-FG) upon irradiation with carbon ion beams to address the problem of quenching of gel dosimeters in high-LET particle therapy. The preparation of the NC-FG in a simple gel preparation device was performed for atmospheric and anoxic environmental conditions and two different container materials. Irradiation was performed with carbon ion beams at energies of 133.39MeV/u or 194.87MeV/u, complemented by a modulated 10 mm carbon ion spread-out Bragg peak (SOBP) irradiation. The R1 profiles obtained from the MRI readout were compared to the treatment planning system and a 2D-ionisation chamber array measurement performed for the mono-energetic and SOBP irradiations, respectively. For different dose levels, no difference in the gel response was obtained for the 3D-printed plastic containers, while the glass vials exhibited a linear dose response. Additional use of the glove box changes produced similar results. For both the mono-energetic and the SOBP irradiation, a linear dose response without quenching was observed. For depths ranging up to the Bragg peak position, the mono-energetic profiles deviated by less than ±15% from the planned dose distribution, excluding the lowest entrance surface dose (ESD). For the SOBP profiles, the deviation from the measured dose distribution was less than ±10% for depths between 20 mm and 33 mm. This study presents a LET-independent measurement of mono-energetic carbon ion Bragg peaks and, for the first time, a LET-independent measurement of a carbon ion SOBP with the NC-FG prepared in a simple gel preparation device under atmospheric conditions. The NC-FG provides a possible solution for 3D dosimetry in carbon ion radiotherapy.

Microscopic and Macroscopic Characterization of Hydrogels Based on Poly(vinyl-alcohol)-Glutaraldehyde Mixtures for Fricke Gel Dosimetry.

In recent decades, hydrogels have emerged as innovative soft materials with widespread applications in the medical and biomedical fields, including drug delivery, tissue engineering, and gel dosimetry. In this work, a comprehensive study of the macroscopic and microscopic properties of hydrogel matrices based on Poly(vinyl-alcohol) (PVA) chemically crosslinked with Glutaraldehyde (GTA) was reported. Five different kinds of PVAs differing in molecular weight and degree of hydrolysis were considered. The local microscopic organization of the hydrogels was studied through the use of the 1H nuclear magnetic resonance relaxometry technique. Various macroscopic properties (gel fraction, water loss, contact angle, swelling degree, viscosity, and Young's Modulus) were investigated with the aim of finding a correlation between them and the features of the hydrogel matrix. Additionally, an optical characterization was performed on all the hydrogels loaded with Fricke solution to assess their dosimetric behavior. The results obtained indicate that the degree of PVA hydrolysis is a crucial parameter influencing the structure of the hydrogel matrix. This factor should be considered for ensuring stability over time, a vital property in the context of potential biomedical applications where hydrogels act as radiological tissue-equivalent materials.

Dosimetric characterization of double network Fricke hydrogel based on PVA-GTA and phenylalanine peptide derivative

A double network hydrogel based on Poly(vinyl-alcohol) (PVA) cross-linked with Glutaraldehyde (GTA) was recently developed by using self-assembling phenylalanine (Phe) peptide derivative (Fmoc-Phe-Phe-OMe), with the aim to improve the mechanical-elastic properties of PVA-GTA hydrogels. In this study, a characterization of the properties of Xylenol Orange based Fricke gel dosimeters obtained by infusing a Fricke solution into the double network hydrogel was performed. The gel dosimeters were irradiated with 6 MV and 15 MV X-rays produced by a medical linear accelerator and investigated by means optical absorbance measurements. The double network hydrogel formulation maintained a satisfactory level of radiological water-equivalence within the investigated radiotherapy range. Fricke gel dosimeters prepared with such network kept the desired properties of independence of the response of the dose rate and energy in the investigated intervals. Furthermore, the addition of self-assembling Phe peptide derivative proved not avoid the motion of radio-inducted ferric ions into the hydrogel, probably maintaining the main characteristics of the standard, no Phe peptide infused, formulation. The time course of formation of the optical response after the irradiation was observed to be similar to what previously measured in traditional PVA-GTA Fricke gel dosimeters, while a decrease of the sensitivity to radiation dose of the order of 30% was found. The extent of the decrease does not seem such as to impair the use of these dosimeters for evaluation of doses typical of radiation therapy applications. The overall dosimetric properties, coupled with the mechanical-elastic characteristics of the double network hydrogel, pave theway to the development of phantoms able both to mimic the deformation of organs possibly occurring during radiotherapy treatments and at the same time to assess the 3D dose distribution within such volumes.

Assessment of dosimetric sensitivity enhancement of xylenol orange Fricke gel by AuNPs: optical and MR imaging investigation

Metallic nanoparticles, such as gold (Au, Z = 79) and silver (Ag, Z = 47) nanoparticles (AuNPs and AgNPs, respectively), possess strong surface plasmonic resonance (SPR) and high atomic number, which makes them ideal candidates for enhancing dosimeter sensitivity. In this study, we have inserted different mass percentages (from 0 to 0.015 wt%) of AuNPs into a gelatinous Fricke-xylenol-orange (FXO-f) gel matrix and irradiated it with doses ranging from 2 to 32 Gy, using a source of x-ray of low energy with an effective energy of 42 keV. Optical absorption increased significantly; sensitivity gains of up to 50% were achieved for the FXO-f gel matrix containing 0.011 wt% AuNPs. To elucidate the mechanism underlying this increased sensitivity, we also evaluated FXO-f gel matrixes containing AgNPs. AgNPs insertion into the FXO-f gel matrix did not enhance sensitivity, which suggested that the AgNPs plasmonic absorption band and the FXO-f gel matrix absorption band at 441 nm overlapped, to increase absorption even after the gel matrix was irradiated. To visualize the dose distribution, we recorded optical tomography and acquired 3D reconstruction maps. In addition, we analyzed the dose enhancement factor (DEF) by using magnetic resonance images. AuNPs insertion into the FXO-f gel matrix resulted in a DEF gain of 1.37, associated with the photoelectric effect originating from the increased number of free radicals.

Study of PVA-GTA Fricke Gel Dosimeters Exposed to 60Co Source

Study of PVA-GTA Fricke Gel Dosimeters Exposed to 60Co Source

Gel dosimetry: MRI

While a vast amount of scientific literature is available on the topic of gel dosimetry with MRI readout, this wealth of information may seem at first overwhelming for medical physicists and newcomers in this rapidly evolving field of research. With this review, my mission is to streamline the wealth of information in the scientific literature and provide a quick guideline for those making their first steps in implementing gel dosimetry in a clinical environment, while still providing a lookout to new and emerging evolutions in the field. In a first section, the physical mechanisms behind the MRI contrast are briefly explained for both Fricke gels and polymer gels. In a subsequent section, an overview is given of the different MRI pulse sequences and pulse sequence optimization will be discussed. Emphasize is placed on the framework and formalism to calculate optimal parameters. The reliability of MRI-based polymer gel dosimetry will be discussed, and a quick beginner’s guide is provided. Finally, a lookout to new and future developments of polymer gel dosimetry will be given.

Rheology modifiers in Fricke – gelatin dosimeters

Fricke gel dosimeters, while very sensitive and versatile, continue to suffer from one major issue – the diffusion of the dose signal. To tackle that problem, here we present the use of inert, linear polymers with very high molecular weights as rheology modifiers that can slow the diffusion in weak gels, based on 4% gelatin. At 0.2% loading of polyacrylamide (with mol. wt. 5×106 Da), a moderate, 20% decrease of the diffusion coefficient was achieved. This effect is compared to that of adding a crosslinker, such as glyoxal or increasing the concentration of gelatin by 1%.

Preliminary dosimetric characterization of EDBreast gel

Purpose. EDBreast gel is an alternative Fricke gel dosimeter, read by Magnetic Resonance Imaging, in which sucrose is added to lower diffusion effects. This paper aims at determining the dosimetric characteristics of this dosimeter. Methods. The characterization has been performed in high energy photon beams. The dose-response of the gel has been evaluated as well as its detection limit, its fading effects, the reproducibility of its response and its stability over time. Its energy and dose-rate dependence has been investigated, and the overall dose uncertainty budget established. Once characterized, the dosimetry method has been applied to a simple reference irradiation case in a 6 MV photon beam, with the measurement of the lateral dose profile of a 2 × 2 cm2 field. The results have been compared with microDiamond measurements. Results. In addition to its low diffusivity, the gel presents a high sensitivity, no dose-rate dependence considering values ranging from 0.66 to 0.79 and an energy response comparable to ionization chambers. However, its non-linear dose-response induces a high uncertainty on the measured dose (8 % () at 20 Gy) and reproducibility issues. The profile measurements displayed discrepancies compared to the microDiamond due to diffusion effects. The appropriate spatial resolution was estimated using the diffusion coefficient. Conclusion. EDBreast gel dosimeter presents interesting characteristics for applications in clinics, but the linearity of its dose-response should be improved to lower the uncertainties and to enhance the reproducibility.

Investigating the performance of susceptibility-weighted images in Fricke gel dosimeter reading and optimizing the related imaging parameters

Fricke gel dosimeters are especially useful in small-field dosimetry and validation of treatment delivery in three-dimensional space with features such as tissue equivalence, non-toxicity, high spatial resolution, non-dependence on energy, and dose rate. The use of basic Magnetic Resonance Imaging (MRI) protocols (T1- and T2-Weighted) for reading Fricke gel dosimeters has always been considered the dominant method in many studies. However, the development and application of advanced MRI techniques for more accurate readings of Fricke gel dosimeters can be useful. Considering that in the main structure of Fricke gel, there are conversions of iron ions to each other, this study aimed to investigate the performance of Susceptibility-Weighted Imaging (SWI) and Quantitative Susceptibility Mapping (QSM) based on magnetic susceptibility in the reading of Fricke gel dosimeters and to optimize the related imaging parameters. For this purpose, a Fricke-Xylenol orange-gelatin was made at five concentrations of iron ammonium sulfate. To obtain gel dosimeter calibration curves, vials containing gel were subjected to irradiation at three different doses by a linear accelerator. The reading of gel dosimeters was performed using MRI imaging in three protocols, T1W, T2W, and SWI, and analyzed with a method unique to each one. Finally, the results obtained from the three protocols were compared with each other. The comparison of calibration curves in three imaging protocols shows that the sensitivity of calibration curves in SWI was about three times its value in T2W, and on the other hand, the reported sensitivity in T1W was very small compared to the other two protocols. The linearity factor was similar between SWI and T1W protocols and higher in T2W. Therefore, it is concluded that in addition to the relaxometry techniques that have been used as a conventional method for reading Fricke gel dosimeter, SWI imaging has high sensitivity and specificity for reading dosimeter gel based on iron.

Dose-response dependencies of Turnbull blue, modified Fricke, VIPET, and Presage® gel dosimeters in high-dose-rate radiation fields

Dose-response dependencies of the radiochromic Turnbull blue (TB), modified Fricke gel (FXO), polymer gel VIPET, and solid-state Presage® dosimeters were investigated, in high-dose-rate radiation fields, using spectrophotometry. A sealed radionuclide source (60Co) with a dose rate up to 9 Gy s−1 was used for irradiation. Standard liquid Fricke solution was used as a reference dosimeter. It was found that the properties of different types of gel dosimeters are affected by the dose rate. The results were compared with the predictions derived from the physical-chemical irradiation response principles. Considerable dose-rate dependencies were observed for all investigated 3D dosimeters. Dose responses of TB, VIPET, and FXO were decreased by 30–40% while the Presage® response were decreased by approximately 20% for dose rates from 7.2 × 10−3 to 9.01 Gy s−1. Presented results could have practical impact on some clinical applications, and therefore proper corrections might be needed.

Radiation Dosimetry by Use of Radiosensitive Hydrogels and Polymers: Mechanisms, State-of-the-Art and Perspective from 3D to 4D.

Gel dosimetry was developed in the 1990s in response to a growing need for methods to validate the radiation dose distribution delivered to cancer patients receiving high-precision radiotherapy. Three different classes of gel dosimeters were developed and extensively studied. The first class of gel dosimeters is the Fricke gel dosimeters, which consist of a hydrogel with dissolved ferrous ions that oxidize upon exposure to ionizing radiation. The oxidation results in a change in the nuclear magnetic resonance (NMR) relaxation, which makes it possible to read out Fricke gel dosimeters by use of quantitative magnetic resonance imaging (MRI). The radiation-induced oxidation in Fricke gel dosimeters can also be visualized by adding an indicator such as xylenol orange. The second class of gel dosimeters is the radiochromic gel dosimeters, which also exhibit a color change upon irradiation but do not use a metal ion. These radiochromic gel dosimeters do not demonstrate a significant radiation-induced change in NMR properties. The third class is the polymer gel dosimeters, which contain vinyl monomers that polymerize upon irradiation. Polymer gel dosimeters are predominantly read out by quantitative MRI or X-ray CT. The accuracy of the dosimeters depends on both the physico-chemical properties of the gel dosimeters and on the readout technique. Many different gel formulations have been proposed and discussed in the scientific literature in the last three decades, and scanning methods have been optimized to achieve an acceptable accuracy for clinical dosimetry. More recently, with the introduction of the MR-Linac, which combines an MRI-scanner and a clinical linear accelerator in one, it was shown possible to acquire dose maps during radiation, but new challenges arise.

Preliminary Study on the Use of Fricke Gel Dosimeter for Verification of IMRT Beam Delivery

The goal of intensity-modulated radiation therapy (IMRT) is to deliver a uniform dose to the tumor with minimal margins around the target, in order to increase local control of the disease while reducing secondary effects. The research performed in this work has shown the potential usefulness of the Fricke-gel dosimeter as a quality assurance (QA) tool to verify IMRT treatments produced by inverse treatment planning. First, the 3D integrating Fricke-gel dosimeter was successfully compared to an accepted dosimetric tool. It was then used to measure relative 3D dose distributions of simple treatment plans with multiple square or rectangular fields and specific inverse-planned IMRT treatment plans. By combining the CT anatomical information and the plan contours with the gel-measured data, it was possible to display the contours on the measured dose and the measured isodose lines on the CT, in addition to measuring dose-volume histograms (DVH) for the plans. This demonstrated the usefulness of the gel dosimeter as a QA tool for IMRT and inverse planning.