Polyacrylamide Gel Dosimeters Research Articles

Feasibility study of using polyacrylamide gel dosimeter as brain equivalent material in therapeutic energy range of X-rays

In this paper the feasibility study of using polyacrylamide gels as a brain equivalent material has been performed. Thus, the parameters of mass energy absorption coefficient (μen/ρ), mass stopping power (S/ρ), effective atomic number (Zeff), mass attenuation coefficient (μeff/ρ) and absorbed dose delivered to polyacrylamide gel (D) were determined for this gel using analytical and Monte Carlo methods and were compared with brain reference material. The calculation results reveal that the relative difference between the calculated μen/ρ, S/ρ, Zeff, μeff/ρ and D for polyacrylamide gel and those for brain are 1.32%, less than 1%, less than 2%, less than 4% and less than 1%, respectively in therapeutic energy range. Consequently, like water, polyacrylamide gel can be also used for designing and construction of head and neck phantoms applied for validation of medical algorithms used in treatment planning systems. The advantage of using polyacrylamide gel as a brain equivalent material in comparison to water is that this gel is a reliable X and gamma rays dosimeter applicable for three dimensional dose distribution determination. Also, other advantages of this gel are its low price, availability and forming it in different sizes and shapes. These properties of the polyacrylamide gel make it beneficial as brain equivalent material in designing and construction of head and neck phantoms.

Evaluations of N-(Isobutoxymethyl) acrylamide gel dosimeter by NMR technique for radiotherapy and uncertainty in dose measurements

N-(Isobutoxymethyl) acrylamide (NIBMA) monomer in gelatin, named NIBMAGAT gel dosimeter, was prepared and investigated by nuclear magnetic imaging (NMR) for radiotherapy in the dose range of 0–30 Gy. NIBMA monomer polymerizes upon irradiation, increasing spin–spin relaxation rate R2. The addition of glycerol as a co-solvent in the gel matrix improved its radiation sensitivity better than the co-solvents of acetone and methanol. The increase of glycerol content by 1% wt/wt enhanced the sensitivity by ˜ 3.1%. This gel has better radiation sensitivity as compared to the polyacrylamide gel (PAG) dosimeter; the sensitivities of NIBMAGAT gel and normoxic polyacrylamide gel (nPAG) are ≈0.13 and ≈0.1 s−1.Gy−1, respectively. By comparing NIBMAGAT gel dosimeter with PAG, nMAG and nPAG gel dosimeters, NIBMAGAT gel dosimeter is less influenced by scanning temperature than the last three dosimeters. The gel is water equivalent and has an energy-independent response from 80 keV to 20 MeV. The overall uncertainty of dose measurement using NIBMAGAT gel is 5.46% at 2σ. Our findings suggest the applicability of using NIBMAGAT gel dosimeter by NMR technique for dose verification/planning in the practice of clinical radiotherapy.

A new less toxic polymer gel dosimeter: Radiological characteristics and dosimetry properties

PurposeA new polymer gel dosimeter recipe was investigated that may be more suitable for widespread applications than polyacrylamide gel dosimeters, since the extremely toxic acrylamide has been replaced with the less harmful monomer 2-Acrylamido 2-Methyl Propane Sulfonic acid (AMPS). MethodsThe new formulation was named PAMPSGAT. The MRI response (R2) of the dosimeters was analyzed for conditions of varying dose, dose rate, and temperature during scanning. Radiological properties of the PAMPSGAT polymer gel dosimeter were investigated. ResultsThe dose-response (R2) of AMPS/Bis appears to be linear over a dose range 10–40 Gy. The percentage of difference between the R2 values for imaging at 15 °C and MRI room temperature is about 4.6% for vial with 40 Gy absorbed dose which decreased to less than 1% for imaging at 20 °C. The percentage difference of Zeff of PAMPSGAT gel and soft tissue was less than 1% in the practical energy range (100 KeV–100 MeV). The electron density of the PAMPSGAT polymer gel was 2.9% higher than that of muscle. Results showed that the sensitivity of PAMPSGAT polymer gel dosimeter irradiated by 60Co (energy = 1.25 MeV) is about 27.7% higher than that of irradiated using a 6 MeV Linac system. ConclusionsTemperature during MRI scanning has a small effect on the R2 response of the PAMPSGAT polymer gel dosimeter. Results confirmed tissue equivalency of the PAMPSGAT polymer gel dosimeter in most practical energy range. The PAMPSGAT polymer gel dosimeter response depends on energy and dose rate.

Characteristics of a novel polymer gel dosimeter formula for MRI scanning: Dosimetry, toxicity and temporal stability of response

The present study intended to investigate the composition of a new polymer gel dosimeter. The new composition would be more suitable for a wide range of applications in comparison to polyacrylamide gel dosimeter since its extremely toxic acrylamide has been replaced with less harmful monomer i.e. 2-Acrylamido-2-MethylPropane Sulfonic acid (AMPS). To this end, the PAGAT gel dosimeter formula was used as a basis to test the new formulation of polymer gel dosimeter with a different monomer (AMPS) instead of acrylamide by using the %6T and %50C to the formula. The new formulation was named PAMPSGAT (Poly AMPS, Gelatin and THPC) polymer gel dosimeter. Moreover, the MRI response (R2) of dosimeters was analyzed in terms of different dose range as well as post-irradiation time. The results indicated that the dose-response (R2) of AMPS/Bis had a linear trend over a wide dose range. Furthermore, the results showed an acceptable temporal stability for the new polymer gel dosimeter.

Radiation-induced refraction artifacts in the optical CT readout of polymer gel dosimeters.

The objective of this work is to demonstrate imaging artifacts that can occur during the optical computed tomography (CT) scanning of polymer gel dosimeters due to radiation-induced refractive index (RI) changes in polyacrylamide gels. A 1 L cylindrical polyacrylamide gel dosimeter was irradiated with 3 × 3 cm(2) square beams of 6 MV photons. A prototype fan-beam optical CT scanner was used to image the dosimeter. Investigative optical CT scans were performed to examine two types of rayline bending: (i) bending within the plane of the fan-beam and (ii) bending out the plane of the fan-beam. To address structured errors, an iterative Savitzky-Golay (ISG) filtering routine was designed to filter 2D projections in sinogram space. For comparison, 2D projections were alternatively filtered using an adaptive-mean (AM) filter. In-plane rayline bending was most notably observed in optical CT projections where rays of the fan-beam confronted a sustained dose gradient that was perpendicular to their trajectory but within the fan-beam plane. These errors caused distinct streaking artifacts in image reconstructions due to the refraction of higher intensity rays toward more opaque regions of the dosimeter. Out-of-plane rayline bending was observed in slices of the dosimeter that featured dose gradients perpendicular to the plane of the fan-beam. These errors caused widespread, severe overestimations of dose in image reconstructions due to the higher-than-actual opacity that is perceived by the scanner when light is bent off of the detector array. The ISG filtering routine outperformed AM filtering for both in-plane and out-of-plane rayline errors caused by radiation-induced RI changes. For in-plane rayline errors, streaks in an irradiated region (>7 Gy) were as high as 49% for unfiltered data, 14% for AM, and 6% for ISG. For out-of-plane rayline errors, overestimations of dose in a low-dose region (∼50 cGy) were as high as 13 Gy for unfiltered data, 10 Gy for AM, and 3.1 Gy for ISG. The ISG routine also addressed unrelated artifacts that previously needed to be manually removed in sinogram space. However, the ISG routine blurred reconstructions, causing losses in spatial resolution of ∼5 mm in the plane of the fan-beam and ∼8 mm perpendicular to the fan-beam. This paper reveals a new category of imaging artifacts that can affect the optical CT readout of polyacrylamide gel dosimeters. Investigative scans show that radiation-induced RI changes can cause significant rayline errors when rays confront a prolonged dose gradient that runs perpendicular to their trajectory. In fan-beam optical CT, these errors manifested in two ways: (1) distinct streaking artifacts caused by in-plane rayline bending and (2) severe overestimations of opacity caused by rays bending out of the fan-beam plane and missing the detector array. Although the ISG filtering routine mitigated these errors better than an adaptive-mean filtering routine, it caused unacceptable losses in spatial resolution.

Preliminary studies on the role and reactions of tetrakis(hydroxymethyl)phosphonium chloride in polyacrylamide gel dosimeters

A major source of dosimetric inaccuracy in normoxic polymer gel dosimeters is local variations in the concentration of oxygen scavenger. Currently, a phosphorus compound, tetrakis(hydroxymethyl)phosphonium chloride (THPC), is the oxygen scavenger of choice in most polymer gel dosimetry studies. Reactions of THPC in a gel dosimeter are not limited to oxygen. It can possibly be consumed in reacting with gelling agent, water free-radicals and polymer radicals before, during and after irradiation, hence affecting the dose response of the dosimeter in several ways. These reactions are not fully known or understood. It is our hypothesis that THPC not only scavenges radical species but also modifies the morphology of the gelatin network and of the polymer, possibly by intervening in the polymerization of monomers. These hypotheses are investigated in an anoxic acrylamide-based gel dosimeter. Scanning electron microscopy results indicate gelatin pores decreasing from 70 to 40 µm and a very different radiation-induced polymer structure in samples containing THPC; Fourier-transform Raman spectroscopy shows a two-fold reduction in the dose constants of monomer consumption; however, a significant change in the relative dose constants of monomer consumption as a function of dose could not be detected.

Mathematical Modeling of the Response of Polymer Gel Dosimeters to HDR and LDR Brachytherapy Radiation

AbstractA model is developed to simulate the response of polyacrylamide gel (PAG) dosimeters to single spherical brachytherapy seeds. The model predicts the amount of polymer formed and crosslink density as a function of radial distance and time, using either a HDR Ir192 seed or a LDR I125 seed. Results indicate that PAG dosimeters should provide accurate dosimetry when used for HDR brachytherapy at distances larger than 4 mm from the center of the seed and that PAG dosimeters will give poor results for LDR dosimetry because of the long time available for the monomer and crosslinker to diffuse toward the seed during polymerization.magnified image

Mathematical Modeling of Depth‐Dose Response of Polymer‐Gel Dosimeters

AbstractA dynamic PDE model is developed to simulate the effects of radiation depth doses on polymer formation in polyacrylamide gel dosimeters. Depth doses are simulated using different types of radiation including Co60 γ and 6 and 15 MV X‐ray photon beams, along with 6, 9, 12, 16 and 20 MeV electron beams. Effects of monomer diffusion (edge enhancement) and temperature are studied. Diffusion results in excess polymer formation at the position of maximum dose (2.6% for Co60 γ‐radiation and less for other types of radiation studied). Temperature increases on the order of 2 °C increase the mass of polymer formed by ≈1.25%. The influence of dose‐rate dependence was also investigated. Results of this work provide insight for developing effective calibration curves using depth‐dose information.magnified image

SU-C-224-01: 3D Dosimetry with Gels and Optical Tomography of Dynamic MLC Tracking Based on an Electromagnetic Transponder System

Purpose: Complex treatment methods such as rotational intensity modulated radiotherapy (rIMRT) offer highly conformal dose delivery but are at risk of being compromised due to target motion. Dynamic MLC tracking has been developed to compensate for target motion online. Due to the complex 3D patterns of tumour motion, tracking would benefit from a full 3D dosimetric verification. In this study we have therefore investigated the use of 3D dosimetry - with gels and optical tomography - for measuring the effect of tracking on 3D dose distributions with the use of a clinically measured prostate trajectory. Methods: Two normoxic polyacrylamide gel dosimeters were irradiated with the same prostate rIMRT treatment plan, delivered with and without tracking while placed on a motion stage that reproduced a patient measured prostate trajectory. A reference gel dosimeter was irradiated without motion or tracking. Dynamic MLC tracking was performed using an electromagnetic transponder system (RayPilot, MicroPos AB, Gothenburg Sweden). After experiments the 3D dose distributions in the two motion experiments were read-out using an optical CT scanner and compared to the stationary reference dosimeter using 3 %/3 mm gamma analyses. Results: 2D 3%/3mm gamma analyses were performed at equidistant slices separated by 1 cm in the transverse plane and the coronal plane of the 3D dosimeters. The weighted average gamma fail-rate was improved from 37% to 14% (transversal plane) and from 30% to 9% (coronal plane) by tracking compared to treatment without tracking. Conclusions: Optical 3D dosimetry is a valuable tool to quantify the impact of motion and motion compensation on the delivered dose distribution to a moving target. Improved knowledge about the dose distribution is obtained due to (i) the three-dimensional nature of the measurements and (ii) the high resolution compared to competing techniques.

Improvement in the accuracy of polymer gel dosimeters using scintillating fibers

We propose a novel method for the absolute calibration of polyacrylamide gel (PAG) dosimeters with one or more reference scintillating fiber dosimeters inserted inside the gel. Four calibrated scintillating fibers were inserted into a cylindrical glass container filled with a PAG dosimeter irradiated with a wedge filtered 6 MV photon beam. Calibration curves using small glass vials containing the same gel as the cylindrical containers were used to obtain a first calibration curve. This calibration curve was then adjusted with the dose measured with one of the scintillating fibers in a low gradient part of the field using different approaches. Among these, it was found that a translation of the gel calibration curve yielded the highest accuracy with PAG dosimeters.

Dose overshoot reduction by tetrakis hydroxymethyl phosphonium chloride in polyacrylamide gel dosimeter

This work deals with an influence of the antioxidant tetrakis (hydroxymethyl) phosphonium chloride (THPC) on the edge enhancing effect of a polyacrylamid gel and THPC (PAGAT) dosimeter. Four batches of PAGAT gel were produced with different THPC concentrations: 0, 1, 2 and 4 mM. It was proved that antioxidant THPC reduce the edge enhancing effects on the cost of reduced sensitivity of the gel dosimeter.

Effect of the exothermal polymerization reaction on polymer gel dosimetric measurements

Discrepancies in polymer gel dosimetric measurements have been observed between containers of different sizes receiving the same radiation dose. We hypothesized that these deviations are caused by a change in the rate of polymerization due to internal heat increase in the gel containers resulting from the exothermic polymerization of monomers. Here, we test this hypothesis in a polyacrylamide gel dosimeter by recording the temperature in glass phantoms of different sizes during and after irradiation. The dose response of the samples was determined with magnetic resonance imaging. The difference of R2 values along the depth of the containers was below ±1%. We discuss that this small difference can be attributed to variations in the rate of gelatin cooling during manufacture rather than to the measured heat increase during irradiation.

Evaluation of the radiation-sensitizer/protector and/or antioxidant efficiencies using Fricke and PAG dosimeters

In this study, our aim is to assess the potential of Fricke and polyacrylamide gel (PAG) dosimeters to quantitatively evaluate the efficiency of potential radiation sensitizers/protectors and antioxidants. These compounds are of importance in radiotherapy as well as in disease prevention and promotion of health. The basic principle of the Fricke dosimeter is the radiation-induced oxidation of Fe2+ to Fe3+ in an aerated aqueous 0.4 M H2SO4. The production of ferric ions is most sensitive to the radical species produced in the radiolysis of water. Using this method, we observed that cystamine (one of the best of the known radioprotectors) can prevent oxydation of Fe2+ from reactive radiolysis species. However, one obvious disadvantage of the Fricke dosimeter is that it operates under highly acidic conditions (pH 0.46), which may degrade biological compounds. In contrast, the pH of the polyacrylamide gel (PAG) dosimeter is almost neutral, such that degradation of compounds is less probable. A change in R2-dose sensitivity was observed in the presence of radiosensitizers/radioprotectors and antioxidants. The protective effect of Trolox (a well-known antioxidant) and thiourea (a radioprotector) was readily observed using the PAG dosimeter. Incorporation of iodinated radiation sensitizers such as NaI and an iodine contrast agent led to a quantifiable sensitizer enhancement ratio. These studies suggest that the Fricke and the PAG dosimeters have the potential to evaluate the efficiency of radiation sensitizers/protectors and antioxidants.

Utilization of nPAG dosimeter for synchrotron radiotherapy: first results

The specificity of synchrotron stereotactic radiotherapy requires new developments in dose calculation and appropriate experimental procedures. The medical beamline of the European Synchrotron Radiation Facility allows quantitative synchrotron radiation CT (SRCT) images to be obtained from a monochromatic x-ray beam. This paper investigates initial experimental dosimetry with normoxic polyacrylamide gel dosimeters. MRI and SRCT readouts are compared.

Polymer gel dosimeters with reduced toxicity: a preliminary investigation of the NMR and optical dose–response using different monomers

In this work, three new polymer gel dosimeter recipes were investigated that may be more suitable for widespread applications than polyacrylamide gel dosimeters, since the extremely toxic acrylamide has been replaced with the less harmful monomers N-isopropylacrylamide (NIPAM), diacetone acrylamide and N-vinylformamide. The new gel dosimeters studied contained gelatin (5 wt%), monomer (3 wt%), N,N′-methylene-bis-acrylamide crosslinker (3 wt%) and tetrakis (hydroxymethyl) phosphonium chloride antioxidant (10 mM). The NMR response (R 2) of the dosimeters was analysed for conditions of varying dose, dose rate, time post-irradiation, and temperature during irradiation and scanning. It was shown that the dose–response behaviour of the NIPAM/Bis gel dosimeter is comparable to that of normoxic polyacrylamide gel (PAGAT) in terms of high dose-sensitivity and low dependence on dose rate and irradiation temperature, within the ranges considered. The dose–response (R 2) of NIPAM/Bis appears to be linear over a greater dose range than the PAGAT gel dosimeter. The effects of time post-irradiation (temporal instability) and temperature during NMR scanning on the R 2 response were more significant for NIPAM/Bis dosimeters. Diacetone acrylamide and N-vinylformamide gel dosimeters possessed considerably lower dose-sensitivities. The optical dose–response, measured in terms of the attenuation coefficient for each polymer gel dosimeter, showed potential for the use of optical imaging techniques in future studies.

Modelling of polyacrylamide gel dosimeters with spatially non-uniform radiation dose distributions

Over the past decade there has been much interest in the development of three-dimensional gel dosimeters to aid in the determination of the distribution and magnitude of absorbed dose in clinical radiation therapy. The most widely used dosimeter for verification of spatial dose distributions is the polyacrylamide gel (PAG) dosimeter. In this article, a detailed fundamental model is developed to describe the chemical and physical phenomena in this gel dosimeter, based on the radiation-induced copolymerization of acrylamide and bisacrylamide monomers in water and gelatin. The model predicts the concentrations of acrylamide, bisacrylamide, pendant double bonds (PDB), cyclized groups and cross-link units as well as temperature at different times and positions within the dosimeter, and accounts for the formation of insoluble microgels. Dosimetry experiments are simulated in which the spatial distribution of the radiation dose is non-uniform in the horizontal direction. The middle section of the dosimeter is irradiated, and the two ends are not, resulting in large concentration gradients near the edges of the irradiated zone. The resulting system of partial differential equations (PDEs) is solved numerically, and a stretching transformation is applied to ensure accurate model predictions. The model is able to simulate the effects of radiation dose, dose rate and dosimeter recipe on edge enhancement and temporal instability, two problems that have plagued the users of polymer gel dosimeters. The mechanistic scheme summarized in the model and the associated simulations provide important knowledge that should assist medical physicists in understanding existing dosimetry systems and in designing improved dosimeters in the future.

Development and optimization of a 2-hydroxyethylacrylate MRI polymer gel dosimeter

In this study, radiation induced changes in a polymer gel dosimeter manufactured using 2-hydroxyethylacrylate (HEA) and N,N′-methylene-bisacrylamide (BIS) were investigated using magnetic resonance imaging (MRI) and FT-Raman spectroscopy. The variation in magnetic resonance relaxation time (T2) with absorbed dose was modelled assuming fast exchange of magnetization. Overall good agreement between the model and experimental data was obtained. However, comparison with FT-Raman data suggests that not all the protons attached to the polymer contribute to the relaxation process. Furthermore, for certain compositions improved agreement with experimental data was achieved when a lower fraction of polymer protons available for exchange with water was assumed in the low dose region. This indicates that the T2 value is influenced by the composition and topology of the formed polymer, which may vary with absorbed dose. The concept of percentage dose resolution (DpΔ,%) was introduced to enable optimization of gel compositions for use in relative dosimetry applications. This concept was applied to demonstrate the effects of varying the gelatine concentration, the total fraction of monomer/crosslinker (%T) and the relative fraction of crosslinker (%C) on gel performance in HEA gels as well as compare the performance of HEA and a standard polyacrylamide gel (PAG). The percentage dose resolution was improved for all HEA gels compared to the PAG dosimeter containing 3% acrylamide and 3% BIS. Increasing the total concentration of monomer was shown to have the largest single effect. In the range of doses of interest for clinical radiation therapy, DpΔ,% for the optimal HEA gel (4% HEA, 4% BIS) was lower than 2.3%, compared to 3.8% for the PAG dosimeter.

The chemistry and physics of polyacrylamide gel dosimeters: why they do and don't work

Three factors that prohibit widespread clinical use of polyacrylamide gel (PAG) dosimeters are polymerization after irradiation ceases, formation of additional polymer near the edges of irradiated zones, and monomer toxicity. Polymerization can occur long after irradiation ceases because polymeric radicals cannot diffuse and terminate with other radicals. Small monomer molecules can diffuse toward trapped polymeric radicals and polymerize. Edge enhancement occurs because acrylamide and bisacrylamide diffuse from regions of high concentration (where the radical concentration is low) to adjacent regions where monomer concentration is low and the radical concentration is high. As monomers diffuse into the irradiated zone, they are polymerized by radicals near the edge. Acrylamide is a neurotoxin and suspected carcinogen that can be absorbed through the skin and inhaled, making it an undesirable monomer for polymer gel dosimetry. Less toxic monomers, such as n-vinyl formamide, should give similar dosimetry results, with less concern for safety. When selecting alternative monomers and cross-linkers for polymer gel dosimetry, it would be advantageous to choose larger molecules that diffuse more slowly, resulting in less edge enhancement. Larger molecules should also lead to improved safety, because they are less easily absorbed through the skin and are less easily vaporized and inhaled.

Modeling chemical and physical phenomena in polyacrylamide gel dosimeters

Polyacrylamide gel (PAG) dosimetry provides a promising means for the calibration and verification of three-dimensional radiation dose delivery in modern conformal radiotherapy. PAG dosimetry is based on the radiation-induced copolymerization of acrylamide and bisacrylamide monomers in water and gelatin. While the basic mechanisms of the radiation-induced polymerization that gives rise to this technique have been studied experimentally, little work has been performed in modeling the polymerization processes. We have reported such models for homogeneous irradiations of PAG dosimeters previously. In this work we present a more complex model that can account for spatial variations in the dose distribution. The models are intended to provide a better understanding of fundamental dosimeter behaviour, so that the use of PAG systems may become more reliable and improved dosimeters may be developed in the future.

Modeling of Free‐radical Crosslinking Copolymerization of Acrylamide and N,N′‐Methylenebis(acrylamide) for Radiation Dosimetry

AbstractOver the past decade there has been much interest in the development of three‐dimensional gel dosimeters to aid in the determination of the distribution and magnitude of absorbed dose in clinical radiation therapy. A widely used dosimeter for verification of spatial dose distribution is the polyacrylamide system, sometimes termed polyacrylamide gel (PAG). In this paper, we develop a model to describe the kinetic mechanisms in this gel dosimeter, based on the radiation‐induced copolymerization of acrylamide and bisacrylamide monomers in water and gelatin. Using the proposed model, the conversion of both monomers and total vinyl groups, the concentrations of pendant double bonds (PDB), cyclized groups and crosslinks along the polymer chains as well as temperature changes were simulated. The model predictions and experimental findings in PAG dosimeters agree for a variety of recipes and spatially uniform radiation conditions. Further experiments are required to obtain accurate kinetic parameter estimates and to verify model predictions.Comparison between the temperatures changes predicted by the model (dotted lines) and experimental data (solid lines) for PAGs irradiated to different doses.imageComparison between the temperatures changes predicted by the model (dotted lines) and experimental data (solid lines) for PAGs irradiated to different doses.