Normoxic Polymer Gel Research Articles

True 3D chemical dosimetry (gels, plastics): Development and clinical role

Since the introduction of volumetric chemical dosimetry with Fricke gel dosimeters in the 1980s, three-dimensional (3D) dosimetry has been a promising technique for the clinic, since it provides a unique methodology for 3D dose measurement of the complex conformal dose distributions achieved by modern techniques such as Intensity Modulated and Volumetric Arc Radiation Therapy. In the last decade, the potential for improved clinical applicability has been advanced by the development of improved 3D dosimeters such as normoxic polymer gel systems, radiochromic plastics (such as PRESAGE) and, recently, newer radiochromic gel dosimeters. Some of these new 3D dosimetry systems were enabled by the availability of optical computed tomography imaging systems for fast dose readout. However, despite its promise, true 3D dosimetry is still not widely practiced in the community. Its use has been confined primarily to select centres of expertise and to specialised quality assurance or commissioning roles where other dosimetry techniques are difficult to implement. In this paper I review some of the current 3D chemical dosimeters available, discuss the requirements for their use and briefly review the roles that these systems can provide to complement the other dose delivery validation approaches available in the clinic. I conclude by describing two roles that may be uniquely served by 3D chemical dosimetry in end-to-end process testing and validation in the complex environment coming into play with the development of Image Guided Adaptive Radiation Therapy.

The use of normoxic polymer gel for measuring dose distributions of 1, 4 and 30mm cones

Abstract This study demonstrates the use of normoxic polymer gel for measuring dose distributions of small fields that lack lateral electronic equilibrium. Two different types of normoxic polymer gel, MAGAT and PAGAT, are studied in a larger field (10 cm×10 cm) and 1, 4 and 30 mm cones to obtain cone factors, dose profiles and percentage depth doses. These results were then compared to KODAK XV film measurements and BEAMnrc Monte Carlo simulations. The results show that the sensitivity of PAGAT gel is 0.090±0.074 s −1 Gy −1 , which may not be suitable for small-field dosimetry with a 0.3 mm resolution scanned using a 3 T MR imager in a dose range lower than 2.5 Gy. There are good agreements between cone factors estimated using KODAK XV film and MAGAT gel. In a dose profile comparison, good dose agreement among MAGAT gel, XV film and MC simulation can be seen in the central area for a 30 mm cone. In penumbra, the distance to agreement is at most 1.2 mm (4 pixel), and less than 0.3 mm (1 pixel) for 4 and 1 mm cones. In a percentage depth dose comparison, there were good agreements between MAGAT and MC up to a depth of 8 cm. Possible factors for gel uncertainty such as MRI magnetic field inhomogeneity and temperature were also investigated.

Comparison of 3D dose distributions for HDR 192Ir brachytherapy sources with normoxic polymer gel dosimetry and treatment planning system

Radiation fluence changes caused by the dosimeter itself and poor spatial resolution may lead to lack of 3-dimensional (3D) information depending on the features of the dosimeter and quality assurance of dose distributions for high–dose rate (HDR) iridium-192 (192Ir) brachytherapy sources is challenging and experimental dosimetry methods used for brachytherapy sources are limited. In this study, we investigated 3D dose distributions of 192Ir brachytherapy sources for irradiation with single and multiple dwell positions using a normoxic gel dosimeter and compared them with treatment planning system (TPS) calculations. For dose calibration purposes, 100-mL gel-containing vials were irradiated at predefined doses and then scanned in an magnetic resonance (MR) imaging unit. Gel phantoms prepared in 2 spherical glasses were irradiated with 192Ir for the calculated dwell positions, and MR scans of the phantoms were obtained. The images were analyzed with MATLAB software. Dose distributions and profiles derived with 1-mm resolution were compared with TPS calculations. Linearity was observed between the delivered dose and the reciprocal of the T2 relaxation time constant of the gel. The x-, y-, and z-axes were defined as the sagittal, coronal, and axial planes, respectively, the sagittal and axial planes were defined parallel to the long axis of the source while the coronal plane was defined horizontally to the long axis of the source. The differences between measured and calculated profile widths of 3-cm source length and point source for 70%, 50%, and 30% isodose lines were evaluated at 3 dose levels using 18 profiles of comparison. The calculations for 3-cm source length revealed a difference of > 3mm in 1 coordinate at 50% profile width on the sagittal plane and 3 coordinates at 70% profile width and 2 coordinates at 50% and 30% profile widths on the axial plane. Calculations on the coronal plane for 3-cm source length showed > 3-mm difference in 1 coordinate at 50% and 70% and 2 coordinates at 30% profile widths. The point source measurements and calculations for 50% profile widths revealed a difference > 3mm in 1 coordinate on the sagittal plane and 2 coordinates on the axial plane. The doses of 3 coordinates on the sagittal plane and 4 coordinates on the axial plane could not be evaluated in 30% profile width because of low doses. There was good agreement between the gel dosimetry and TPS results. Gel dosimetry provides dose distributions in all 3 planes at the same time, which enables us to define the dose distributions in any plane with high resolution. It can be used to obtain 3D dose distributions for HDR 192Ir brachytherapy sources and 3D dose verification of TPS.

Dosimetry study of diagnostic X-ray using doped iodide normoxic polymer gels

In radiotherapy, polymer gel dosimeters are used for three-dimensional (3D) dose distribution. However, the doses are within the Gy range. In this study, we attempted to develop a low-dose 3D dosimeter within the mGy range for diagnostic radiology. The effect of the iodinated compound was used as a dose enhancement sensitizer to enhance the dose sensitivity of normoxic polymer gel dosimeters. This study aims to use N-isopropylacrylamide(NIPAM)-based and methacrylic acid (MAGAT)-based gels to evaluate the potential dose enhancement sensitizer, as well as to compare two gels that may be suitable for measuring diagnostic radiation doses. The suitable formulation of NIPAM gel [5% (w/w) gelatin, 5% (w/w) NIPAM, 3% (w/w) N,N′-methylenebisacrylamide (BIS), 5mM tetrakis (hydroxymethyl) phosphonium chloride (THPC), and 87% (w/w) deionized distilled water] and MAGAT gel (4% MAA, 9% gelatin, 87% deionized water, and 10mM THPC) were used and loaded with clinical iodinated contrast medium agent (Iobitridol, Xenetix® 350). Irradiation was conducted using X-ray computed tomography. The irradiation doses ranged from 0mGy to 80mGy. Optical computed tomography was the employed gel measurement system. The results indicate that the iodinated contrast agent yields a quantifiable dose enhancement ratio. The dose enhancement ratios of NIPAM and MAGAT gels are 3.35±0.6 and 1.36±0.3, respectively. The developed NIPAM gel in this study could be suitable for measuring diagnostic radiation doses.

High resolution dosimetry in monoenergetic proton beam therapy on a normoxic polymer gel: the importance of high spatial resolution for reduced Bragg-Peak-quenching

Proton ion beam therapy demands for high resolution dosimetry due to the high dose gradients present in lateral confinement and final Bragg-peak. In polymer gels the reduction of the linear dose response in the area of the Bragg-peak is reported (Bragg-peak quenching), which is assumed to be mainly due to the high linear energy transfer (LET). We here investigate the impact of the spatial resolution in T2-mapping for accurate Magnetic Resonance Imaging (MRI)-based polymer gel dosimetry in the Bragg-peak for monoenergetic ion beams. We implemented MR-protocols for T2-mapping at microscopic resolution on a High-Field 7T human MR-scanner using an insert gradient system and sensitive rf-coils. The best results are obtained for an optimzed polymer gel based on THPC with an optimized MR-protocol for reduced measurement time and sufficient SNR at 0,547 mm pixel size. The dose in the fine Bragg-peak could be measured correctly for a monoenergetic proton beam as confirmed by Monte Carlo dose simulations. Such high spatial resolutions at minimum are necessary for an accurate measurement of the dose in the sharp Bragg-peak for monoenergetic ion beams. We demonstrate that at higher pixel size the dose levels may be underestimated due to spatial averaging in MRI-based polymer gel dosimetry.

Revealing the impact of radiation-induced refractive index changes in polymer gel dosimeters

This study examines effects of radiation-induced refractive index (RI) changes in polymer gel dosimeters. A prototype fan-beam optical computed tomography scanner was used to image a normoxic polymer gel dosimeter that was irradiated with two simple irradiation patterns–one single beam, one cross beam. A combed fan-beam was used for rayline tracing. Scans revealed that notable rayline errors occur when a steep, side-to-side dose gradient (i.e. RI gradient) is encountered. When the gradient occurs in the plane of the detector system, distinctive streaks in images are observed. When the gradient occurs perpendicular to the plane of the detector system, much more severe image errors are observed.

Characterization of the essential dosimetric properties of cosolvent-free polymer gel dosimeters: Recent progress in x-ray CT based normoxic polymer gel dosimetry

This work evaluates the temporal stability, spatial stability, batch reproducibility and dose rate dependence of a new cosolvent-free polymer gel dosimeter optimized for use with x-ray computed tomography readout. Temporal and spatial stability investigations reveal the new gel formulation should be imaged between 15-36 hours after irradiation. Intra- and inter-batch reproducibility were found to be excellent over the entire range of doses examined. A dose rate dependence was found for gels irradiated with machine dose rates between 100-600MU/min. An intensity-modulated radiation therapy treatment validation is also presented to illustrate an example clinical application using the new gel formulation.

Application of MAGAT polymer gel dosimetry in breast balloon

The purpose of this study is to characterize the response of MAGAT normoxic polymer gel for breast brachytherapy applications using two balloon applicators (MammoSite® and Contoura®) and verify the dose distribution with a commercial treatment planning system (BrachyVision® version 8.9.15). We present the fabrication, irradiation and readout of the gel used for the work described herein.

Recent developments with a prototype fan-beam optical CT scanner

The latest design of a prototype fan-beam optical computed tomography scanner is presented. A new beam creation system consists of a 635 nm laser diode module with variable, DC voltage-controlled beam intensity. A change in scanner alignment allows for the elimination of ring artefacts caused by data corruption that is spaced symmetrically across the detector array. These artefacts, as well as a pair of streaking artefacts caused by flask seams, are removed in sinogram space. A flask registration technique has been developed that allows for accurate, reproducible dosimeter placement. Protocol investigations with gel dosimeters have indicated the importance of: i) proper cooling techniques during gel manufacture, and ii) scanning the dosimeter while it is at room temperature. Latest reconstructions of a normoxic polymer gel dosimeter are presented as an indicator of current system performance.

Radiological properties of MAGIC normoxic polymer gel dosimetry

For a polymer gel dosimeter to be of use in radiation dosimetry, it should display water-equivalent radiological properties. In this study, the radiological properties of the MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) normoxic polymer gels were investigated. The mass density (ρ) was determined based on Archimedes' principle. The weight fraction of elemental composition and the effective atomic number (Zeff) were calculated. The electron density was also measured with 90° scattering angle at room temperature. The linear attenuation coefficient (μ) of unirradiated gel, irradiated gel, and water were determined using Am-241 based on narrow beam geometry. Monte Carlo simulation was used to calculate the depth doses response of MAGIC gel and water for 6MV photon beam. The weight fractions of elements composition of MAGIC gel were close to that for water. The mass density was found to be 1027 ± 2 kg m−3, which is also very close to mass density of muscle tissue (1030 kg m−3) and 2.7% higher than that of water. The electron density (ρe) and atomic number (Zeff) were found to be 3.43 × 1029 e m−3 and 7.105, respectively. The electron density measured was 2.6% greater than that for water. The atomic number was very close to that for water. The prepared MAGIC gel was found to be water equivalent based on the study of element composition, mass density, electron density and atomic number. The linear attenuation coefficient of unirradiated gel was very close to that of water. The μ of irradiated gel was found to be linear with dose 2-40 Gy. The depth dose response for MAGIC gel from a 6 MV photon beam had a percentage dose difference to water of less than 1%. Therefore it satisfies the criteria to be a good polymer gel dosimeter for radiotherapy.

MAGAT Gel Dosimetry Validation in RapidArc^TM Treatment Using Cone-beam CT

This study presents the three-dimensional (3D) measurement of the dose distribution delivered using the RapidArc^TM technique and MAGAT polymer gel with cone-beam CT (CBCT) as the dose reading tool. 3D verification of clinical prostate quality assurance (QA) plan was performed and analyzed. The MAGAT normoxic polymer gel consisted of methacrylic acid (monomer purity ＞ 98%), gelatin from procine skin (type A, Sigma Aldrich), and tetrakis (hydroxymethyl) phosphonium choloride (THPC) (80% solution in water). A linear accelerator (Varian) with CBCT was used for radiation delivered and CBCT as dose reader. Corresponding QA plans of RapidArc^TM plans of two prostate cancer cases were selected for dose comparison measurements. 3D gamma comparisons, dose profiles, and 3D volumes were used for evaluation. The dose response curve of the MAGAT polymer gel was acquired as x = 9.79y-13154 with R^2 = 0.9794, where y is the CT number and x is the radiation dose in cGy. The 3D gamma analysis was performed using comparisons of volumes in 90% and 80% isodose between treatment planning system calculation and gel phantom measurement for the prostate cases. In the 90% isodose covered volume, 96.3% of points were within 5%/5 mm (dose difference/distance to agreement) and 91.2% of points were within 3%/3 mm. In the 80% isodose volume, the pass rates were 98.7% and 96.1%, respectively. Excellent 3D dose measurement quality for the RapidArc^TM delivery technique was achieved with the MAGAT polymer gel and CBCT scan.

Effect of Gold Nanoparticle on Percentage Depth Dose of 18 MV X-Ray in MAGICA Polymer Gel Dosimeter

Radiation-sensitive polymer gels are among the most promising three-dimensional dose verification tools and tissue-like phantom developed to date. This study is an investigating of percentage depth dose enhancement within the gel medium with the use of conformal distribution gold nanoparticle as contrast agents by high atomic number material. In this work, the normoxic polymer gel dosimeter MAGICA tissue-equivalence was first theoretically verified using MCNPX Monte Carlo code and experimentally by percentage depth dose curves within the gel medium. Then gold nanoparticles (GNPs) of 50 nm diameter with different concentrations of 0.1 mM, 0.2 mM, and 0.4 mM were embedded in MAGICA gel and irradiated by 18 MV photon beam. Experimental results have shown dose increase of 10%, 2% and 4% in 0.1 mM, 0.2 mM and 0.4 mM concentrations, respectively. Simulation results had good agreement in the optimum concentration of 0.1 mM. The largest error between experimental and simulation results was equal to 9.28% stood for 0.4 mM concentration. The results showed that the optimum concentration of gold nanoparticles to achieve maximum absorbed dose in both experimental and simulation was 0.1 mM and so it can be used for clinical studies.

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.

Optical and NMR dose response of N-isopropylacrylamide normoxic polymer gel for radiation therapy dosimetry

BackgroundApplication of less toxic normoxic polymer gel of N-isopropyl acrylamide (NIPAM) for radiation therapy has been studied in recent years. AimIn the current study the optical and NMR properties of NIPAM were studied for radiation therapy dosimetry application. Materials and methodsNIPAM normoxic polymer gel was prepared and irradiated by 9MV photon beam of a medical linac. The optical absorbance was measured using a conventional laboratory spectrophotometer in different wavelengths ranging from 390 to 860nm. R2 measurements of NIPAM gels were performed using a 1.5T scanner and R2–dose curve was obtained. ResultsOur results showed R2 dose sensitivity of 0.193±0.01s−1Gy−1 for NIPAM gel. Both R2 and optical absorbance showed a linear relationship with dose from 1.5 to 11Gy for NIPAM gel dosimeter. Moreover, absorbance–dose response varied considerably with light wavelength and highest sensitivity was seen for the blue part of the spectrum. ConclusionOur results showed that both optical and NMR approaches have acceptable sensitivity and accuracy for dose determination with NIPAM gel. However, for optical reading of the gel, utilization of an optimum optical wavelength is recommended.

Fabrication of a Normoxic Polymer Gel Dosimeter and its Dose Distribution Characteristics

This study evaluated the performance of tissue equivalent polymer gel dosimetry as a tool for 3-dimensional dose quality assurance to using Magnetic Resonance Imaging (MRI) measure the dose characteristics of therapeutic radiation using and applying it to intensity modulated radiation therapy (IMRT). The polymer gel under normoxic conditions was synthesized using Gelatin, methyacrylic acid (MAA), hydroquinone (HQ) and tetrakis hydroxymethyl phosphonium chloride (THPC), and a polymer gel dosimeter was fabricated by placing the gel into each phantom appropriate for experimental use. The dose reaction experiment employed Co gamma-rays and a 6-MV photon beam with a dose range of 0 ∼ 14 Gy. The dose characteristics of the polymer gel were compared with those of an ion chamber, Gafchromic EBT2 film, by using MRI. The linear correlations coefficient for the polymer gel, the ion chamber and the film were r = 0.970, r = 0.999, and r = 0.972, respectively. The percentage depth doses at a depth of 50 mm were 85.8% and 84.6% for the polymer gel and the ion chamber, respectively, giving a difference of 1.2%. On the other hand, the percentage depth dose at the depth of 100 mm was 66.2% and 63.7% for the polymer gel and the ion chamber, respectively, indicating a 2.5% gap. The flatness and the symmetry of the gel dosimeter on the cross plane were 3.80% and 3.58%, respectively. The gel dosimeter was applied to an IMRT. The doses measured in the 100% dose domain of the therapeutic plan was 96.2%, 103.0%, and 100% for the axial, sagittal and coronal planes, respectively, highlighting the applicability of the gel dosimeter as a 3-dimensional dosimeter.

SU‐E‐T‐199: Three‐Dimensional Dose Verification Using Normoxic Polymer Gel Dosimeters for Tomotherapy

The aim of this study is to evaluate the feasibility of using MAGAT as a near real‐time 3‐dimensional dose measurement device for tomotherapy. MAGAT is a new type of normoxic polymer gel dosimeter, which responses well to absorbed dose and can be easily made in the presence of normal oxygen surroundings. Its dose response was measured by irradiating MAGAT‐gel‐filled testing vials with tomotherapy and its linear relationship with dose was present from 0 to 6.5 Gy. One group of gel samples were measured in near real‐time, in which the gel phantom was read right after the irradiation. The other group was measured 12 hours after irradiation so the gel phantom can be exposed to oxygen. Several post‐imaging processing filters including Nagao, Guess, median, mean, min and max, were applied on megavoltage computed tomography (MVCT) images for better discrimination on dose responses. Our results show that dose responses for MVCT with real‐time and 12‐hour delayed measurement were 4.76 and 4.69 ΔSI．cGy‐1, respectively, and show no significant difference (p‐value = 0.72). For study of the filtering effects, Gauss, median and mean filters offer better linear correction coefficients of dose response. In conclusion, the MAGAT polymer gel dosimeter read from MVCT imaging is a promising method for dose verification in clinical tomotherapy.

Severe dose inaccuracies caused by an oxygen-antioxidant imbalance in normoxic polymer gel dosimeters

Two oxygen scavengers have been successfully tested to produce normoxic polymer gel dosimeters under normal atmospheric conditions. The first is ascorbic acid and the second is a chloride (also sulfate) salt of tetrakis (hydroxymethyl) phosphonium. These antioxidants, added to the dosimeter during gel preparation, chemically remove dissolved oxygen that otherwise inhibits propagation of the polymerization reaction during irradiation of the dosimeter. These gel dosimeters are radiosensitive after manufacture under normoxic conditions. However, we show herein that the accuracy of the dosimetric measurement is compromised due to chemical reactions of the antioxidant with radicals. In addition, we provide evidence that both antioxidant and oxygen act as radical scavengers that affect the amount of polymer formed in the gel dosimeter. This can result in important dose inaccuracies in both methacrylic acid-based and acrylamide-based normoxic dosimeter gels.

Normoxic polymer gel dosimetry using less toxic monomer of N-isopropyl acrylamide and X-ray computed tomography for radiation therapy applications

Background Polymer gel dosimetry has been used extensively in radiation therapy for its capability in depicting a three dimensional view of absorbed dose distribution. However, more studies are required to find less toxic and more efficient polymers for application in radiotherapy dosimetry. Aim The purpose of this work was to evaluate the N-isopropyl acrylamide (NIPAM) gel dosimetric characteristics and optimize the protocol for X-ray computed tomography (CT) imaging of gel dosimeters for radiation therapy application. Material and methods A polymer gel dosimeter based on NIPAM monomer was prepared and irradiated with 60Co photons. The CT number changes following irradiation were extracted from CT images obtained with different sets of imaging parameters. Results The results showed the dose sensitivity of Δ N CT (H) = 0.282 ± 0.018 (H Gy −1) for NIPAM gel dosimeter. The optimized set of imaging exposure parameters was 120 kV p and 200 mA with the 10 mm slice thickness. Results of the depth dose measurement with gel dosimeter showed a great discrepancy with the actual depth dose data. Conclusion According to the current study, NIPAM-based gel dosimetry with X-ray CT imaging needs more technical development and formulation refinement to be used for radiation therapy application.

Impact of oxygen on the accuracy and precision of normoxic polymer gel dosimeters

Diffusion of oxygen in normoxic polymer gel dosimeters was studied. Gel samples in glass tubes were exposed to oxygen for different periods and then irradiated. Oxygen inhibited polymerization at the surface and polymerization occurred at larger depths. However the interface between these two regions revealed a local increase of polymerization. R2 values in these distinct regions are analyzed in terms of the concentration of oxygen and of antioxidant. Perhaps counter-intuitively, a small concentration of oxygen increases the sensitivity of the gel by consuming antioxidant molecules. The presence of antioxidant molecules is shown to cause a decrease of sensitivity, presumably through interactions with water free radicals and/or polymer radicals. Mixing and filling procedures as well as oxygen penetration from plastic caps or walls of gel containers are potential sources of oxygen-related discrepancies in normoxic polymer gel dosimeters. This effect may cause dosimetric inaccuracies of up to 60%.

Development of PDRESS (Patient Specific Dose Real Evaluation Systems) using a TENOMAG Gel and Optical CT (VISTA TM) in Clinical IMRT Prostate Case

The aims of this study, we present the preliminary results of 3 dimensional dose evaluation software (PDRESS, patient specific dose real evaluation systems). In this work, we compared planned 3D dose distribution with measured 3D dose distribution using a novel normoxic polymer gel dosimeter (TENOMAG) and a commercial cone-beam optical CT scanner (VISTATM, Modus Medical Devices, Inc., London, ON, Canada) to verify the 3D dose distribution in intensity-modulated radiation therapy (IMRT) prostate case. And we developed PDRESS using the Xelis Flatform which is developed by INFINITT Corporation is used to display the 3D dose distribution by loading the DICOM RT Data which is exported from RTP and optical-CT reconstructed VFF file. Data analysis is achieved by comparing the RTP data with the VFF data using profile, gamma map, and DTA. The profiles showed good agreement between RTP data, gel dosimeter, and gamma distribution and the precision of the dose distribution is within ± 5%. The results from this study show that there are no significantly discrepancies between the calculated dose distribution from treatment plan and the measured dose distribution from a TENOMAG gel scanned with an optical CT scanner. The 3D dose evaluation software (PDRESS) which is developed in this study evaluates the accuracy of the three dimensional dose distributions.