Normoxic Polymer Gel Dosimeter Research Articles

Investigating the Sensitivity of New Formulation MAGAT and NIPAM Polymer Gels in the Radiation Therapy Dosimetry.

Background: Normoxic polymer gels have been used as a three dimensional (3D) dosimeter in radiation therapy, recently. The sensitivity of these gels is important in dosimetry and their improvement can be also useful.Objective: In this study, different modalities of gel reading were used and the structure of gel changed due to the best improvement of sensitivity. The sensitivities of the new formulation of Methacrylic acid gel (MAGAT) and N-isopropyl acrylamide (NIPAM) polymer gel dosimeters were studied using two different reading methods of magnetic resonance imaging (MRI) and X-ray computed tomography (X-ray CT).Material and Methods: In this experimental study, in addition to making the NIPAM polymer gel dosimeter, a new formulation of normoxic polymer gel dosimeter, which named MAGAT gel, was investigated. The gels were irradiated with 6 MV in low doses, including1, 1.5, 1.75, 2 and 2.5 Gy. MRI and X-ray CT did the reading of gel dosimeters a day after irradiation using an elevated protocol.Results: The dose sensitivities of 0.92 HGy-1 and 0.47 HGy-1 were obtained for new MAGAT and NIPAM polymer gel dosimeters, respectively, based on the X-ray CT reading modality. The use of MRI reading modality and the dose sensitivities were 0.74 S-1Gy-1 and 0.27 S-1Gy-1 for new MAGAT and NIPAM polymer gel dosimeters, respectively.Conclusion: The new formulation of MAGAT polymer gel with a suitable protocol of gel reading has a better response.

Novel 3D polymer gel dosimeters based on N-(3-Methoxypropyl)acrylamide (NMPAGAT) for quality assurance in radiation oncology

New anoxic and normoxic polymer gel dosimeters containing different concentrations of N-(3-Methoxypropyl) acrylamide (NMPA) are introduced in this investigation for radiotherapy treatment planning dose validation. NMPA dosimeters were irradiated to 20 Gy using the 10 MV photon beam of a medical linear accelerator under a constant dose rate. The polymerization of NMPA dosimeters occurs and increases monotonically with increasing absorbed dose. Nuclear magnetic resonance (NMR) was used to investigate the degree of polymerization of gels. The dose response in terms of spin–spin relaxation rate (R2) of NMPA dosimeters increased with the addition of glycerol. The influence of the x-ray radiation energy, dose, and dose rate, as well as the stability of the irradiated NMPA gel dosimeters were studied in order to show this preliminary study is indicative of a promising low-toxicity polymer gel dosimeter.

Dose integration and dose rate characteristics of a NiPAM polymer gel MRI dosimeter system

The normoxic polymer gel dosimeter based on N-isopropyl acrylamide (NiPAM) is a promising full 3D-dosimeter with high spatial resolution and near tissue equivalency. NiPAM gel samples were irradiated to different doses using a linear accelerator. The absorbed dose was evaluated using MRI and statistical significance of the analysed data was calculated. The analysis was carried out using an in-house developed software. It was found that the gel dosimeter responded linearly to the absorbed dose. The gel exhibited a dose rate dependence, as well as a dependence on the sequential beam irradiation scheme. A higher dose rate, as well as a higher dose per sequential beam, resulted in a lower dose response.

New normoxic N-(Hydroxymethyl)acrylamide based polymer gel for 3D dosimetry in radiation therapy

A novel composition of normoxic polymer gel dosimeters based on radiation-induced polymerization of N-(Hydroxymethyl)acrylamide (NHMA) is introduced in this study for 3D dosimetry for Quality Assurance (QA) in radiation therapy. Dosimeters were irradiated by 6, 10 and 18MV photon beams of a medical linear accelerator at various dose rates to doses of up to 20Gy. The dose response of polymer gel dosimeters was studied using nuclear magnetic resonance (NMR) spin-spin relaxation rate (R2) of hydrogen protons within the water molecule. Also, we measured gel response using absorption spectroscopy and found that this novel gel can be successfully utilized for both MRI- and OCT- (Optical Computed Tomography) based 3D dosimetry. We investigated dosimetric properties of six different compositions of the new NHMA-based gel in terms of dose rate, radiation beam quality and stability of dose-dependent polymerization after irradiation. We found no significant effects of these parameters on the novel gel dosimeter performance in both relaxation rate and absorbance measurements.

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.

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.

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.

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.

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.

SU-E-T-197: Normoxic Polymer Gel Dosimetry for Intensity Modulated Radiation Therapy

Purpose: To evaluate a tissue equivalent normoxic polymergeldosimeter as a tool for 3‐dimensional dosequality assurance for intensity modulated radiation therapy(IMRT) by measuring the dosecharacteristics of therapeuticradiation by Magnetic Resonance Imaging(MRI). Methods: Polymergel under normoxic conditions was synthesized using gelatin, methyacrylic acid (MAA), hydroquinone (HQ) and tetrakis hydroxymethyl phosphonium chloride (THPC), and polymergeldosimeter was fabricated by placing it into each phantom for the appropriate experiments. Co‐60 gamma‐rays and a 6 MV photon beam were used for the dose reaction experiments. The dose range was 0∼20 Gy. The dosecharacteristics of the polymergeldosimeter were compared with those of the ion chamber and Gafchromic EBT2 film using 3.0T MRI. Results: The linear correlation coefficient was r2=0.970, r2=0.999 and r2=0.972 for the polymergeldosimeter,ion chamber and film, respectively. The percentage depth dose at a depth of 50 mm was 85.8% and 84.6% for the polymergeldosimeter and ion chamber, respectively. The flatness and symmetry of the geldosimeter on the cross‐plane were 3.80% and 3.58%, respectively. After applying the geldosimeter to IMRT, the dosages measured in the 100% dose domain of therapeutic planning for the axial, sagittal and coronal planes were 96.2%, 103.0% and 100%, respectively. Conclusions: The results show a relatively high level of precision in the high dose range in IMRT, which is important in clinic sites. The possibility of this as a 3‐dimensional dosequality assurance tool of a polymergeldosimeter was confirmed This work was supported by National Research Foundation of Korea Grant funded by the Korean Government.

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.

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.

Preliminary Dose Calibration Results Using a TENOMAG Polymer Gel Dosimeter and Optical CT (VISTATM)

We present a preliminary result of dose calibration for 3D dose evaluation using gel dosimetry in radiotherapy. In this work, we measured the percentage depth dose (PDD) and dose linearity using a normoxic polymer gel dosimeter (TENOMAG) and a commercial cone-beam optical CT scanner (VISTATM, Modus Medical Devices, Inc., London, ON, Canada). We confirmed that the PDD measured for 1 and 2 Gy had a build up region at 0.4 cm depth. However, the PDD measured for 3 and 4 Gy had not a build up region. In all cases, the PDD has not greatly decreased with depth above 4 cm depth. Moreover we confirmed that the optical density proportionally increased with dose at 1, 1.5 and 2 cm depth, while the response has not greatly increased with dose for other depths. In this study, we confirmed that we will be able to perform the dose calibration using optical density obtained from a TENOMAG gel data scanned with an optical CT scanner.

A novel application of normoxic polymer gel dosimeters for near real-time dose measurement using cone-beam computed tomography

The aim of this study is to evaluate the feasibility of using cone-beam CT system as a near real-time measurement device in dose estimation with normoxic polymer gel dosimetry (MAG). Each vial was filled with MAG gel and irradiated with uniform doses of 0–10 Gy to generate dose response curves. After irradiation, a cone-beam CT was used to perform the 3D dose measurement. In this study, two groups of gel samples were irradiated and measured in two ways for comparison: near real-time measurement, in which the gel phantom was read right after the irradiation, and delayed measurement, in which the measurement was performed 30 min, 4 h and 1 day after the irradiation for the gel phantom to be exposed to oxygen. All groups were also performed with and without a full bowtie filter to estimate the influence of a full bowtie filter to dose response. The linear dose response curves with and without a full bowtie filter for the four different CT imaging times were within a range 0.044–0.049Δ N CT cGy −1 and 0.061–0.063Δ N CT cGy −1, respectively, with no significant difference at different imaging times. Nevertheless, dose response curves with the full bowtie filter were higher than those without, with p-value <0.05 for all the different imaging times tested. Normoxic polymer gel dosimetry combined with cone-beam CT provides a useful method for near real-time dose measurement.

Dose Response Characteristics in Polymer Gel for the Composition Ratio

Although a gel dosimeter is not widely used, it has many potential merits for 3D dosimetry in the development of radiotherapy techniques. Because conventional gel dosimeters tend to sensitively polymerize with oxygen. A new polymer gel that solves these problems was introduced recently. The gel dosimeter contains anti-oxidants that eliminate oxygen chemically instead of using complicated facilities. The type and the concentration of the monomer affect the polymerization of the polymer gel dosimeter. Therefore, the polymerization of the normoxic polymer gel dosimeter is affected by anti-oxidant, the monomer, the constituent ratio, and the condition of the monomer, so many researchers have concentrated on the anti-oxidant, the type of monomer, and the constituent ratio. In this study, a normoxic polymer gel dosimeter was composed using anti-oxidants in a laboratory, and the dose response and sensitivity were measured. The results of this study are as follow the threshold R2 values were reduced, and the radio sensitivity was reduced with the increasing MAA ratio. Otherwise, the increase in the gelatin represented an increase and decrease in the threshold value of R2 values and the radio sensitivity, respectively. Therefore, in this study, 6 − 8 % MAA ratios and an 8 % gelatin ratio, considering the aging effect of the gel, were the optimal values.

Study on energy dependence of PAGAT polymer gel dosimeter evaluated using X-Ray CT

The normoxic polymer gel dosimeter evaluated with X-Ray computed tomography has emerged as a promising tool for measuring the dose delivered during radiotherapy in three dimensions. This study presents the dependence of PAGAT normoxic polymer gel sensitivity to different photon and electron energies. PAGAT polymer gel was prepared under normal atmospheric condition and irradiated with different photon energies of 1.25 MeV from Co-60 and 6 MV and 15 MV from linear accelerator and electron energies of 6, 9, 12, 15, 18 and 21 MeV from linear accelerator. Evaluation of dosimeter was performed with an X-Ray CT scanner. Images were acquired with optimum scanning protocols to reduce the signal-to-noise ratio. The averaged image was subtracted from the unirradiated polymer gel image for background. Central axis depth dose (PDD) curves obtained for each energy and polymer gel dosimeter measurements were in good agreement with diode and film measurements. Hounsfield (HU) – dose response curve for each photon and electron energy were derived from the PDD curve obtained from the gel dosimeter measurements. From the study it was clear that the HU-dose response curve was linear in the region 1–10 Gy. The dosimeter sensitivity was defined as a slope of these linear HU-dose response curves and found that the sensitivity of polymer gel decreases with increase in both photon and electron energies. This trend in dependence of PAGAT gel dosimeter sensitivity to different photon and electron energies was not dosimetrically significant. However, to evaluate the test phantom exposed with one energy using the calibration curve derived at another energy can produce clinically significant error.

Polymer gels for in-phantom dose imaging in radiotherapy

Normoxic polymer gel dosimeters are studied, with the aim of achieving a valid and advantageous method for in-phantom 3D dose determinations. Developments were carried out in the application of such dosimetric material to the method based on dosimeter gel layers that has shown good reliability for absorbed dose imaging in radiotherapy. The technique has been improved, in particular taking care of minimizing the oxygen infiltration into the gel matrix in order to suitably avoid its effect of inhibiting the polymerization process after exposure. A suitable choice of the material of dosimeter walls has brought to achieve good steadiness in time of dosimeter sensitivity and satisfactory results in dose imaging and depth-dose profiling.