Polymer Gel Dosimeters Research Articles

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.

Errors introduced by dose scaling for relative dosimetry

Some dosimeters require a relationship between detector signal and delivered dose. The relationship (characteristic curve or calibration equation) usually depends on the environment under which the dosimeters are manufactured or stored. To compensate for the difference in radiation response among different batches of dosimeters, the measured dose can be scaled by normalizing the measured dose to a specific dose. Such a procedure, often called “relative dosimetry”, allows us to skip the time‐consuming production of a calibration curve for each irradiation. In this study, the magnitudes of errors due to the dose scaling procedure were evaluated by using the characteristic curves of BANG3 polymer gel dosimeter, radiographic EDR2 films, and GAFCHROMIC EBT2 films. Several sets of calibration data were obtained for each type of dosimeters, and a calibration equation of one set of data was used to estimate doses of the other dosimeters from different batches. The scaled doses were then compared with expected doses, which were obtained by using the true calibration equation specific to each batch. In general, the magnitude of errors increased with increasing deviation of the dose scaling factor from unity. Also, the errors strongly depended on the difference in the shape of the true and reference calibration curves. For example, for the BANG3 polymer gel, of which the characteristic curve can be approximated with a linear equation, the error for a batch requiring a dose scaling factor of 0.87 was larger than the errors for other batches requiring smaller magnitudes of dose scaling, or scaling factors of 0.93 or 1.02. The characteristic curves of EDR2 and EBT2 films required nonlinear equations. With those dosimeters, errors larger than 5% were commonly observed in the dose ranges of below 50% and above 150% of the normalization dose. In conclusion, the dose scaling for relative dosimetry introduces large errors in the measured doses when a large dose scaling is applied, and this procedure should be applied with special care.PACS numbers: 87.56.Da, 06.20.Dk, 06.20.fb

On the output factor measurements of the CyberKnife iris collimator small fields: Experimental determination of the correction factors for microchamber and diode detectors

Purpose:To measure the output factors (OFs) of the small fields formed by the variable aperture collimator system (iris) of a CyberKnife (CK) robotic radiosurgery system, and determine the correction factors for a microchamber and four diode detectors.Methods:OF measurements were performed using a PTW PinPoint 31014 microchamber, four diode detectors (PTW‐60017, −60012, −60008, and the SunNuclear EDGE detector), TLD‐100 microcubes, alanine dosimeters, EBT films, and polymer gels for the 5 mm, 7.5 mm, 10 mm, 12.5 mm, and 15 mm iris collimators at 650 mm, 800 mm, and 1000 mm source to detector distance (SDD). The alanine OF measurements were corrected for volume averaging effects using the 3D dose distributions registered in polymer gel dosimeters. correction factors for the PinPoint microchamber and the diode dosimeters were calculated through comparison against corresponding polymer gel, EBT, alanine, and TLD results.Results:Experimental OF results are presented for the array of dosimetric systems used. The PinPoint microchamber was found to underestimate small field OFs, and a correction factor ranging from 1.127 ± 0.022 (for the 5 mm iris collimator) to 1.004 ± 0.010 (for the 15 mm iris collimator) was determined at the reference SDD of 800 mm. The PinPoint correction factor was also found to increase with decreasing SDD; values equal to 1.220 ± 0.028 and 1.077 ± 0.016 were obtained for the 5 mm iris collimator at 650 mm and 1000 mm SDD, respectively. On the contrary, diode detectors were found to overestimate small field OFs and a correction factor equal to 0.973 ± 0.006, 0.954 ± 0.006, 0.937 ± 0.007, and 0.964 ± 0.006 was measured for the PTW‐60017, −60012, −60008 and the EDGE diode detectors, respectively, for the 5 mm iris collimator at 800 mm SDD. The corresponding correction factors for the 15 mm iris collimator were found equal to 0.997 ± 0.010, 0.994 ± 0.009, 0.988 ± 0.010, and 0.986 ± 0.010, respectively. No correlation of the diode correction factors with SDD was observed.Conclusions:This work demonstrates an experimental procedure for the determination of the correction factors required to obtain small field OF results of increased accuracy.

SU-E-T-103: Three-Dimensional Measurements of Dose and LET from a Proton Beam via Polymer Gel Dosimetry.

To obtain and assess the accuracy of 3D dose distributions and LET maps from a passively-scattered proton beam through the use of LET-dependent and LET-independent polymer gel formulations. Dose measurements were performed with BANG3-Pro2 (MGS Research, Inc, Madison, CT) polymer gel dosimeters. The dosimeters were mixed from kits and were poured into 14.5 cm high by 15 cm diameter acrylic cylinders. For LET measurements, a new BANG3-Pro variant intended for use as an 'LET- meter' was employed. Initial irradiations were performed using a 200 MeV passively scattered proton beam with a 4 cm spread out Bragg peak (SOBP), delivering a physical dose of 3 Gy to the center of the SOBP. Dosimeters were read out using the OCTOPUS-IQ (MGS Research, Inc, Madison, CT) optical CT scanner. The optical densities measured in the gel were compared against ion chamber data to assess the accuracy of the dosimeter. Initial analysis indicates 80% of points measured along the central axis agreed with ion chamber data at the ±5%/±3mm level in the sensitive region of the gel. An artifact in the reconstructed image produced inaccurate readings beyond the depth of 50% dose on the distal edge of the SOBP, resulting in the majority of the agreement failures. BANG3-Pro2 polymer gel dosimeters demonstrate promise as a 3D dosimeter for use in proton therapy. The dosimetric data obtained to date will be used as the baseline measurement against which the LET-sensitive BANG3-Pro formulation will be compared for the measurement of proton LET.

SU-E-T-472: Characterization of the Very High Energy Electrons, ISO - 250 MeV (VHEE) Beam Generated by ALPHA-X Laser Wakefield Accelerator Beam Line for Utilization in Monte Carlo Simulation for Biomedical Experiment Planning.

Progress in the development of compact high-energy pulsed laser- plasma wakefield accelerators is opening up the potential for using Very High Energy Electron (VHEEs) beams in the range of 150 - 250 MeV for biomedical studies. Initial experiments using VHEE for this purpose have been carried out using the ALPHA-X laser-plasma wakefield accelerator beam line at the University of Strathclyde, Glasgow, UK. The purpose of this investigation is to use Monte Carlo simulations to plan experiments and compare with characterization of the interaction of the VHEE beam using a dosimeter. An experiment using the VHEE beam to irradiate a muscle-equivalent BANG polymer gel dosimeter has been carried out. Simulations have been used to prepare for the experiments. These were undertaken using the expected average energy for a pulse set and an energy spread approximated by Gaussian distribution. The model was implemented in FLUKA Monte Carlo code with follow up modeling using the Geant4 toolkit. The results have been compared with 1mm̂3 voxel laser CT based measurements of the dose deposited in the BANG dosimeter and with measurement of the induced radioactivity. The results of the measured dose from induced radioactivity have been compared with data from the FLUKA simulations. The beam model based on an average energy of particles in irradiation gives an acceptable estimate of the induced radioactivity and the dose deposited in the BANG dosimeter. Comparison with the dosimeter scanned profiles shows that the structure of the spectra of VHEE beams in the experiment and secondary scattered particles in the beam line should be accounted for in any model. Such model description of the VHEE beam for the ALPHA-X beam line has been developed. Monte Carlo simulations using the FLUKA code is an efficient way to plan a VHEE experiment and analyze data from measurements.

SU-E-T-150: The Basic Dosimetric Properties of NIPAM Polymer Gel Dosimeter.

To asses a series of basic dosimetric properties such as reproducibility, linearity, tissue equivalency, dose rate and energy independency for NIPAM polymer gel dosimeter. The NIPAM gel was manufactured according to the method, described by senden et al (2006). The gels were irradiated approximately 2 h after manufacturing and MR images of the gel were made 24 h after irradiation. Transverse relaxation rates (R2=T2?1 ) were obtained from the signal decay data using the proper data analyzer. In order to investigate the absorbed dose response reproducibility, the experiment was repeated three times using the same batch of monomer, irradiation method, scanning parameters and conditions, also with analyzing two set of the gel with different batches of chemical the effect of different batches were investigated . For assessing if the NIPAM gel dosimeter response is dependent on different photon energies, two sets of NIPAM gel were irradiated using a 9 MV linear accelerator and a 60co. The effect of different dose rate on gel response was studied in SSD of 80, 90, 100, 110 and 120 and radiation beam were calibrated to give 5Gy in each SSD. To investigate the linearity of the gel, the vials were irradiated from 1 to 35 Gy. In order to verify tissue equivalency, effective atomic number and relative electron density of NIPAM dosimeter were calculated using CT number, and compared with tissue. This polymer gel found to be tissue equivalent. The results showed that the dose response of NIPAM polymer gel is highly reproducible in same and different batches of chemical and its response was linear up to 26 Gy. Energy and dose rate had no effect on NIPAM gel response. NIPAM gel dosimeter appears to be a promising dosimeter in all aspects of dosimetric properties which were assessed in this study.

SU-E-T-99: Small Field Output Factor Measurement Using MAGIC Gel Dosimeter in 3T MRI.

Small field dosimetry is very important because of radiation therapy techniques that use small fields such as IMRT, gamma and body radiosurgery, cyberknife and tomotherapy. We investigated use of a MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) gel dosimeter to quantitatively measure small field output factors (OFs) for 6MV x rays. In this work, MAGIC (Gelatin 9%; Methacrylic acid 4%; CuSO4 0.1mM; Ascorbic ascid 2mM; Glucose 10%) gel phantoms were developed to measure the 6MV x-ray output factors for 1×1 up to 10×10 cm2 square fields (Varian 2100C linear accelerator). For comparison, 3 ion chambers (PTW:TN30013, Exradin A12, Capintec PR-05P), Gafchromic film (EBT2), and TLD (LiF-100) were used to measure the small field OFs under identical experimental conditions: 6cm depth (solid water), SAD=100cm, SSD = 94cm, 6MV, 512 MUs per irradiation. Relative OFs were normalized to a reference field (10×10 cm2 @ SAD =100cm). MAGIC gel dosimeters were scanned in a 3T GE signa® EXCITETM clinical scanner using a Spin Echo pulse sequence for dose distribution readout (pixel size = 0.4mm, slice thickness 3mm, TR = 4000ms, TE = 10ms and 110ms, respectively). Gel dose distributions were then calculated using custom Matlab code. 6MV x-ray OFs versus field size for all detectors were graphically compared. The MAGIC polymer gel dosimeter OF for a 1×1 cm2 field is 0.612 (+/- 5%), approximately 2% different from the OFs measured using small volume dosimeters (TLD, EBT2 film and the Capintec PR-05P ion chamber). Larger ion chamber (PTW:TN30013 and Exradin A12, both ∼ 0.6cc) OFs were low (OF = 0.26 for 1×1cm2 ) due to nonequilibrium and partial volume conditions. MAGIC gel dosimeter with 3T MRI scanner as a read-out makes it an ideal tool for small field dosimetry.

SU-E-J-56: Localization Error in Gamma Radiosurgery with 3T MR Due to Fiducial Box Chemical Shift from Incorrect Liquid Filling Materials.

Use of a 3D MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) polymer gel dosimeter to evaluate potential localization and positioning errors due to the use of incorrect liquid filling material (baby oil instead of copper sulfate) for a gamma radiosurgery (GSR) MRI fiducial box. Two phantoms were used to evaluate geometric errors due to chemical shift for 'baby oil' fiducial markers: 1) a cylindrical grid phantom with Teflon rod insert; 2) a human skull phantom with a MAGIC (Gelatin 9%; Methacrylic acid 4%; CuSO4 0.1mM; Ascorbic acid 2mM; Glucose 10%) gel capsule to simulate an intracranial target. Both phantoms were stereotactically MR and CT scanned for GSR treatment planning using a 'baby oil' GSR MRI fiducial box and a standard CT fiducial box. The gel capsule was given 15Gy @ 50% isodose line using the 4mmcollimator based on 'baby oil' fiducial box MR images. Then, the skull phantom with capsule was scanned in 3T GE Signa® EXCITETM clinical scanner (Spin Echo pulse sequence) for 3D dose distribution read-out. Dose distribution errors were analyzed based on gel results for the combined MR and CT plans. For comparison, deviation of grid phantom teflon rods were directly measured from MR and CT stereotactic images. Gel dose profile measurements indicate a 3.8mm shift in the phase encoding direction for the 'baby oil' fiducial marker for 3T MRI based GSR treatment planning. A 3.7mm shift in Teflon rod position was also confirmed through direct comparison of CT and MR images. Both results showsignificant target localization and positioning errors can be caused if inappropriate filling materials are used for GSR MRI fiducial boxes. Thisstudy also indicates that MAGIC polymer gel can be used as a versatile toolfor GSR 3D quality assurance.

Effect of inorganic salt on the dose sensitivity of polymer gel dosimeter

The effect of inorganic salts, non-transition metal chlorides, on the dose sensitivity of methacrylic-acid-based polymer gel dosimeter is investigated. Dose-R2 responses are obtained from magnetic resonance imaging data. Temperature increase due to exothermic polymerization reaction in the gel is also measured directly during irradiation. As a result, substantial increases in R2 response are observed in the polymer gel dosimeter containing inorganic salt, especially with MgCl2. The sensitivity of the gel with 1.0M MgCl2 is approximately 2.8 times higher than that of without MgCl2. As the salt concentration increases, an increase of polymerization rate is also observed via the temperature measurements. These results indicate that inorganic salt acts as an accelerator for radiation-induced free-radical polymerization in methacrylic-acid-based gel.

Evaluation of the Effects of Inhomogeneities on Dose Profiles Using Polymer Gel Dosimeter and Monte Carlo Simulation in Gamma Knife

Introduction Polymer gel dosimeters offer a practical solution to 3D dose verification for conventional radiotherapy as well as intensity-modulated and stereotactic radiotherapy. In this study, EGSnrc calculated and PAGAT polymer gel dosimeter measured dose profiles from single shot irradiation with 18 mm collimator of Gamma Knife in homogeneous and inhomogeneous phantoms were compared with each other. Materials and Methods The head phantom was a custom-built 16 cm diameter plexiglas sphere. Inside the phantom, there were two cubic cutouts for inserting the gel vials and inhomogeneities. Following irradiation with the Gamma Knife unit, the polymer gel dosimeters were scanned with a 1.5 T MRI scanner. For the purpose of simulation the simplified channel of 60Co source of Gamma Knife BEAMnrc and for extracting the 3D dose distribution in the phantom, DOSXYZnrc codes were used. Results Within high isodose levels (>80%), there are dose differences higher than 7%, especially between air inserted and PTFE inserted phantoms, which were obtained using both simulation and experiment. This means that these values exceed the acceptance criterion of conformal radiotherapy and stereotactic radiosurgery (i.e., within some isodose levels, less than 93% of prescription dose are delivered to the target). Conclusion The discrepancies observed between the results obtained from heterogeneous and homogeneous phantoms suggest that Leksell Gamma Knife planning system (LGP) predictions which assume the target as a homogeneous material must be corrected in order to take care of the air- and bone-tissue inhomogeneities.

An experimental investigation into the effect of periodic motion on proton dosimetry using polymer gel dosimeters and a programmable motion platform

Organ motion in proton therapy affects treatment dose distribution during both double-scattering (DS) and uniform-scanning (US) deliveries. We investigated the dosimetric impact of target motion using three-dimensional polymer gel dosimeters and a programmable motion platform. A simple one-beam treatment plan with 16 cm range and 6 cm modulation was generated from the treatment planning system (TPS) in both the DS and US modes. One gel dosimeter was irradiated with a stationary DS beam. Two other gel dosimeters were irradiated with the DS and US beams while they moved in the same sinusoidal motion profile using a programmable motion platform. The dose distribution of the stationary DS delivery agreed with the TPS plan. Dosimetric comparisons between DS motion delivery and the MATLAB-based motion model showed insignificant differences. Dose–volume histograms of a cylindrical target volume inside the gel dosimeters showed target coverage degradation caused by motion. A three-dimensional gamma index calculation (3% and 3 mm) confirmed different dosimetric impacts from DS and US with the same target motion. This polymer-gel-dosimeter-based study confirmed the dosimetric impact of intrafraction target motion and its interplay with temporal delivery of different energy layers in US proton treatments.

Verification of the 3D dose distribution in spinal radiosurgery by using a BANG3® polymer gel dosimeter

In intensity-modulated radiosurgery (IMRS) treatment, radiation delivery techniques require the ability to accurately verify complex three-dimensional (3D) dose distributions. This study was designed to evaluate and verify dosimetry generated from gels, films, and treatment planning systems. In this study, commercially available BANG3® polymer gel was used to confirm the accuracy of the treatment plan and to assess the dosimetric uncertainty of the radiosurgery procedure. BANG3® gels that are read with R2 magnetic resonance (MR) imaging mapping are useful options. The gel is a tissue equivalent, and the relaxation ratio measured using MR imaging is proportional to the dose absorbed in the gel. A cylindrical container (5″ deep, 7″ high) filled with BANG3® gel was mounted in a cubic phantom (The EASY CUBEr, Euromechanics, Schwarzenbruck, Germany). We then carried out the same process using the gel and gafchromic film as would be used for a patient with metastatic T-spine cancer by using a Novalis Radiosurgery system (Brain LAB Inc., Germany). Our experimental results provided the dose distribution and the radiation delivery precision. Comparisons of the measured and the calculated relative dose distributions showed good agreement in the high-dose region with differences of 2 mm. BANG3® polymer gel dosimetry can be useful for the verification of clinical treatment radiosurgery plans.

Simulation and experimental verificationof dose distributions of electron beams

Accurate dose calculations are very important in radiotherapy treatment planning. The Monte Carlo method has proven to be an accurate and reliable method for simulation of the dose distributions from electron beams and can provide theoretically more accurate distributions than conventional planning system algorithms (e.g. the Generalized Gaussian Pencil Beam algorithm) for more complex target configurations. The aim of this work was to assess the parameters and spectrum of a given electron beam (Varian CLINAC 2100 C/D) from experimental depth dose distribution and then simulate dose distributions in an experimental arrangement with a 3D polymer gel dosimeter. The comparison of data from experimental measurements, Monte Carlo calculations and the planning system enabled us to verify experimentally the simulation model and technique as well as the planning system results. Both simple and more complex systems (e.g. dose distributions close to different material interfaces) can be studied. The generation and influence of the “bremsstrahlung” photons in the electron beams was also analyzed using the simulation technique. The computations, experimental methods used and results obtained are presented and discussed.

Spatial correlation of proton irradiation‐induced activity and dose in polymer gel phantoms for PET/CT delivery verification studies

This work demonstrates a novel application of BANG3-Pro2 polymer gel dosimeter as a dosimetric phantom able to accurately capture both dose and induced activity. BANG3-Pro2 dosimeters were irradiated with a clinical proton beam using an unmodulated beam and a spread-out Bragg peak (SOBP) modulation, the latter with a Lucite compensator to introduce a range offset in one quadrant of the circular field. The dosimeters were imaged in a nearby positron emission tomography/computed tomography (PET/CT) unit starting within 5 min of beam-off. Induced positron emission (PE) activity along the central axis of the beam was compared to analytical calculations. Dose distributions were read out using an optical CT scanner and were validated against ion chamber measurements and the treatment plan. The offset between the distal fall-off of dose and activity (50% level) was determined over the entire irradiated field. Lateral profiles of PE were correlated to measured dose for the unmodulated beam delivery. Measured profiles of PE activity along the central beam axis were found to be within 10% of the predictions of analytical calculations. The depth-dose profiles agreed with the reference values (ion chamber or treatment plan) within 3%. The offset between the depth profiles of dose and activity for the unmodulated beam was 8.4 ± 1.4 mm. For the compensator-based SOBP delivery, the distribution of offsets throughout the field was found to be bimodal, with the mean of 8.9 ± 2.8 mm for the thinner region of the compensator and 4.3 ± 2.5 mm for the thicker region. For the pristine beam delivery, lateral profiles of dose and activity were found to exhibit fair spatial correlation throughout the beam range, with the mean 2D gamma index of 0.42 and 91% of the evaluated pixels passing the test. This work presents the first demonstration of simultaneous and accurate experimental measurement of three-dimensional distributions of dose and induced activity and lays the groundwork for further investigations using BANG3-Pro2 as a dosimetric phantom in PET/CT delivery verification studies.

Monte Carlo verification of polymer gel dosimetry applied to radionuclide therapy: a phantom study

This study evaluates the dosimetric performance of the polymer gel dosimeter ‘Methacrylic and Ascorbic acid in Gelatin, initiated by Copper’ and its suitability for quality assurance and analysis of I-131-targeted radionuclide therapy dosimetry. Four batches of gel were manufactured in-house and sets of calibration vials and phantoms were created containing different concentrations of I-131-doped gel. Multiple dose measurements were made up to 700 h post preparation and compared to equivalent Monte Carlo simulations. In addition to uniformly filled phantoms the cross-dose distribution from a hot insert to a surrounding phantom was measured. In this example comparisons were made with both Monte Carlo and a clinical scintigraphic dosimetry method. Dose-response curves generated from the calibration data followed a sigmoid function. The gels appeared to be stable over many weeks of internal irradiation with a delay in gel response observed at 29 h post preparation. This was attributed to chemical inhibitors and slow reaction rates of long-chain radical species. For this reason, phantom measurements were only made after 190 h of irradiation. For uniformly filled phantoms of I-131 the accuracy of dose measurements agreed to within 10% when compared to Monte Carlo simulations. A radial cross-dose distribution measured using the gel dosimeter compared well to that calculated with Monte Carlo. Small inhomogeneities were observed in the dosimeter attributed to non-uniform mixing of monomer during preparation. However, they were not detrimental to this study where the quantitative accuracy and spatial resolution of polymer gel dosimetry were far superior to that calculated using scintigraphy. The difference between Monte Carlo and gel measurements was of the order of a few cGy, whilst with the scintigraphic method differences of up to 8 Gy were observed. A manipulation technique is also presented which allows 3D scintigraphic dosimetry measurements to be compared to polymer gel dosimetry measurements without generating misleading errors due to the limited spatial resolution.

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

Dosimetric Evaluation of Linac Photon Small Fields using MAGIC Polymer Gels

Introduction: In radiotherapy, methods of treatment planning are becoming increasingly more complicated. This requires verification of the doses delivered to increasingly smaller and more precise regions. Radiotherapy techniques are continuously employing smaller and smaller field sizes to deliver tighter radiation doses with higher therapeutic ratios, generating interest among researchers to provide reliable dosimetry for beams and treatment plans collimated to small field sizes. In this study, the dosimetry of these fields was evaluated in clinical applications by using polymer gel dosimetry. Material and Methods: The MAGIC polymer gel was used in this study. The gel samples were manufactured and poured into phantoms and calibration vials and were irradiated with a 6 MV x-ray beam. The R2 maps of the dose distributions were obtained from the gel MR images. The depth dose distributions and dose profile measurements were measured in different fields at a depth of 5 cm in gel, and were compared against another technique using a pinpoint chamber. Results: Comparison of the results of gel and pinpoint chamber measurements showed largest differences between the dose profile measurements in the low dose regions (near the edges). In these regions, the pinpoint-chamber measured penumbra width was at most 3.2 mm wider than those given by gel dosimeters. For a 30 × 30 mm2 field, maximum difference between gel and pinpoint chamber was 2 mm within the depth of maximum dose region, and were 2 mm, 3 mm and 2 mm for 20 × 20 mm2, 10 × 10 mm2 and 5 × 5 mm2 fields, respectively. The maximum differences within the buildup region for 30 × 30 mm2, 20 × 20 mm2, 10 × 10 mm2 and 5 × 5 mm2 fields were 9%, 4.5%, 2% and 9%, respectively. Discussion and Conclusion: The differences in the depth dose distributions and dose profile measurements between MAGIC polymer gel and pinpoint chamber are attributable to different factors. The dosimetry of these fields using ionization chambers has limitations regarding uncertainty, due to the relatively large sizes of ionization chambers with respect to small field sizes and the lack of charged-particle equilibrium in such small fields. Pinpoint chambers are too large for very small photon fields and smaller dosimeters should be used. Fewer limitations in the dosimetry of small fields exist when using polymer gel dosimeters. Hence polymer gel dosimeters are suitable for dosimetry of photon small fields.

Investigating potential physicochemical errors in polymer gel dosimeters

Measurement errors in polymer gel dosimetry can originate either during irradiation or scanning. One concern related to the exothermic nature of polymerization reaction was that the heat released in polymer gel dosimeters during irradiation modifies their dose response. In this paper, the effect of heat released from the exothermal polymerization reaction on the dose response of a number of dosimeters was studied. In addition, we investigated whether heat-generated geometric distortion existed in newly proposed gel dosimeters that contain highly thermoresponsive polymers. Our results suggest that despite a significant internal temperature increase in some gel compositions, their dose responses are not affected when oxygen is well expelled mechanically from the gel mixture. We also report on significant pre-irradiation instability in some recently developed polymer gel dosimeters but that geometric distortions were not observed. Data obtained by a set of small calibration vials are compared to those obtained from larger phantoms, and potential physicochemical causes of deviations between them are identified.

How do monomeric components of a polymer gel dosimeter respond to ionising radiation: A steady-state radiolysis towards preparation of a 3D polymer gel dosimeter

Ionising radiation-induced reactions of aqueous single monomer solutions and mixtures of poly(ethylene glycol) diacrylate (PEGDA) and N, N′-methylenebisacrylamide (Bis) in a steady-state condition are presented below and above gelation doses in order to highlight reactions in irradiated 3D polymer gel dosimeters, which are assigned for radiotherapy dosimetry. Both monomers are shown to undergo radical polymerisation and cross-linking, which result in the measured increase in molecular weight and radius of gyration of the formed polydisperse polymer coils. The formation of nanogels was also observed for Bis solutions at a low concentration. In the case of PEGDA-Bis mixtures, co-polymerisation is suggested as well. At a sufficiently high radiation dose, the formation of a polymer network was observed for both monomers and their mixture. For this reason a sol–gel analysis for PEGDA and Bis was performed gravimetrically and a proposition of an alternative to this method employing a nuclear magnetic resonance technique is made. The two monomers were used for preparation of 3D polymer gel dosimeters having the acronyms PABIG and PABIG nx. The latter is presented for the first time in this work and is a type of the formerly established PABIG polymer gel dosimeter. The elementary characteristics of the new composition are presented, underlining the ease of its preparation, low dose threshold, and slightly increased sensitivity but lower quasi-linear range of dose response in comparison to PABIG.

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.