Polymer Gel Dosimeters Research Articles

Features of PABIGnx 3D Polymer Gel as an Ionising Radiation Dosimeter.

This work presents the features of the PABIGnx 3D polymer gel dosimeter. It consists of two cross-linkers: poly(ethylene glycol) diacrylate (PEGDA), as one biacrylic component, and N,N′-methylenebisacrylamide (MBA), which is another cross-linker often used in 3D dosimeters. Additionally, it contains oxygen scavenges of copper sulfate pentahydrate and ascorbic acid. All ingredients are embedded in a physical gel matrix of gelatine. Upon irradiation, the biacrylic cross-linking agents (PEGDA and MBA) undergo radical polymerisation and cross-linking, which is manifested by the appearance of the opacity of the intensity related to the absorbed dose. PABIGnx was irradiated with an oncological source of ionising radiation, and analysed by using a nuclear magnetic resonance (0.5 T). The following characteristics were obtained: (i) linear and dynamic dose-response of 0.5 to ~18 Gy and 40 Gy, respectively, (ii) dose sensitivity of 0.071 ± 0.001 Gy−1 s−1, (iii) integral 3D dose distribution for at least 24 days after irradiation, (iv) adequate batch-to-batch reproducibility, (v) dose-response independent of irradiation with 6 MV photons, 15 MV photons, 6 MV photons FFF of 0.0168–0.1094 Gy/s dose rates, and (vi) soft tissue equivalence. The study showed that the features of PABIGnx confirm its suitability for use in 3D radiotherapy dosimetry.

Induced Radionuclides and Their Activity Concentration in Gel Dosimeters Irradiated by Carbon Ion Beam.

Radioactivity was measured in a micellar gel dosimeter, a polymer gel dosimeter, and water was irradiated by carbon ion beams at various beam energy conditions. Monte Carlo simulation was also performed to estimate the radioactivity. Short-lived positron-emitting nuclides were observed immediately after irradiation, but they decayed rapidly into the background. At 24 h post-irradiation, the dominant measured radioactivity was of 7Be. The simulation also showed minor activity of 24Na and 3H; however, they were not experimentally observed. The measured radioactivity was independent of the type of gel dosimeter under all irradiation conditions, suggesting that the radioactivity was induced by the interaction of carbon ions with water (the main component of the gel dosimeters). The ratio between the simulated and measured radioactivity was within 0.9–1.5. The activity concentration of 7Be was found to be less than 1/10 of the value derived using the exemption concept proposed by the International Atomic Energy Agency. This result should be applicable to irradiated gel dosimeters containing mainly water and 0–4 wt.% C and 0–1.7 wt.% N.

Evaluation of MAGIC-f polymer gel dosimeter for dose profile measurement in small fields and stereotactic irradiation

Background and purposeThe use of small fields in modern radiotherapy treatment techniques is increasingly growing, which has aroused the interest of researchers to use more polymer gel in small field dosimetry. Polymer gel dosimeters contain radiation-sensitive materials that are polymerized under irradiation as a function of absorption dose, and the spatial information of the radiation dose distribution is extracted by imaging of polymer gel. In this study, the performance and capability of MAGIC-f polymer gel dosimeter were evaluated for small field dosimetry and the temporal stability was investigated. MethodsMAGIC-f polymer gel was manufactured and it was poured into phantoms and calibration vials, and then phantoms were irradiated with 6 MV photons with small fields. The values of R2 maps and dose distribution in irradiated gel phantoms were obtained using MRI images taken from the gel phantoms at 1 day and 14 days after irradiation. Also, EBT3 film was used for the measurement of dose profiles at the same field shapes. The transverse dose profiles measured with polymer gel dosimeter were compared with the results of EBT3 film. ResultsThe slope of fitted lines on dose-response data at 1 day and 14 days after irradiation were equal to 0.6005 and 0.6147 respectively. By comparing the results of MAGIC-f gel and EBT3 film, the relative difference in FWHM for profile curves was observed 12.1%, 6.1%, and 4.3% for 0.5, 1.0, and 1.5 cm field size respectively. The gamma index pass rates between dose profile of polymer gel and film with criteria of 1%/0.5 mm were more than 77.1%. In the case of temporal stability, the penumbra measured by MAGIC-f at 14 days after irradiation was almost equal to that at 1 day after irradiation but in terms of FWHM, it was greater at 14 days. ConclusionThe results of this study show that the MAGIC-f polymer gel has good reliability and stability characteristics for small field dosimetry but it must be benchmarked against other dosimeters to become a three-dimensional dosimeter in small field dosimetry.

Improved Dose Response of N-(hydroxymethyl)acrylamide Gel Dosimeter with Calcium Chloride for Radiotherapy.

The impact of calcium chloride (CaCl2) on the performance of N-(hydroxymethyl)acrylamide (NHMA) polymer gel dosimeter is studied in this article. The dosimeter was exposed to doses of up to 10 Gy with radiation beam-energy of 10 MV and dose-rates of 300 cGy/min. The relaxation rate (R2) parameter was utilized to explore the performance of irradiated NHMAGAT gels. The dose response in terms of R2 increased from 0.29 to 0.63 Gy−1·s−1 with increasing calcium chloride concentration from 0 to 1000 mM. The results show no substantial impact of dose-rates as well as radiation energies on NHMAGAT samples. For the steadiness of irradiated NHMAGAT dosimeters, it was found that there is no apparent variation in R2 (less than ±3%; standard deviation) up to 3 days. The overall uncertainty of the gel dosimeter with calcium chloride is 4.96% (double standard deviation, 95% confidence level).

Improvement on the performance N-(3-methoxypropyl)acrylamide polymer-gel dosimeter by the addition of inorganic salt for application in radiotherapy dosimetry

Improvement on the performance N-(3-methoxypropyl)acrylamide polymer-gel dosimeter by the addition of inorganic salt for application in radiotherapy dosimetry

Characterization of Improved PASSAG Polymer Gel Dosimeter Using Magnetic Resonance Imaging

Characterization of Improved PASSAG Polymer Gel Dosimeter Using Magnetic Resonance Imaging

Real time 4D Radiation Gel Dosimetry on the Australian MRI-Linac

4D radiation dosimetry using a highly radiation-sensitive polymer gel dosimeter with real-time quantitative MRI readout is presented as a technique to acquire the accumulated radiation dose distribution during image guided radiotherapy (IGRT) on an MRI-Linac. Optimized T2 weighted TSE scans are converted into quantitative ΔR2 maps and subsequently to radiation dose maps. The potential of real-time 4D radiation dosimetry in a theragnostic MRI-Linac is demonstrated in test tubes, for a square beam in a cylindrical gel phantom, for a simple step-and-shoot irradiation in a head phantom and a dynamic arc treatment on a cylindrical gel phantom using a rotating couch. The optimal sequence parameters for maximal dose resolution in the dynamic MRI acquisition will be presented and the trade off between MRI scanning speed and dose resolution will be discussed. A further improvement in temporal resolution using a keyhole imaging approach is the focus of future research.

OD34 - An innovative polymer gel dosimeter in a 3D printed head phantom for stereotactic radiosurgery dose verification: a feasibility study

OD34 - An innovative polymer gel dosimeter in a 3D printed head phantom for stereotactic radiosurgery dose verification: a feasibility study

Stopping power and CSDA range calculations of electrons and positrons over the 20 eV–1 GeV energy range in some water equivalent polymer gel dosimeters

In this study, the stopping power, CSDA range and radiation yield calculations of electrons and positrons over the 20 eV–1 GeV energy range in some water equivalent polymer gel dosimeters were performed. For collision stopping power calculations of electrons and positrons the effective charge concept proposed by Sugiyama were considered. Here, both the effective charge of incident electrons and positrons (z*) and the effective charge (Z*) and the effective mean excitation energy (I*) of the target material were calculated. For the density effect correction the Fano model was selected. For the radiative stopping power analytical model based on the ratio between the collision and the radiative stopping power discribed by Attix was considered. For CSDA range and radition yield the continuous slowing down approximation (CSDA) was considered and the calculations were performed using numerical integral methods. Because of their water equivalent and 3D dose distribution properties, MAGIC and MAGAS polymer gels were selected as a target materials. The calculations were performed by programming all the equations discribed in this study as a computational code. The results of the stopping power, range and radiation yield were compared with those of ESTAR program and PENELOPE Monte Carlo modelling. Some deviations in low and high energy region between the calculated and reference data were observed. However, the similarity between calculated and reference data is remarkable. For the collision stopping power and CSDA range a good agreement between the calculated and reference data was observed for energies >1 keV. Whereas, for the radiative stopping power a good agreement was observed for energies >100 keV.

Effect of lithium chloride inorganic salt on the performance of N-(Hydroxymethyl)acrylamide polymer-gel dosimeter in radiation therapy

Effect of lithium chloride inorganic salt on the performance of N-(Hydroxymethyl)acrylamide polymer-gel dosimeter in radiation therapy

The impact of various amounts of fabricating components on the response of PASSAG polymer gel dosimeter: An optimization study

Polymer gel dosimeters are used to measure three-dimensional dose distribution with high spatial resolution. Recently, a novel polymer gel dosimeter, known as PASSAG, has been introduced in which has several advantages over other polymer gel dosimeters; however, its response-dose sensitivity is relatively low. It has been shown that the sensitivity of polymer gel dosimeters can be changed by various amounts of components used in their fabrication. This study aims to assess the dosimetric characteristics of PASSAG gel dosimeter for different amounts of components in its formulation. For this purpose, different formulations of PASSAG gel dosimeter were first fabricated, and then they were irradiated using 6 MV X-rays in a 0–10 Gy dose range. After the irradiation process, the responses (R2) of gel dosimeters were read out by a 1.5 T MRI scanner during one-month post irradiation time. According to the results, the maximum R2-dose sensitivity belonged to PASSAG gel dosimeter with 5% gelatin, 4% AMPS sodium salt (as monomer), and 4% Bis (as crosslinker) that showed a 31% increased sensitivity compared to conventional PASSAG gel dosimeter (5% gelatin, 3% monomer, and 3% crosslinker). Moreover, an excellent linear R2-dose response was observed for optimized PASSAG gel dosimeter. The dose resolution values of optimized PASSAG gel dosimeter also varied between 0.14 and 0.42 Gy in a 0–10 Gy dose range. Other findings revealed that the R2 values of the optimized gel dosimeter had a relative stability during 15–18 °C and 20–24 °C scanning temperatures. Furthermore, a temporal stability in the R2 values of the optimized gel dosimeter was observed during 15–30 days post irradiation time. The R2-dose sensitivity of optimized gel dosimeter also varied during 30 days post-irradiation time (from 31.00% to 47.97%).

Laser Light Trapping Phenomenon in a 3D Radiotherapy Polymer Gel Dosimeter.

This paper aims to explain the phenomenon of laser light trapping (LLT) in a 3D polymer gel dosimeter. A VIC-T polymer gel dosimeter containing 17% N-vinylpyrrolidone, 8% N,N′-methylenebisacrylamide, 12% tert-butyl alcohol, 5% gelatine, 0.02% hydroquinone and 14 mM tetrakis(hydroxymethyl)phosphonium chloride was used in this study. It was exposed to green laser light with a wavelength of 532 nm. A film was recorded during the exposure. After exposure, Raman spectroscopy was used to study the reactions taking place inside the dosimeter. The obtained results were used to explain what the LLT phenomenon is, what are the consequences for the dosimeter in which such a phenomenon occurs, and what dosimeter components play an important role in the occurrence of LLT. In addition, the conditions under which 3D polymer gel dosimeters can be measured using optical computed tomography at short wavelengths of visible laser light are indicated.

The Effect of N, Nʹ-Methylene-bis-acrylamide Removal from a Polymer Gel Dosimeter Formulation: Spin–Spin Relaxation Rate Investigation and Post-irradiation Time Instability

The Effect of N, Nʹ-Methylene-bis-acrylamide Removal from a Polymer Gel Dosimeter Formulation: Spin–Spin Relaxation Rate Investigation and Post-irradiation Time Instability

Experimental characterization and Monte Carlo simulations of the dose enhancement on the millimeter scale of PAGAT infused with gadolinium

This study reports on the determination of the K-edge dose enhancement factor achieved in a polymer gel dosimeter based on acrylamide, known as PAGAT, infused with a high concentration of a clinical gadolinium-based compound. PAGAT dosimeters infused with gadolinium concentrations of 10 mg/mL and 20 mg/mL are characterized in terms of dose-response and sensitivity, obtaining the dose enhancement between two beam qualities for these Gd concentrations of clinical interest. Additionally, the dose enhancement ratio between these two concentrations was evaluated for each beam quality. To this aim, Gadodiamide solution, a Gd-based agent commonly used in clinics for magnetic resonance scanning, is used to manufacture Gd-infused dosimeters that are further irradiated with a 50 kVp X-ray beam in order to assess dose enhancement contributions exclusively from absorption from lower edges (L, M, etc.); subsequently, a 160 kVp spectrum optimized by yttrium and indium filtration is used to evaluate K-edge contributions. Gel dosimeter samples are prepared in spectrophotometric vials, and their readout is performed by optical density measurements, which is correlated with the absorbed dose. An appreciable increase was found in the K-edge dose enhancement factor (160kVp/50 kVp) for the samples with concentrations of 20 mg/mL and 10 mg/mL ranging from 1.20 to 1.61 and 1.10–1.41, respectively. These results could be interpreted as the contribution to the absorbed dose due the increment in the secondary radiation emitted from Gd atoms by K-edge interactions. Similarly, a relevant increase was observed in the dose enhancement when the Gd concentration is incremented from 10 mg/mL to 20 mg/mL obtaining factors of around 1.10–1.33 and 1.34–1.47 for the beam qualities 50 kVp and the optimized 160 kVp, respectively. Monte Carlo simulations using the PENELOPE main code are implemented aimed at complementing and comparing with the experimental results, showing differences of less than 4% for the K-edge dose enhancement factor and differences of less than 3% for the dose enhancement ratio for the studied beam qualities. In this regard, the results from the simulations support the reliability of the experimental measurements with this type of dosimeters within the studied concentration range. Finally, it is noticeable that the appreciable dose enhancement reported through this work was obtained infusing mass Gd as part of the Gadodiamide compound, already used as a contrast agent for magnetic resonance imaging, thus ensuring its suitability for clinical practices in humans.

Polymer gel containing N,N′-methylene-bis-acrylamide (BIS) as a single monomer for radiotherapy dosimetry

In this study, the first polymer gel that contains N,N′-methylene-bis-acrylamide (BIS) crosslinker without the need of adding another radiosensitive monomer has been introduced as a new low toxic polymer gel dosimeter. Various concentrations (0–0.9 M) of magnesium chloride (MgCl2) were added to BIS gel to improve the dose-response of the gel dosimeter. The gel samples were irradiated in a water phantom to different doses using 6 MV Linac X-ray beams with 200 cGy/min dose rate. The samples were read out using nuclear magnetic resonance (NMR) spectroscopy to measure the relaxation rates (R2) of the scanned samples by a standard Multi-Spin-Echo sequence. The optimum formulation gels were irradiated to different dose-rates (200 and 500 cGy/min) with a fixed radiation-energy of 6 MV, while other samples were irradiated to different radiation-energy (6 and15 MV) with a fixed dose rate of 200 cGy/min to investigate the influence of these important parameters on the performance of gel dosimeters. The dose sensitivity of the composition containing BIS, gelatin and antioxidant agent (BISGAT) polymer gel was very small (0.07 Gy−1s−1) compared to the available normoxic polymer gels such as NIMPAGAT. A significant increase in dose sensitivity (0.33 Gy−1s−1) of BISGAT dosimeter was achieved after adding 0.9 M MgCl2 to the gel which is greater than the standard composition of NIPAMGAT (i.e. BIS and NIPAM monomer) polymer gel with dose sensitivity of 0.16 Gy−1 s−1.The temporal stability of irradiated samples to different doses changed insignificantly (0.07%) in the period up to 6 days. The BISGAT gel with MgCl2 was independent of dose-rate (200 and 500 cGy/min) as well as radiation energy (6 and 15 MV). Additionally, the gel is water equivalent and has theoretical energy-independent response from 0.1 to 20 MeV for the optimum composition. The gel dosimetry accuracy was evaluated in this study by calculating the overall uncertainty and found to be 7.04% (2σ, 95% confidence level).

Performance of a new commercial high-definition 3D patient specific quality assurance system for CyberKnife robotic radiotherapy and radiosurgery

Conventional two dimensional and low-definition measurement techniques for dosimetric verification of radiotherapy treatment deliveries are no longer adequate in the era of hypofractionation and extremely high dose gradients. New quality assurance (QA) tools with 3D capability and high definition are urgently needed. The purpose of this study was to evaluate the performance of one of the first such commercial systems as applied to CyberKnife (CK) radiotherapy/radiosurgery system.This study employed the recently developed commercial 3D patient-specific quality assurance (PSQA) system. Tissue-equivalent radiochromic polymer gel dosimeters, sealed in light protective thin-wall glass spheres (CrystalBalls), were mounted in a high-precision acrylic QA stand. Patient treatment plans were superimposed on the simulator CT scan of one selected CrystalBall and were irradiated in CK machine. CrystalBalls were then scanned using a laser CT scanner (OCTOPUS, MGS Research, Inc., DBA 3D Dosimetry, Madison, CT, USA). The simulator CT scan and the laser CT scan of the CrystalBall were registered in 6D using the VOLQA (MGS Research, Inc., DBA 3D Dosimetry, Madison, CT, USA) software. Planned and delivered dose distributions were compared by means of 3D gamma passing rates at user-selected acceptance criteria. Cumulative 3D dose-volume histograms and overlays of orthogonal profiles and isodoses both in 2D and in 3D were also generated, along with voxel statistics for dose difference and distance-to-agreement criteria. Twelve stereotactic radiosurgery/stereotactic radiotherapy/stereotactic body radiation therapy patients were selected randomly such that each set of two represented one of the six diagnoses: brain metastasis, trigeminal neuralgia, spine metastasis, prostate cancer, lung cancer and liver cancer. The validation of the 3D system was carried out by comparing the 2D gamma passing rates for all the patients using EBT3 films sandwiched between PMMA slabs and then analyzed with PTW VeriSoft software.The 3D gamma passing rate averaged over all patients studied was (90.5 ± 6.7) % at acceptance criteria of 2% (local) dose difference and 2 mm distance-to-agreement down to 20% isodose. For 3% (local) dose difference and 3 mm distance-to-agreement (also down to 20% cutoff dose), the average passing rate was (96.0 ± 3.1) %.The results of the study presented here form part of the information needed to assess suitability of a system and hence, the results suggest that the new high-definition 3D CrystalBall system can be a useful PSQA tool as a part of quality assurance for CK.

MAGIC-f gel dosimeter reading: a comparison between an in-house optical CT and MRI imaging

Introduction: According to new developments in radiation therapy techniques, accurate ‎dose verification in three-dimensions has become more critical. Polymer gel dosimeters ‎‎(PGDs) are valuable tools to be used for this purpose. Nowadays, various imaging ‎modalities are employed to read out the gels. This study was aimed to investigate the ‎measured dose distribution recorded in MAGIC-f PGD with optical computed tomography ‎‎(OCT) by comparison with MRI. ‎Material and methods: We developed an in-house CCD based cone-beam OCT scanner. A ‎phantom of MAGIC-f PGD was used to measure a four-field box dose distribution. MRI and ‎OCT scanners were performed for gel readouts. Both measurement results were compared ‎by gamma index analysis with various acceptance criteria. The temporal stability of the gel ‎was also evaluated with the OCT readout system.‎Results: The percentage of isodose lines from two measured datasets agreed well together. ‎The pass rates were 99.02%, 96.8%, and 89.8% with 5%/5mm, 4%/4mm, and 3%/3mm ‎criteria, respectively, at the phantom's central axial slice. ‎Conclusion: The results indicate that the performance of this OCT system is almost the ‎same with acceptable discrepancies to the MRI as accepted standard readout modality, and ‎it can be used for three-dimensional dose verifications.‎

Improved performance of N-(Hydroxymethyl)acrylamide gel dosimeter using potassium chloride for radiotherapy

In this study, polymer-gel dosimeters containing N-(Hydroxymethyl) acrylamide (NHMA) with different concentrations of potassium chloride (KCl) were developed and introduced for use in radiotherapy. The gels were irradiated with a megavolt X-ray photon beam up to 10 Gy using a medical linear accelerator. The irradiated NHMA_KCl dosimeters were evaluated by nuclear magnetic resonance (NMR), in terms of relaxation rate (R2). The results show that the dose sensitivity of polymer gel was improved by 33% when KCl concentration was increased from 0 to 1.9 wt%. The decrease in R2 values with increasing scanning temperature was 1.6% per °C at 6 Gy. The dosimeter was found stable within a period of 2–120 h after irradiation and is independent of dose-rate in the range of 50–600 cGy/min and independent of photon beam energy between 6 and 15 MV within 7.5% overall uncertainty.

Substantial influence of magnesium chloride inorganic salt (MgCl2) on the polymer dosimeter containing N-(Hydroxymethyl)acrylamide for radiation therapy

The influence MgCl2 on the nuclear magnetic resonance (NMR) signal of radiation-induced polymerization of N-(Hydroxymethyl)acrylamide polymer gel (NHMAGAT) dosimeter was investigated in this study. The gels were irradiated via a 6 MV X-ray radiation beam (produced by medical linear accelerator) with doses ranging from 0.25 to 5 Gy. NMR spectroscopy with 0.5 T was used as a tool for measuring the relaxation rate (R2) of gel samples before and after irradiation. The sensitivity of NHMAGAT with a concentration of 1000 mM MgCl2 was found to be approximately 5.1 times higher than the same gel without MgCl2 in the range dose response of 0–2 Gy. Moreover, the key factors like dose-rate, scanning temperature, radiation-energy, the stability of the polymerization post-irradiation were studied.

Investigation of Verification Methods of Field Matching Using Polymer Gel Dosimeter in Proton Therapy

In our proton beam therapy center, we use imaging plates (IP) for dose verification of field matching in irradiation (called patch-field technique). In this study, a polymer gel dosimeter, which can perform three- dimensional dose distribution measurement, was used as a new tool for the verification of the patch field irradiation method corresponding to the rectangular irradiation. The results of measurements of the PAGAT gel dosimeter to irradiate to two rectangular fields called as patch irradiation fields, which were created using the treatment planning system, were evaluated and compared to the results of IP and plan in profiles near the boundary of two fields. In the case of no gap between the two fields, the relative dose using the gel dosimeter was 10.1% higher compared to that measured with the IP in the midpoint of two fields. In case of overlap (called hot region), the result of gels was 6.3% higher than that of IP. In the case of space (called cold region), the result of gels was 14.9% higher than that of IP. The results of the difference between gel and plan in the midpoint of two fields were 14.2% (no gap), -5.0% (hot), and 10.5% (cold). We found that the gel dosimeter was a 3-D dosimetric tool and possibility method for dose verification of patch fields. In this study, the results were preliminary and included several error factors. In the future, it is necessary to develop a dosimeter with improved and more precise measurements.