Polymer Gel Dosimeters Research Articles

Effect of Gold Nanoparticles on Dose Enhancement in Brachytherapy Using a Polymer Gel Dosimeter

The objective of the present study was to measure and evaluate the effect of gold nanoparticles (GNPs) on dose enhancement in brachytherapy with and 192Ir source using polymer gel dosimeters. To test the dose enhancement effect of GNPs during brachytherapy using 192Ir radioisotope, we used gamma rays in the same amount to irradiate gel dosimeters containing different concentrations and sizes of GNPs to measure their doses. Radiation was applied to 10 polymer gel dosimeters and control gel dosimeters that contained GNPs with different sizes (10 nm and 100 nm) and concentrations (0.1, 0.2, 0.3, 0.4, and 0.5 mM) under the same conditions. Dose enhancement factors (DEFs) were found to be 1.011, 1.014, 1.015, 1.026, and 1.029 when the concentrations of GNP were 0.1, 0.2, 0.3, 0.4, and 0.5 mM, respectively, with the size of the GNPs at 10 nm. When the GNP size was increased to 100 nm, the DEFs were 1.070, 1.088, 1.1890, 1.212, and 1.244 when concentrations of the GNPs were 0.1, 0.2, 0.3, 0.4, and 0.5 mM, respectively, showing dose enhancements higher than those of 10-nm-sized GNPs. When GNP size was 100 nm and its concentration was 0.3 mM or higher, the DEFs were higher than 1.15. The bigger the GNP size and the higher the concentration were, the larger the dose enhancement effect was. Such a dose enhancement effect of GNPs obtained in the present study can be utilized as basic data for clinical applications to enhance treatment effects. If such data are utilized in a treatment plan system, a treatment plan taking dose enhancement effect into account can be established at the planning stage.

Improvement of the sensitivity of PASSAG polymer gel dosimeter by urea

The severe toxicity in compound of polymer gel dosimeters has been reported as one of their major limitations for utilization in clinical applications. Recently, PASSAG polymer gel dosimeter has been introduced as a safe polymer gel dosimeter. Despite the excellent dosimetric results reported for this gel dosimeter, its R2-dose sensitivity is relatively low. Therefore, the present study is aimed to improve the sensitivity of PASSAG gel dosimeter by adding urea to its structure. Moreover, it was tried to obtain the optimal amount of urea for the new gel dosimeter. After preparation of the PASSAG-U (PASSAG and Urea) gel dosimeters, they were irradiated using 6 MV photon energy and their responses were read by a 1.5 T MRI scanner. Then, the R2-dose response and the R2-dose sensitivity of the PASSAG-U gel dosimeters with various percentages of the urea were assessed at a 0–10 Gy dose range, various scanning temperatures (15–24 °C), and post irradiation times (1–30 days). The radiological properties of PASSAG-U gel dosimeters confirmed soft tissue and water equivalence of the new gel dosimeters. Compared to the PASSAG gel dosimeter, the R2-dose sensitivities of PASSAG-U gel dosimeters with 1%, 3%, and 5% urea were improved by 12.14%, 25.15%, and 27.90%, respectively. Although the addition of urea improves the R2-dose sensitivity of the gel dosimeter, it leads to the degradation of dose resolution (especially for 5% urea). Moreover, the dosimetric evaluation of characteristics related to the PASSAG-U gel dosimeters with various urea concentrations resulted to following conclusions: 1) the optimal amount of urea was determined 3%; 2) there was a stability in the R2 values for 18–22 °C scanning temperatures; 3) there was a temporal stability at the response of PASSAG-U gel dosimeters from 14 to 30 days after irradiation; 4) the R2-dose sensitivity of PASSAG-U gel dosimeters varied over post irradiation time.

Three-dimensional radiochromic and polymer gel dosimeters with Pluronic F-127 matrix – a review of current research

A co-polymer of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic F-127, PEO-PPO-PEO) was proposed as a physical gel matrix, substituting gelatine for three-dimensional polymer gel dosimeters and as a matrix for the preparation of new radiochromic gel dosimeters. Two polymer gel dosimeters and seven radiochromic gel dosimeters were obtained with this new matrix. In this review we summarise the main advantages of using Pluronic F-127 for manufacturing radiation dosimeters and the best performing new Pluronic dosimeters.

Making and assessing 3D dosimeters

Several 3D dosimeters are commercially available. However, there are many circumstances that require a customized 3D dosimeter. Examples include feasibility tests of non-standard treatment modalities, inhomogeneous tissue configurations, unique shapes and sizes and teaching. In this session, general approaches for preparing radiochromic dosimeters, Fricke and polymer gel dosimeters, micelle gel and silicone dosimeters were presented. Advise will be given to developers of new 3D dosimeters. For optical readout, light absorption and scatter can limit the practical size of dosimeters. Specifically, increasing from 5 to 15 cm diameter dosimeters is optically challenging. Strategies to maximize initial optical transmission were presented. For MRI readout, the dose resolution is determined by both the dosimeter sensitivity and the pulse sequence parameters and the accuracy is determined by the sensitivity of the dosimeter to temperature and dose rate, next to imaging performance. For X-ray CT imaging, the dose resolution is determined by the sensitivity of the dosimeter which largely depends on the polymer density that can be achieved. The importance of characterizing the dosimeter in terms of dose sensitivity and stability, spatial integrity, dose rate and fractionation dependence, oxygen and ambient light sensitivity, temperature sensitivity and thermal history were emphasized. The dosimeter requirements also dictate the types of vessels and scanners appropriate for readout. For example, the preferred dosimeter formulation may include a compound that is incompatible with the preferred vessel.

Spatial dose distribution analysis of Co-60 HDR brachytherapy of cervical cancer using an AQUAJOINT®-based VIPET polymer gel dosimeter

High-dose rate brachytherapy treatment has become more complicated with the use of three-dimensional image-guided brachytherapy (3D-IGBT) to prepare treatment plans using 3D images, such as those obtained from computed tomography (CT). In planning a 3D-IGBT, spatial measurement of dose distribution verification is recommended. In this study, the spatial dose distribution of intrauterine cavity irradiation by a Co-60 sealed brachytherapy source was acquired using an AQUAJOINT® polymer gel dosimeter. A CT/magnetic resonance imaging (MRI) compatible applicator was inserted into the gel in a bottle. It was irradiated and visualized using MRI. The image was converted to an R2 image using a DD-System (R-TECH INC, Japan), and dose distribution was evaluated using the dose–R2 response curve. The obtained dose distribution, dose profile of the radiotherapy treatment planning system, isodose curve, and gradient passing were calculated. In the dose profile, the dose difference was large near the applicator (high-dose region) and was less than ±2% in the middle-dose region. The isodose curves showed good agreement in the region of 2–6 Gy near the prescription point. The gamma analysis was 87.594% on the sagittal cross-section and 93.711% on the coronal cross-section based on the dose difference (DD)/distance to agreement (DTA) of 3%/3 mm.

A chemical evolution of NVP-containing VIPAR-family 3D polymer gel dosimeters – a brief overview

NVP-containing three-dimensional polymer gel dosimeters for radiotherapy dosimetry have been developed for more than 20 years. There have been 11 main modifications of the originally proposed VIPAR polymer gel dosimeter that altogether have amounted to 12 gel dosimeter formulae. This communication is to summarise the main chemical changes made to the VIPAR dosimeter over these years of research. The newest NVP-polymer gel dosimeters are also introduced.

MRI-based iPAGAT polymer gel dosimetry using fast recovery spin echo sequences

In an estimation of iPAGAT polymer gel dosimeters using a magnetic resonance imaging (MRI) system, the dosimetric characteristics were investigated with two echo sequences: a spin echo (SE) and a fast recovery fast spin echo (FRFSE) sequence. FRFSE can shorten total scan time compared to SE because of its short repetition time (TR) and long echo train length (ETL). Although the R2-dose response measured from FRFSE was decreased compared to SE, both responses were fitted quadratic curve with a high correlation coefficient. In addition, their calibrated dose distributions showed high conformity to the planned data in the three-dimensional (3D) gamma index, dose volume histogram (DVH) and surface rendering analyses; however, results of SE were a little superior to FRFSE. In conclusion, it is suggested that FRFSE can present the accurate characteristics of polymer gel dosimeters under the optimum parameters.

Basic characteristics of an AQUAJOINT®-based VIPET polymer gel dosimeter

The basic characteristics of a VIPET-type polymer gel dosimeter were assessed using AQUAJOINT® as a hydrogel matrix. The dosimeter exhibited a good dose response. The threshold dose was 0.5–0.6 Gy and the linear section of the response curve, in the range of 1.7 – 2.7 Gy, had a minimum dose response of 0.05–0.06 Gy. The dose-rate dependence was determined by the delivery of 200 MU for dose rates between 100 and 600 MU/min; the resulting value of R2 was 3.222 ± 0.036 (mean ± 1SD). A slight change in the value of R2 in the integration of the divided doses was observed; the R2 value was 3.059 ± 0.015 (mean ± 1SD) for a total irradiation of 200 MU. Gel samples of different volumes were irradiated and compared with a PAGAT polymer gel. The change in sensitivity of the AQUAJOINT®-based VIPET was smaller than that of the PAGAT.

Gel dosimetry measurement of dose enhancement bismuth-based nanoparticles in radiation therapy

Recently bismuth-based nanoparticles as a promising radiosensitizer have drawn great attention in radiation therapy. To prove physical dose enhancement effect of the nanoparticles, gel dosimeters can be considered as an ideal method. This study aims to prove the applicability of bismuth ferrite nanoparticles as a magnetic localized dose enhancement agent by gel dosimetry method. Bismuth ferrite nanoparticle was synthesized by the conventional sol-gel method. Then we investigated dose enhancement property of the nanoparticles with gel dosimetry. MAGIC Polymer Gel dosimeters with nanoparticles were prepared and irradiated. According to gel dosimetry assay, for 0.5 mg/ml concentration of bismuth ferrite nanoparticles dose enhancement factor were obtained as 2 and 1.6 at 160 keV and average energy of 380 keV, respectively. Moreover, radiosensitiser effect of bismuth ferrite nanoparticles in presence of a low dose rate brachytherapy source (125-I) was investigated by Monte Carlo method. Whereas bismuth ferrite nanoparticles have magnetic property, we made a biodegradable spacer (fiducial) brachytherapy loaded with the nanoparticles for delivering nanoparticles and drug by applying an external magnetic field.

Investigation of lung tumour peripheral doses using normoxic polymer gel and film dosimetry techniques

This paper describes the investigation of lung tumour peripheral doses for 6MV, 6MV FFF, 10MV FFF and 15MV conformal arc therapy (CAT) beams calculated in the Monaco® TPS and delivered using a flattening filter free capable Elekta® AgilityTM linac. Two independent high resolution dosimetry techniques were used for investigations. Measurements were performed using the normoxic PAG polymer gel dosimeter and GAFchromicTM EBT3 film and compared against the calculated dose plane from the Monaco treatment planning system. Both measurement methodologies indicate that Monaco is overestimating the lung tumour peripheral dose for the beams investigated in this study. It is shown that when using 10MV FFF in lung for arc deliveries, there is a dosimetric compromise compared with 6MV and 6MV FFF.

Response overshoot: a challenge for the application of polymer gel dosimeters

In this research response overshoots of the half irradiated PAGAT polymer gel dosimeter at variations post irradiation times were traced out. Results showed that 5 h after irradiation the measured dose by polymer gel dosimeter in boundary region are compatible with delivered dose. However, more elapsed time post irradiation caused misevaluation of dose. It was observed that response overshoot caused a maximum error of 28% in dose evaluation. It was concluded that more care is necessary when a high gradient dose distribution is evaluated by means of a polymer gel dosimeter.

Raman spectroscopy as a tool to evaluate oxygen effects on the response of polymer gel dosimetry

Currently, advanced dosimeters like polymer gels are capable of obtaining reliable and accurate 3D dose distributions from correlations with the different polymerization degrees induced by incident radiation. Samples of polymer gel dosimeters are commonly read out using magnetic resonance imaging or optical methods like visible light transmission or laser computed tomography. Alternatively, this work proposes and evaluates the implementation of Raman spectroscopy to provide direct information on the effect of oxygen permeating through the walls of phantoms on the polymerization initiated by irradiation in three types of polymer gel dosimeters, namely NIPAM, ITABIS and PAGAT. The aim of the present study is to provide better and complete interpretations using three different containers, adequate for integral, 2D and 3D dose mapping. Moreover, Raman spectroscopy has been used to analyze the well-known effect of oxygen inhibition on the different polymer gel dosimeters remarking the importance of avoiding air exposition during sample storage and readout. Dose-response curves for different polymer gels were obtained in terms of measurements with a calibrated ionization chamber. Additionally, dedicated Monte Carlo simulations were performed aimed at characterizing dose for different X-ray irradiation setups, providing also suitable information to evaluate oxygen diffusion through the sample wall. The obtained results were contrasted with optical transmission readout as well as Monte Carlo simulations attaining very good agreements for all dosimeter types.

Design and characteristics of an agar additive polymer gel dosimeter

Herein, we investigate the use of agar and gelatin in a polymer gel dosimeter. The polymer gel is enclosed in a vinyl film to obtain a dosimeter of arbitrary shape and maintain the shape at room temperature. The resulting polymer gel dosimeter could preserve its shape across a wide temperature range. Excluding the surface region, the obtained dose distribution was within 3% of that determined in an ionization chamber.

3.2.6 LET Effects of Polymer Gel Dosimeters Based on Radiation-crosslinked Gel

3.2.6 LET Effects of Polymer Gel Dosimeters Based on Radiation-crosslinked Gel

3-D Quality Assurance in CyberKnife Radiotherapy Using a Novel N-(3-methoxypropyl) Acrylamide Polymer Gel Dosimeter and Optical CT

In radiotherapy, dosimetry tools are needed to record and verify complex 3-D dose distributions. A novel composition of polymer gel dosimeter based on radiation-induced polymerization of N-(3-methoxypropyl) acrylamide (NMPA) is presented and studied as a 3-D dose verification quality assurance tool (QA) in radiotherapy treatment planning. A 160 mm diameter crystal glass round bottom flask (CrystalBall) containing NMPA gel was used as a phantom. The gel was irradiated by a CyberKnife robotic radiotherapy system and scanned optically with a red light source using an optical computed tomography OCT scanner with sub-millimeter spatial resolution. Additionally, a commercial polymer gel dosimeter of BANG-GEL-QA™ was irradiated and scanned to compare its performance with that of the novel gel dosimeter. The dose distributions of a CyberKnife robotic SRS/SBRT treatment planning system (TPS) for a patient with brain metastasis were compared to the measurements of both polymers using 2-D and 3-D gamma analysis with a criteria of 5% dose difference and 2 mm distance-to-agreement. A gamma pass rate of 95% was calculated within a diameter of at least 32 mm for the 2-D and 3-D gamma analysis of both polymer gels. This suggests that the novel NMPA polymer dosimeter can provide an accurate 3-D verification of complex dose distribution in SRS/SBRT radiotherapy treatments. This study demonstrates that the newly developed CrystalBall NMPA phantom can provide a promising 3-D quality assurance tool for radiation therapy.

Quantitative magnetic resonance elastography for polymer-gel dosimetry phantoms

Commonly dose-responses of conventional dosimetric methods are affected by a saturation dose and are known to be limited when the delivered dose is relatively high. In contrast, elastic properties of polymer-gel dosimeter phantoms play major roles in a new dosimetry technique using magnetic resonance elastography (MRE). A single volume of polymer-gel dosimeter solution containing methacrylic and ascorbic acid in gelatin initiated by copper was prepared. The material was subsequently stored in cylindrical containers for future use as a biological tissue-mimicking phantom material. The phantom material was irradiated with gamma rays, where absorbed doses of 10–50 Gy were delivered. To study the dynamic elastic behaviour, periodic mechanical external forces of 100–400 Hz were applied to generate shear waves in the samples. The radiation-induced changes in the shear modulus of the samples were estimated from wave-displacement images and converted to elastograms. The smallest and largest shear modulus values were approximately 2.10 ± 0.64 and 35.26 ± 2.85 kPa, respectively. The dynamic elastic response of the polymer-gel dosimeters showed an increased dependency with frequency. A linear relationship (R2 = 0.996) was observed between the integrated area and the absorbed dose of the samples. The elastograms clearly showed that the largest shear modulus values were in the irradiated region of the polymer-gel dosimeter phantoms. Quantitative values of the shear modulus of polymer-gel dosimeters were estimated using MRE.

Evaluation of the Dosimetric Accuracy of Brain Stereotactic Radiotherapy by Using a Hybrid Quality Assurance (QA) Toolkit

We evaluated the feasibility of a magnetic resonance (MR) tumor model and the accuracy of 3D dosimetry for evaluating brain stereotactic radiotherapy (SRT). We first validated the stability of the BANG3 polymer gel dosimeter and performed a pre-clinical verification of this device for brain SRT. Gel-filled vials were irradiated with 6-MV beams to generate a dose calibration curve in relation to the R2 (1/T2) values on 9.4 T MR images. The dose linearity, inter-reproducibility and intra-reproducibility, and dose-rate dependence of the gel dosimeters were evaluated. We also developed an image tumor model for pre-clinical testing. A calibration kit for the gel dosimeter and two spherical tumor representations were fabricated by using 3D printing. Two planning target volumes (PTVs) were contoured onto the MR images of the tumor models comprised of dosimetric gel. To evaluate the geometric and dosimetric accuracy, we created a treatment plan such that the D95s for PTV1 and shifted PTV2 were more than the prescribed dose. The shifted PTV2 was produced by an intentional shift of 5 mm from the true target position. A 2 arc VMAT plan was created to deliver 35 Gy in 5 fractions. After irradiation, calibration vials and tumor model phantoms were scanned by using 9.4 T MRI, and the acquired images were then analyzed using ImageJ and DCMTK software libraries. Scanned MRI images of the tumor models were imported into a treatment planning system and registered to CT images. We also compared the agreement of results between the planned and the measured data in 1D (ion chamber), 2D (2D film), and 3D (gel dosimeter). The best dose linearity achieved was 0.99 (R2) at 180 TE (ms) under which conditions the reproducibility and dose rate dependence were less than 2.2% and 3.5%, respectively. The mean dose differences between the treatment plan and the ion-chamber results were 3.3%, 0.4%, and 4.5%, respectively, for PTV1 and 2 and for the shifted PTV. Moreover, the point dose differences between the treatment plan and the gel results were 0.9%, 2.6% and 3.7%, respectively, for PTV1 and 2 and for the shifted PTV. The gamma passing rates with 3%/3 mm criteria were greater than 99% for all plans. The isodose distributions and profiles showed good qualitative agreement between the gel dosimeter, EBT film and RTP data for all PTVs. Our present study results, thus, indicate that an MR tumor model can effectively evaluate the geometric and dosimetric accuracy of brain SRT. Image tumor models using gel dosimeters are, therefore, useful for validating the 3D dose distributions for patient-specific whole treatment processes. We are planning to improve this image tumor model by facilitating respiratory mobility and utilizing more complex tumor shapes for liver stereotactic body radiotherapy (SBRT) cases.

Examination of THPC as an oxygen scavenger impacting VIC dosimeter thermal stability and comparison of NVP-containing polymer gel dosimeters

This work reports on the impact of tetrakis(hydroxymethyl)phosphonium chloride (THPC) on the properties of a VIC gel dosimeter (VIC is an abbreviated acronym of VIPARCT). THPC was used as a substitute oxygen scavenger in VIC (17% N-vinylpyrrolidone, 8% N,N′-methylenebisacrylamide, 12% tert-butyl alcohol, 7.5% gelatine, 0.02% hydroquinone and an oxygen scavenger of 0.007% ascorbic acid and 0.0008% CuSO4 × 5H2O). THPC reduced the gelation time of VIC from hours to minutes. The best composition (VIC-T) contained 14 mM THPC and a reduced gelatine concentration (5%) with respect to VIC, which allowed for gelation in about 3 min. VIC-T was characterised by the same dose sensitivity (0.176 ± 0.003 Gy−1 s−1 for VIC-T and 0.171 ± 0.002 Gy−1 s−1 for VIC), dose threshold (0.5 Gy) and dynamic dose range (0.5‒50 Gy) as VIC, and a lower linear dose range (20 Gy for VIC-T, 30 Gy for VIC) (0.47 T NMR measurements). VIC-T was stable for at least 10 days after irradiation, and 3D dose distribution was stable for over 4 months after irradiation. The dose response of VIC-T was independent of the radiation dose rate, type and energy of radiation for 6 and 15 MV photons and 12 MeV electrons. This is an improvement with respect to VIC which showed a different dose response for 6 MV photons than for 12 MeV electrons and 15 MV photons. Raman spectroscopy showed similarity in the rate of radiation-induced conversion of monomers in VIC and VIC-T, indicating interaction of THPC with gelatine in VIC-T, and showed ageing of gelatine in both dosimeters. Differential scanning calorimetry showed VIC-T stability at 0 °C–80 °C (VIC: 0 °C‒29.5 °C). The chemical polymerisation and crosslinking of gelatine with THPC is reported, the mechanism of which was analysed in detail. A comparison of N-vinylpyrrolidone-containing dosimeters is presented in this work.

Evaluation of Gold Nanoparticle Size Effect on Dose Enhancement Factor in Megavoltage Beam Radiotherapy Using MAGICA Polymer Gel Dosimeter

Background: Gold nanoparticles (GNPs) are among the most promising radiosensitive materials in radiotherapy. Studying the effective sensitizing factors such as nanoparticle size, concentration, surface features, radiation energy and cell type can help to optimize the effect and possible clinical application of GNPs in radiation therapy. In this study, the radiation sensitive polymer gel was used to investigate the dosimetric effect of GNP size in megavoltage (MV) photon beam radiotherapy.Material and Methods: GNPs with the size of 30nm, 50nm and 100nm in diameter were used. Transmission electron microscope (TEM) and dynamic light scattering (DLS) were applied to analyze the size of nanoparticles. The MAGICA polymer gel was synthesized and impregnated with different sizes of GNPs. The samples were irradiated with 6MV photon beam and 24 hours after irradiation, they were read using a Magnetic Resonance Imaging (MRI) scanner. Macroscopic Dose Enhancement Factor (DEF) was measured to compare the effect of GNP size. The MAGICA response of the 6MV x-ray beam was verified comparing Percentage Depth Dose (PDD) curve extracted from polymer gel dosimetry and Treatment Planning System (TPS).Results: MAGICA polymer gel dose response curve was linear in the range of 0 to 10 Gy. DEFs by adding 30nm, 50nm and 100nm GNPs were 1.1, 1.17 and 1.12, respectively. PDD curves of polymer gel dosimeter and treatment planning system were in good agreement.Conclusion: The results indicated a substantial increase in DEF uses a MV photon beam in combination with GNPs of different sizes and it was inconsistent with previous radiobiological studies. The maximum DEF was achieved for 50nm GNPs in comparison with 30nm and 100nm leading to the assumption of self-absorption effect by larger diameters. According to the outcomes of this work, MAGICA polymer gel can be recommended as a reliable dosimeter to investigate the dosimetric effect of GNP size and also a useful method to validate the current radiobiological and simulation studies.

High dose sensitivity of N-(isobutoxymethyl)acrylamide polymer gel dosimeters with improved monomer solubility using acetone co-solvent

Normoxic N-(isobutoxymethyl)acrylamide polymer gel (NIBMAGAT) dosimeters have been prepared in deionized water with acetone as a co-solvent for radiotherapy with very low levels of turbidity. The polymer gel dosimeters were irradiated in a cubic water phantom using a 6 MV X-ray radiation beam of a medical linear accelerator for doses from 1 to 20 Gy. A 0.5 Tesla nuclear magnetic resonance spectroscopy was used to measure the transverse relaxation rate (R2) of un-irradiated and irradiated gel samples. The polymer dosimeter presents a linear dose response for doses up to 10 Gy with significant R2 sensitivity of 0.4776 s−1 Gy−1. A higher NIBMA monomer concentration results from higher NIBMA solubility in water and acetone enhances significantly (i.e. more than six times) the performance of NIBMAGAT polymer gel dosimeters for radiotherapy dosimetry. The key parameters such as dose rate, radiation beam energy and post irradiation stability up to 10 days were investigated. We found these parameters had no significant effect on the dose response and dose sensitivity of NIBMAGAT gel dosimeters.