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

Enhancement of the dosimetric properties of N-vinyl caprolactam polymer gel dosimeter for clinical practice

In this study, the impact of Lithium Chloride inorganic salt sensitizer on the performance of N-vinyl caprolactam polymer-gel dosimeter was evaluated in terms of dose-response combined with spin-spin relaxation rate (R2) obtained from nuclear magnetic resonance relaxometry technique. The irradiation experiments were conducted using a medical linear accelerator, and the improved polymer gel dosimeters were exposed to various doses, photon beam energies, and dose rates. The signal development of the dosimeters was analyzed using a 0.5 T nuclear magnetic resonance instrument. The dose response of the improved gel at different concentration of co-monomers and at different types of gelatins was investigated. Results showed that the R2 sensitivity of the dosimeter was improved with an increase in Lithium Chloride concentration. With 6 % Lithium Chloride, the sensitivity was improved by more than one and half to two times in the linear dose response rang of (0-10 Gy). Furthermore, no significant impact was seen from varying dose rates and photon energy with the improved dosimeter, and R2 was not affected by changing the irradiation temperature from 10 to 30 °C. Additionally, the dose-response and hence R2 data decreased with increasing scanning temperature and the response was stable for up to five days after irradiation. The polymer gel dosimetry accuracy was estimated by calculating the overall uncertainty and found to be 4.06 % (2σ, 95 % confidence level).

Magnetic properties of polymeric acrylic acid hydrogel dosimeter for radiotherapy applications

The present study introduces the first magnetic characterization of a hydrogel dosimeter comprising acrylic acid synthesized within a polyvinyl alcohol matrix. The study aims to accurately assess ionizing radiation dose distributions, making it a valuable tool for radiotherapy treatment. The hydrogel was irradiated to a 1-60 Gy dose range using a medical linear accelerator with dose rates of 100-600 MU/min and radiation beam energies of 6, 10, and 15 MV. The developed dosimeter was synthesized by irradiation-triggered polymerization, and the polymerization degree was indirectly quantified by monitoring the positive alterations in the nuclear magnetic resonance spin‒spin relaxation rate. The polymeric hydrogel dosimeter demonstrated an exceptional dose response with an NMR sensitivity of 0.26 Gy⁻¹s⁻¹, which is 20 times more than the sensitivity of the same gel when measured optically in our previous study. Moreover, it exhibited consistent performance regardless of the beam energy or dose rate.

Advancements in Tissue-Equivalent Gel Dosimeters

Tissue-equivalent hydrogel dosimeters represent a class of tools that hold significant promise, particularly in the precise measurement of three-dimensional dose distributions in radiotherapy. Due to their physical properties closely resembling those of human soft tissue, these dosimeters effectively replicate the energy transfer phenomena resulting from radiation interactions, such as atomic ionization and scattering by nuclei or electrons. Consequently, tissue-equivalent dosimeters, characterized by their linear energy transfer properties, have been extensively applied in medical physics, radiation oncology, and nuclear safety. Future advancements focusing on developing more stable, less toxic, normoxic, and cost-effective dosimeters could enable their broader adoption. This review provides a comprehensive overview of the key characteristics that make hydrogel dosimeters tissue-equivalent, highlighting their benefits, limitations, and primary application areas. Additionally, it explores current advancements in polymeric gel technology and discusses future directions aimed at optimizing their performance and accessibility for broader adoption.

Three-dimensional radiation dosimetry of carbon ion beams using surfactant hydrogels: Fundamental investigation

BACKGROUND

In carbon ion radiotherapy, accurate measurement of the three-dimensional (3D) absorbed dose distribution is critical for effectively targeting tumors. Although micellar gel dosimeters exhibit considerable potential for measuring 3D absorbed dose distributions, few studies have focused on radiotherapy using carbon ion beams.

PURPOSE

This study investigated the applicability of the surfactant hydrogel dosimeter (SHD), a micellar gel dosimeter, to measuring a 3D dose absorbed through carbon ion beam irradiation.

METHODS

A cubic target region of 34 mm per side was established at a depth of 46 mm below the upper surface of an SHD specimen. Scanning irradiation was performed using a pencil beam of carbon ions at the Ion-beam Radiation Oncology Center in Kanagawa ("i-ROCK"), Japan, under irradiation conditions set by the treatment planning system ("Monaco for Carbon", Ver. 5.20, Elekta AB, Sweden) to create a spread-out Bragg peak within the target. The physical dose was set to 10 Gy at the isocenter, situated at the center of the target. The SHD responsiveness was measured twice using optical computed tomography (CT) ("Vista 15", Modus Medical Devices, Canada) for three irradiated specimens, and six types of measured optical attenuation coefficient (OAC) were obtained. To assess whether the OAC represented the absorbed dose expected in the treatment plan, we compared the relative distribution of the OAC and that of the absorbed dose. Relative fraction (RF) was used to measure the difference between the relative value of the OAC and that of the absorbed dose. Moreover, the distribution of OH radical (•OH) concentration obtained by Monte Carlo simulation ("PHITS" ver. 3.24 JAEA, Japan) and that of the OAC were compared.

RESULTS

In the direction of beam travel, the relative distribution of the OAC was lower than that of the absorbed dose. This discrepancy could be attributed to a decrease in the concentration of •OH produced by irradiation owing to the recombination reaction, which does not accurately reflect the absorbed dose. By contrast, the distributions in the plane perpendicular to the beam travel were consistent. The RF increased from ± 3% to ± 13% along the beam travel direction. The small RF in the plane perpendicular to the beam travel could be attributed to the constant distribution of linear energy transfer, regardless of the irradiation position, and the generation of radicals proportionally to the absorbed dose. The increase in RF along the beam travel direction was ascribed to ring artifacts in the irradiated region.

CONCLUSION

The measurement of the absorbed dose distribution in the beam travel direction should be improved. The observed discrepancy is attributed to the reduced reactivity of the SHD due to a high liner energy transfer near the Bragg peak. However, the absorbed dose distribution can be effectively evaluated in the plane perpendicular to the direction of beam travel.

Optimizing the feasibility of polyvinyl alcohol-potassium iodine gel for medical dosimeter

This work aims to improve the post stabilty of reusable potassium iodide hydrogel dosimter. A reusable and low-cost radiochromic dosimeter containing a gel matrix of polyvinyl alcohol, potassium iodide dye, froctose as reducing agent and glutaraldehyde as cross-linking agent was developed for dose calibration in radiotherapy. The gel samples were exposed to different absorbed doses using a medical linear acceleration. UV-vis Spectrophotometry was utilized to investigate the changes in optical-properties of irradiated gels with regard to peak wavelength of 353 nm. The stability of the gel (one of the most limitation of using this dosimeter) was improved significantly by the addition of certain concentrations of dimethyl sulfoxide. The two-dimensional optical imaging system of charge-coupled-device (CCD) camera with a uniform RGB light-emitting-diode (LED) array source was used for diffusion coefficient purpose using two dimensional gel template. The value of diffusion coefficient reported is significant and highly reduced compared with other dosimeters reported in the literatures. Moreover, heating the improved gels to certain temperatures results in resetting their optical properties, which makes it possible to reuse for multiple times.

Radiation Dosimetry by Use of Radiosensitive Hydrogels and Polymers: Mechanisms, State-of-the-Art and Perspective from 3D to 4D

Gel dosimetry was developed in the 1990s in response to a growing need for methods to validate the radiation dose distribution delivered to cancer patients receiving high-precision radiotherapy. Three different classes of gel dosimeters were developed and extensively studied. The first class of gel dosimeters is the Fricke gel dosimeters, which consist of a hydrogel with dissolved ferrous ions that oxidize upon exposure to ionizing radiation. The oxidation results in a change in the nuclear magnetic resonance (NMR) relaxation, which makes it possible to read out Fricke gel dosimeters by use of quantitative magnetic resonance imaging (MRI). The radiation-induced oxidation in Fricke gel dosimeters can also be visualized by adding an indicator such as xylenol orange. The second class of gel dosimeters is the radiochromic gel dosimeters, which also exhibit a color change upon irradiation but do not use a metal ion. These radiochromic gel dosimeters do not demonstrate a significant radiation-induced change in NMR properties. The third class is the polymer gel dosimeters, which contain vinyl monomers that polymerize upon irradiation. Polymer gel dosimeters are predominantly read out by quantitative MRI or X-ray CT. The accuracy of the dosimeters depends on both the physico-chemical properties of the gel dosimeters and on the readout technique. Many different gel formulations have been proposed and discussed in the scientific literature in the last three decades, and scanning methods have been optimized to achieve an acceptable accuracy for clinical dosimetry. More recently, with the introduction of the MR-Linac, which combines an MRI-scanner and a clinical linear accelerator in one, it was shown possible to acquire dose maps during radiation, but new challenges arise.

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).

Fluorescent organic particle doped polymer-based gel dosimeter for neutron detection

The purpose of this work is to develop a material capable of detecting neutrons produced by photodisintegration in a linear accelerator for its medical use. In this study, we have developed a gel-like material doped with fluorescent organic particles. PPO at 1 wt% is used as primary dopant and POPOP as secondary one at 0.03 wt%. A set of four samples is produced, with boric acid concentrations of 0, 400, 800 and 1200 ppm. The viscoelastic properties of the material are characterized with rheological measurements, finding a gel-like behavior, i.e., a material that can keep its original shape if no stresses are applied, but can also be deformed by applying a moderate shear rate. Furthermore, the material was irradiated with gamma, electron, and neutron emission sources from ¹³⁷Cs, ²²Na, ⁶⁰Co, ²¹⁰Po, ⁹⁰Sr and ²⁴¹AmBe, and its response was measured in two different experimental settings, in two different institutions, for comparative purposes. From these measurements, one can clearly establish that the new material detects neutrons, electrons, and gammas within the MeV regions and below. Thus, our findings show that the developed material and its properties make it a promising technology for its use in a neutron detector.

Hydrogels for Three-Dimensional Ionizing-Radiation Dosimetry

Radiation-sensitive gels are among the most recent and promising developments for radiation therapy (RT) dosimetry. RT dosimetry has the twofold goal of ensuring the quality of the treatment and the radiation protection of the patient. Benchmark dosimetry for acceptance testing and commissioning of RT systems is still based on ionization chambers. However, even the smallest chambers cannot resolve the steep dose gradients of up to 30-50% per mm generated with the most advanced techniques. While a multitude of systems based, e.g., on luminescence, silicon diodes and radiochromic materials have been developed, they do not allow the truly continuous 3D dose measurements offered by radiation-sensitive gels. The gels are tissue equivalent, so they also serve as phantoms, and their response is largely independent of radiation quality and dose rate. Some of them are infused with ferrous sulfate and rely on the radiation-induced oxidation of ferrous ions to ferric ions (Fricke-gels). Other formulations consist of monomers dispersed in a gelatinous medium (Polyacrylamide gels) and rely on radiation-induced polymerization, which creates a stable polymer structure. In both gel types, irradiation causes changes in proton relaxation rates that are proportional to locally absorbed dose and can be imaged using magnetic resonance imaging (MRI). Changes in color and/or opacification of the gels also occur upon irradiation, allowing the use of optical tomography techniques. In this work, we review both Fricke and polyacrylamide gels with emphasis on their chemical and physical properties and on their applications for radiation dosimetry.