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

Pilot scale validation campaign of gel dosimetry for pre-treatment quality assurance in stereotactic radiotherapy

Purpose: Complex stereotactic radiotherapy treatment plans require prior verification. A gel dosimetry system was developed and tested to serve as a high-resolution 3D dosimeter for Quality Assurance (QA) purposes.Materials and Methods: A modified version of a polyacrylamide polymer gel dosimeter based on chemical response inhibition was employed. Different sample geometries (cuvettes and phantoms) were manufactured for calibration and QA acquisitions. Irradiations were performed with a Varian Trilogy linac, and analyses of irradiated gel dosimeters were performed via MRI with a 1.5 T Philips Achieva at 1 mm3 or 2 mm3 isotropic spatial resolution. To assess reliability of polymer gel data, 54 stereotactic clinical treatment plans were delivered both on dosimetric gel phantoms and on the Delta4 dosimeter. Results from the two devices were evaluated through a global gamma index over a range of acceptance criteria and compared with each other.Results: A quantitative and tunable control of dosimetric gel response sensitivity was achieved through chemical inhibition. An optimized MRI analysis protocol allowed to acquire high resolution phantom dose data in time -frames of approximate to 1 h. Conversion of gel dosimeter data into absorbed dose was achieved through internal calibration. Polymer gel dosimeters (2 mm3 resolution) and Delta4 presented an agreement within 4.8 % and 2.7 % at the 3 %/1 mm and 2 %/2 mm gamma criteria, respectively.Conclusions: Gel dosimeters appear as promising tools for high resolution 3D QA. Added complexity of the gel dosimetry protocol may be justifiable in case of small target volumes and steep dose gradients

Preliminary investigation of X-ray imaging for dose extraction of BANG® polymer gel in intensity modulated radiation therapy

This study investigated gel dosimetry with X-ray CT imaging as a possible means for extracting dose information from a 3D gel dosimeter. Currently Optical CT and MRI are the popular means of dose extraction, but X-ray CT imaging has the advantage of being more convenient and cost effective. The dosimetric system was based on the BANG® polymer gel (MGS Research) and an ordinary clinical X-ray CT unit. The gel system was analyzed for its effectiveness in detecting absorbed dose from a 10 MV Linac unit. This study investigated calibration doses up to 8 Gy and two fractions of an IMRT treatment plan for a total dose of 4.22 Gy. The irradiation plans were generated by the Varian Eclipse® treatment planning system and delivered at OHSU. One week post irradiations the BANG® polymer gels were analyzed using X-ray CT imaging at OHSU. The imaging parameters were unique to this investigation. Post irradiation the BANG® gel dosimeter responded to the absorbed doses through the process known as polymerization. The BANG® polymer gel dosimeter changed chemically and physically. A density change occurring in the location of irradiation allowed for detection via X-ray Imaging. A tube potential of 120 kV was selected for better signal to noise ratio and thin image slices of 1 mm was used for greater spatial resolution. The use of X-ray imaging with these specific imaging parameters proved to be convenient and effective. Images of the gel which received no radiation were studied to build a background evaluation. Post irradiation images were evaluated for dose response. This study showed that X-ray imaging was able to detect the change within the gel due to irradiation with a response of about 0.66 ± 0.03 pixel values per Gy. The BANG® gel dosimeter was characterized for its response to radiation, dose sensitivity, dose resolution, and dose distribution. We found our system to have a high dose sensitivity of 0.96 ± 0.6 H/Gy. The X-ray CT images were able to differentiate between doses with a resolution of 39% within the mean dose. From these finding we were able to build dose distributions and dose maps for our calibration and treatment phantoms. The conclusion of this preliminary investigation found X-ray CT imaging to be successful for dose extraction purposes. We note that there are still areas in gel dosimetry which need additional research such as development in software or code to integrate, analyze the dose response, and compare the results with predicted dose distribution generated by the treatment plans. Using X-ray CT will certainly decrease the cost of the 3D Gel dosimetry systems and with increased clinical use 3D gel dosimetry will soon allow for better quality assurance of radiotherapy treatments

Development of a new dosimetry technique

In order to minimize the damage inflicted in healthy tissue during radiotherapy, it is vital to verify the dose absorbed by the patient. A common approach is the use of dosimetry gels in which local radiolysis processes are induced through radiation. As the subsequent read-out using Magnetic Resonance Imaging techniques is time-consuming, the here presented thesis investigates a new approach based on light scattering to determine the absorbed dose. More precisely, a laser sheet with sinusoidal intensity modulation is employed to scan the dosimetry gel slice-wise and scattered light is detected at a 90 degree angle which allows the determination of the local extinction coefficient in the sample. The idea is to relate the local extinction coefficient to the absorbed dose. In order to achieve this task, a setup for the data acquisition is built and the algorithm necessary for the calculation is written. Based upon first measurements investigating the applicability of the new approach, one can state that the new technique is of great potential as it allows the visualization of the irradiation structures with high resolution.Radiotherapy is a common technique employed when treating cancer patients. However, the radiation can also potentially inflict great damage in the healthy tissue surrounding the tumor. Consequently, it is of great importance to verify that the radiation received by the patient is absorbed in the intended location and has the right dose. In order to implement such a so called dosimetric verification, a gelatine probe that behaves similar to human tissue when irradiated is produced. More importantly, the irradiation induces a change of structure in the gelatine sample which allows one to draw conclusions concerning the absorbed dose. Currently, the most common approach to determine the absorbed dose from structural changes in the gelatine is to use Magnetic Resonance Imaging which is the same technique used to image tiny fractures in bones or to do brain scans. However, this technique also has a few disadvantages such as high costs associated with it as well as its time-consuming nature. Therefore, this thesis investigates a new approach to draw conclusions concerning the absorbed radiation dose from structural changes in the gelatine sample. In this new technique, a blue laser beam is first compressed into a very thin sheet which is subsequently sent into the gelatine. The particles in the gelatine scatter light into all directions and a camera detects the amount of light that is scattered at a 90 degree angle. The brighter the image of the sample in a specific point, the more light has been scattered in that specific point. Most importantly, different structures in the sample scatter the light by different amounts. The laser sheet is then moved in steps through the gelatine, thereby scanning the whole sample. At each step, the camera acquires a new image which is employed to calculate a physical quantity called extinction coefficient for each point of the sample. The extinction coefficient describes how likely light is to be scattered when passing through a specific point in the sample. Thus, the value of this extinction coefficient depends on the structure of the sample and thereby also on the absorbed radiation dose. The work presented here aims to investigate the applicability of the new technique. The result of this thesis work is a complete setup to acquire the data as well as the algorithm necessary to calculate the extinction coefficient in each point of the sample. Based on the results, one can state that the new technique is of great potential. Not only does it allow one to image the irradiated structures in the gelatine with a very high precision, it also shortens the time necessary for data acquisition as compared to other currently employed techniques

Evaluation of the region-specific risks of accidental radioactive releases from the European Spallation Source

The European Spallation Source (ESS) is a neutron research facility under construction in southern Sweden. The facility will produce a wide range ofradionuclides that could be released into the environment. Some radionuclides are of particular concern such as the rare earth gadolinium-148. In this article, the local environment was investigated in terms of food production and rare earth element concentration in soil. The collected data will later be used to model thetransfer of radioactive contaminations from the ESS

Use of gold nanoparticles in MAGIC-f gels to 18 MeV photon enhancement

Objective(s): Normoxic MAGIC-f polymer gels are established dosimeters used for three dimensional dose quantifications in radiotherapy. Nanoparticles with high atomic number such as gold are novel radiosensitizers used to enhance doses delivered to tumors. The aim of this study was to investigate the effect of gold nanoparticles (GNPs) in enhancing percentage depth doses (PDDs) within the MAGIC-f gel exposed to linear accelerator (linac) high energy photon beams. Materials and Methods: The MAGIC-f gel was fabricated based on its standard composition with some modifications. The PDDs in tubes containing the gel were calculated by using a common Monte Carlo code (Geant4) followed by experimental verifications. Then, GNPs with an average diameter of 15 nm and a concentration of 0.1 mM were embedded in the gel, poured into falcon tubes and irradiated with 18 MeV beams of an Elekta linac. Finally, similar experimental and Monte Carlo (MC) calculations were made to determine the effect of using GNPs on some dosimetric parameters of interest.Results: The results of experimental measurements and simulated MC calculations showed a dose enhancement factor (DEF) of 1.12±0.08 and 1.13±0.04, respectively due to the use of GNPs when exposed to 18 MeV linac energies.Conclusion: The results indicated that the fabricated MAGIC-f gel could be recommended as a suitable tool for three dimensional dosimetric investigations at high energy radiotherapy procedures wherein GNPs are used

High Performance Optical Computed Tomography for Accurate Three-Dimensional Radiation Dosimetry

Optical computed tomography (CT) imaging of radiochromic gel dosimeters is a method for truly three-dimensional radiation dosimetry. Although optical CT dosimetry is not widely used currently due to previous concerns with speed and accuracy, the complexity of modern radiotherapy is increasing the need for a true 3D dosimeter. This thesis reports technical improvements that bring the performance of optical CT to a clinically useful level. New scanner designs and improved scanning and reconstruction techniques are described. First, we designed and implemented a new light source for a cone-beam optical CT system which reduced the scatter to primary contribution in CT projection images of gel dosimeters from approximately 25% to approximately 4%. This design, which has been commercially implemented, enables accurate and fast dosimetry. Second, we designed and constructed a new, single-ray, single-detector parallel-beam optical CT scanner. This system was able to very accurately image both absorbing and scattering objects in large volumes (15 cm diameter), agreeing within ∼1% with independent measurements. It has become a reference standard for evaluation of optical CT geometries and dosimeter formulations. Third, we implemented and characterized an iterative reconstruction algorithm for optical CT imaging of gel dosimeters. This improved image quality in optical CT by suppressing the effects of noise and artifacts by a factor of up to 5. Fourth, we applied a fiducial-based ray path measurement scheme, combined with an iterative reconstruction algorithm, to enable optical CT reconstruction in the case of refractive index mismatch between different media in the scanner’s imaged volume. This improved the practicality of optical CT, as time-consuming mixing of liquids can be avoided. Finally, we applied the new laser scanner to the difficult dosimetry task of small-field measurement. We were able to obtain beam profiles and depth dose curves for 4 fields (3x3 cm2 and below) using one 15 cm diameter dosimeter, within 2 hours. Our gel dosimetry depth-dose curves agreed within ∼1.5% with Monte Carlo simulations. In conclusion, the developments reported here have brought optical CT dosimetry to a clinically useful level. Our techniques will be used to assist future research in gel dosimetry and radiotherapy treatment techniques