3D Dosimeter Research Articles

Using 3D dosimetry to quantify the Electron Return Effect (ERE) for MR-image-guided radiation therapy (MR-IGRT) applications

Image-guided radiation therapy (IGRT) using computed tomography (CT), cone-beam CT, MV on-board imager (OBI), and kV OBI systems have allowed for more accurate patient positioning prior to each treatment fraction. While these imaging modalities provide excellent bony anatomy image quality, MRI surpasses them in soft tissue image contrast for better visualization and tracking of soft tissue tumors with no additional radiation dose to the patient. A pre-clinical integrated 1.5 T magnetic resonance imaging and 7 MV linear accelerator system (MR-linac) allows for real-time tracking of soft tissues and adaptive treatment planning prior to each treatment fraction. However, due to the presence of a strong magnetic field from the MR component, there is a three dimensional (3D) change in dose deposited by the secondary electrons. Especially at nonhomogeneous anatomical sites with tissues of very different densities, dose enhancements and reductions can occur due to the Lorentz force influencing the trajectories of secondary electrons. These dose changes at tissue interfaces are called the electron return effect or ERE. This study investigated the ERE using 3D dosimeters.

Development of a methodology to study the effect of magnetic field on dose distributions in an MR-linac, using PRESAGE® and Monte Carlo calculations

At the Royal Marsden Hospital (RMH) and the Institute of Cancer Research (ICR) a new MR-linac is being installed and commissioned. Modifications to absorbed dose patterns will occur, because the paths of secondary electrons are deflected by Lorentz forces. In this paper, we describe a methodology to measure the effects of magnetic field on dose distributions, using the PRESAGE® 3D dosimeter and Monte Carlo (MC) simulations. A poly(methyl methacrylate) (PMMA) phantom has been developed to be positioned between the poles of an electromagnet and accommodate cylindrical samples of PRESAGE®. This phantom will be used to study the influence of different magnetic field strengths on radiation deposition from a Cobalt-60 (60Co) beam. Both the orientation of the samples with respect to the magnetic field and radiation beam, and the size of the air gap can be changed. Preliminary MC simulations with two PRESAGE® cylinders separated by an air gap, gave a good insight about the dosimetric effects that can be obtained with our newly-developed phantom.

Dosimetric comparison of DEFGEL and PAGAT formulae paired with an MRI acquisition

Normoxic Polyacrylamide Gels, or nPAGs, are 3D gel dosimeters that measure dose through the process of radiation-induced polymerization. Two nPAG formulae are DEFGEL and PAGAT, which are very similar, but differ mainly due to different weight fractions of monomers. The dosimetric resolutions of the two formulae when paired with a Spin-Echo (SE) MRI sequence and a monoexponential fit were compared over a range of 0-15 Gy. It was found that in the dose range 0-6 Gy the PAGAT formula generally showed a much finer dose resolution, while the DEFGEL formula showed a finer resolution from 8-15 Gy.

Tetrazolium salt monomers for gel dosimetry II: Dosimetric characterization of the ClearView™ 3D dosimeter

ClearView is a colourless radiochromic 3D dosimeter that is chemically and mechanically stable before and after irradiation. Upon irradiation, colour is generated from the radiochemical reduction of a tetrazolium salt suspended within a gellan gum gel. Here we present the dosimetric characterization of small and large volume samples (4 mL and 1 L, respectively) from three different batches of ClearView. The dose sensitivity was linear up to 80 Gy and constant within a batch but varied among batches between 3.4×10-3 and 4.1×10-3 Gy-1cm-1. Once generated, the radiation signal within the large samples remained unchanged for at least 58 days. Both electrons and photons produced the same signal at different energy levels; however, at dose rates below 400 cGy/min, a drop in sensitivity was observed. Overall, ClearView is best suited for relative dosimetry up to 80 Gy at high dose rates.

A prototype optical-CT system for PRESAGE 3D dosimeter readout

This work introduces the Duke Integrated-lens Optical Scanner (DIOS), a prototype optical-CT system designed for convenient and low-cost readout of PRESAGE 3D dosimeters. A key novelty of the DIOS is the incorporation of a multi-purpose light-collimating tank (the LC-tank). The LC-tank collimates light from a point source, maintains parallel ray geometry through a dosimeter mounted inside the tank, and refocuses emergent light onto a CCD detector. A second purpose is to dramatically reduce the amount of refractive matched fluid required in prior optical-CT scanners. This is achieved by substituting large quantities of refractive-matched fluid with solid RI-matched polyurethane. The advantages of DIOS include eliminating the need for expensive telecentric lenses, and eliminating the impracticality of large volumes of RI matched fluid. The DIOS is potentially more susceptible to stray-light artifacts. Preliminary phantom testing shows promising agreement between PRESAGE/DIOS readout and prior commissioned optical-CT scanners, as well as with Eclipse dose calculations.

Treatment planning and 3D dose verification of whole brain radiation therapy with hippocampal avoidance in rats

Despite increasing use of stereotactic radiosurgery, whole brain radiotherapy (WBRT) continues to have a therapeutic role in a selected subset of patients. Selectively avoiding the hippocampus during such treatment (HA-WBRT) emerged as a strategy to reduce the cognitive morbidity associated with WBRT and gave rise to a recently published the phase II trial (RTOG 0933) and now multiple ongoing clinical trials. While conceptually hippocampal avoidance is supported by pre-clinical evidence showing that the hippocampus plays a vital role in memory, there is minimal pre-clinic data showing that selectively avoiding the hippocampus will reduce radiation-induced cognitive decline. Largely the lack of pre-clinical evidence can be attributed to the technical hurdles associated with delivering precise conformal treatment the rat brain. In this work we develop a novel conformal HA-WBRT technique for Wistar rats, utilizing a 225kVp micro-irradiator with precise 3D-printed radiation blocks designed to spare hippocampus while delivering whole brain dose. The technique was verified on rodent-morphic Presage® 3D dosimeters created from micro-CT scans of Wistar rats with Duke Large Field-of-View Optical Scanner (DLOS) at 1mm isotropic voxel resolution. A 4-field box with parallel opposed AP-PA and two lateral opposed fields was explored with conformal hippocampal sparing aided by 3D-printed radiation blocks. The measured DVH aligned reasonably well with that calculated from SmART Plan Monte Carlo simulations with simulated blocks for 4-field HA-WBRT with both demonstrating hippocampal sparing of 20% volume receiving less than 30% the prescription dose.

Optical and X-ray computed tomography scanning of 3D dosimeters

Optical computed tomography (CT) scanners can measure attenuation in large-volume radiochromic dosimeters. Scattered light within the dosimeters is the major factor limiting dynamic range. Scattered light limits the use of large volume polymerization gels for small field dosimetry with broad-beam scanning geometries. Planar and single ray scanning geometries generate and accept less stray light due to scatter in the dosimeter. Transparent, radiochromic dosimeters provide higher dynamic range for accurate transmission imaging. X-ray CT scanning is applicable for large volume polymerization gel dosimeters.

OC-0258: Investigation of magnetic field effects on 3D dosimeters for MR-IGRT applications

OC-0258: Investigation of magnetic field effects on 3D dosimeters for MR-IGRT applications

Active heating of large gel dosimeters

Temperature uniformity of 3D dosimeters is key factor for accurate measurements. For gel dosimeters larger than 1 litre the time to change from storage to room temperature can be several hours. Local thermal histories within certain dosimeter materials can also cause measurable differences in ionizing radiation response. Passive heating by immersion in a water bath may not be optimum. Active uniform heating of 15 cm diameter, hydrogel, dosimeters using a near-infrared, incandescent light source reduced the time required to heat a hydrogel from 4 to 20°C uniformly from greater than five hours to less than 3 hours.

Preliminary investigation of the dosimetric properties of ClearView™ dosimeter

ClearView™ is a single-use radiochromic 3D dosimeter designed by Modus Medical Devices Inc. Some of the dosimetric characteristics of this gel, such as response as a function of dose and as a function of energy were investigated. Moreover, a 400 mL jar was irradiated with a 1x1 cm2 field and PDD and cross-profiles were compared to those obtained with active detectors. Small volumes of gels were found to exhibit a linear response to dose in the dose range 10-80 Gy. Promising PDD and cross profiles were obtained. Artefacts obtained in high resolution for the PDD are suspected to come from small inhomogeneities in the gel.

SP-0414: Experience with the ESTRO mobility grant; proton irradiation of a 3D dosimeter

SP-0414: Experience with the ESTRO mobility grant; proton irradiation of a 3D dosimeter

On the experimental validation of model-based dose calculation algorithms for 192Ir HDR brachytherapy treatment planning

There is an acknowledged need for the design and implementation of physical phantoms appropriate for the experimental validation of model-based dose calculation algorithms (MBDCA) introduced recently in 192Ir brachytherapy treatment planning systems (TPS), and this work investigates whether it can be met. A PMMA phantom was prepared to accommodate material inhomogeneities (air and Teflon), four plastic brachytherapy catheters, as well as 84 LiF TLD dosimeters (MTS-100M 1 × 1 × 1 mm3 microcubes), two radiochromic films (Gafchromic EBT3) and a plastic 3D dosimeter (PRESAGE). An irradiation plan consisting of 53 source dwell positions was prepared on phantom CT images using a commercially available TPS and taking into account the calibration dose range of each detector. Irradiation was performed using an 192Ir high dose rate (HDR) source. Dose to medium in medium, , was calculated using the MBDCA option of the same TPS as well as Monte Carlo (MC) simulation with the MCNP code and a benchmarked methodology. Measured and calculated dose distributions were spatially registered and compared. The total standard (k = 1) spatial uncertainties for TLD, film and PRESAGE were: 0.71, 1.58 and 2.55 mm. Corresponding percentage total dosimetric uncertainties were: 5.4–6.4, 2.5–6.4 and 4.85, owing mainly to the absorbed dose sensitivity correction and the relative energy dependence correction (position dependent) for TLD, the film sensitivity calibration (dose dependent) and the dependencies of PRESAGE sensitivity. Results imply a LiF over-response due to a relative intrinsic energy dependence between 192Ir and megavoltage calibration energies, and a dose rate dependence of PRESAGE sensitivity at low dose rates (<1 Gy min−1). Calculations were experimentally validated within uncertainties except for MBDCA results for points in the phantom periphery and dose levels <20%. Experimental MBDCA validation is laborious, yet feasible. Further work is required for the full characterization of dosimeter response for 192Ir and the reduction of experimental uncertainties.

Radiochromic and radiofluorogenic 3D solid polymer dosimeter; effect of the photoinitiator

Due to the recent increase in the complexity of external radiotherapy treatments, there is a need for a dosimeter capable of rendering a 3D dose profile to verify the absorbed dose with high spatial resolution. We are developing a solid and moldable 3D dosimeter material with the objective of determining the absorbed dose by measuring its fluorescence instead of the absorbance, which is a more established method. Measuring fluorescence could potentially provide higher sensitivity and spatial resolution, which is critical for 3D dosimetry. In this study, absorbance and fluorescence responses to gamma radiation are presented for doses up to 1 kGy. Since the material is cured by a photopolymerization process, the effect of the photoinitiator is also analyzed.

Introduction of This Special Issue on 3D Gel Dosimeters

Introduction of This Special Issue on 3D Gel Dosimeters

Preface to the Special Issue on 3D Gel Dosimeters

Preface to the Special Issue on 3D Gel Dosimeters

Chemically tuned linear energy transfer dependent quenching in a deformable, radiochromic 3D dosimeter

Most solid-state detectors, including 3D dosimeters, show lower signal in the Bragg peak than expected, a process termed quenching. The purpose of this study was to investigate how variation in chemical composition of a recently developed radiochromic, silicone-based 3D dosimeter influences the observed quenching in proton beams. The dependency of dose response on linear energy transfer, as calculated through Monte Carlo simulations of the dosimeter, was investigated in 60 MeV proton beams. We found that the amount of quenching varied with the chemical composition: peak-to-plateau ratios (1 cm into the plateau) ranged from 2.2 to 3.4, compared to 4.3 using an ionization chamber. The dose response, and thereby the quenching, was predominantly influenced by the curing agent concentration, which determined the dosimeter’s deformation properties. The dose response was found to be linear at all depths. All chemical compositions of the dosimeter showed dose-rate dependency; however this was not dependent on the linear energy transfer. Track-structure theory was used to explain the observed quenching effects. In conclusion, this study shows that the silicone-based dosimeter has potential for use in measuring 3D-dose-distributions from proton beams.

Review of gel dosimetry: a personal reflection

Gel dosimeters are manufactured from radiation sensitive chemicals which, upon irradiation, polymerize as a function of the absorbed radiation dose. These gel dosimeters have the capacity to record radiation dose distribution in three-dimensions (3D) compared to one and two-dimensional dosimeters. 3D dosimeters are radiologically soft-tissue equivalent and may be evaluated using magnetic resonance imaging (MRI), optical-computerized tomography (optical-CT), x-ray CT, ultrasound or vibrational spectroscopy.

Optical computed tomography in PRESAGE® three-dimensional dosimetry: Challenges and prospective.

With the advent of new complex but precise radiotherapy techniques, the demands for an accurate, feasible three-dimensional (3D) dosimetry system have been increased. A 3D dosimeter system generally should not only have accurate and precise results but should also feasible, inexpensive, and time consuming. Recently, one of the new candidates for 3D dosimetry is optical computed tomography (CT) with a radiochromic dosimeter such as PRESAGE®. Several generations of optical CT have been developed since the 90s. At the same time, a large attempt has been also done to introduce the robust dosimeters that compatible with optical CT scanners. In 2004, PRESAGE® dosimeter as a new radiochromic solid plastic dosimeters was introduced. In this decade, a large number of efforts have been carried out to enhance optical scanning methods. This article attempts to review and reflect on the results of these investigations.

Dosimetric comparison of 3D chemical dosimeters for use in modern RT/SRS quality assurance

Introduction QA of modern radiotherapy/radiosurgery techniques is quite challenging, requiring dosimetric systems with fine spatial resolution, water equivalence, lack of energy/dose rate dependence and, hopefully, 3D dose registering. Purpose A comparative dosimetric assessment of two 3D dosimeters, one custom and one commercially available, towards a dosimetric system with favorable characteristics for radiotherapy/radiosurgery QA. Materials and methods MRI scanned VIP Polymer Gel and Optical-CT scanned PRESAGE radiochromic dosimeters were calibrated side by side. Both were subjected to the same irradiation scheme, in an extensive dose range. The dosimetric characteristics compared included dose sensitivity, dose resolution, dose rate dependence, useful dose range, minimum detectable dose and, ultimately, the uncertainty budget of the measured dose distribution. Results & conclusion Both 3D dosimetric systems exhibited measured dose uncertainty appropriate for clinical dosimetry. The dose sensitivity of PRESAGE/OCT was higher resulting to improved dose resolution. This adds up to the increased ease of use of PRESAGE/OCT and availability of a dedicated optical scanner. However the increased noise in comparison to Polymer Gel/MRI and the occurrence of concentric ring and significant edge artifacts render this favorable characteristics debatable. Moreover indications of dose rate and energy dependence due to higher density have to be further investigated in order to end up with a dosimetric system to be used in the verification of complex radiotherapy/radiosurgery treatment plans. Disclosure Research conducted with financing by “IKY Postdoc Fellowships of Excellence in Greece – Siemens Programme” in the framework of the Hellenic Republic – Siemens Settlement Agreement.

TH-AB-BRA-11: Using 3D Dosimeters for the Investigation of the Electron Return Effect (ERE) in MR-Guided Radiation Therapy: A Feasibility Study

Purpose: To demonstrate the capability of 3D radiochromic PRESAGE and Fricke-type dosimeters to measure the influence of magnetic fields on dose distribution, including the electron return effect (ERE), for MR-guided radiation therapy applications. Methods: Short cylindrical 3D dosimeters with PRESAGE and Fricke-type formulations were created in-house prior to irradiations in a 1.5T/7MV MR-linac. Each dosimeter was prepared with a concentric cylindrical air cavity with diameters of 1.5 cm and 2.5 cm, and the diameters of the dosimeters were 7.2 cm and 8.8 cm for PRESAGE and Fricke-type respectively. The dosimeters were irradiated within the bore of the MR-linac with the flat face of the dosimeters perpendicular to the magnetic field. Dosimeters were irradiated to approximately 9 Gy and 29 Gy to the center of dosimeters with a 15×15 cm2 field. The PRESAGE dosimeter was scanned using an optical-CT 2 hours post-irradiation; the Fricke-type dosimeter was immediately imaged with the MR component of the MR-linac post-irradiation. Results: Axial slices of the dose distributions show a clear demonstration of the dose enhancement due to the ERE above the cavity and the region of reduced dose below the cavity. The regions of increased and reduced dose are rotated with respect to the radiation beam axis due to the average directional change of the electrons. Measurements from line profiles show the dose enhanced up to ∼0.5 cm around the cavity by up to a factor of 1.3 and 1.4 for PRESAGE and Fricke-type dosimeters respectively. Conclusion: PRESAGE and Fricke-type dosimeters are able to qualitatively measure the ERE with good agreement with previously published simulation and 2D dosimetry demonstrations of the ERE. Further investigation of these 3D dosimeters as promising candidates for quality assurance of MR-guided radiation therapy systems is encouraged to assess changes in response and measurement accuracy due to the magnetic field.