Beddar, Sam

Abstract Background Incorporating image guidance into ion beam therapy is critical for minimizing beam range uncertainties and realizing the modality's potential. One promising avenue for image guidance is to capture transmission ion radiographs (iRads) before and/or during treatment. iRad image quality is typically maximized using a single‐event imaging system, which involves tracking individual ions, albeit the approach is generally not suited to clinical beam settings. An alternative faster and clinically compatible method is integrated mode imaging, where individual pencil beam data is acquired, rather than single ion data. To evaluate the usefulness of transmission ion imaging for image guidance, it is crucial to evaluate the image quality of integrated mode iRad systems. Purpose We report extensive image quality metrics of integrated mode carbon ion radiographs (cRads) and compare them with proton radiographs (pRads). Methods iRads were obtained at the Marburg Ion Beam Therapy Center using a plastic volumetric scintillator equipped with CCD cameras. The detector captures orthogonal views of the 3D energy deposition in the scintillator from individual pencil beams. Four phantoms were scanned using a field of view and a beam spacing of 1 mm. First, 9 tissue‐substitute inserts were used to evaluate water equivalent thickness (WET) accuracy. Radiographs of those inserts were reconstructed for beam spacings ranging from 1 to 7 mm to evaluate the impact of spacing on quantitative accuracy. For spatial resolution, custom 3D printed line pair (lp) modules ranging from 0.5 to 10 lp/cm were scanned. To evaluate low contrast detectability, a custom 3D printed low contrast module consisting of 20 holes with depths ranging from 1 to 8 mm and diameters from 1 to 10 mm was scanned. iRads of an anthropomorphic head phantom were also obtained. Results Spatial resolution and low contrast detection are systematically improved for cRads compared to pRads. Image resolution was 3.7 lp/cm for cRads and 1.7 lp/cm for pRads in the center of the field of view. Spatial resolution was found to vary with the object's location in the field of view. While pRads could mostly resolve low contrast holes of 10 mm in diameter, cRads could resolve holes of up in 4 mm diameter. WET accuracy is similar for both ion species, with a root mean squared error of approximately 1 mm. WET accuracy was stable (maximum of 0.1 mm increase) across beam spacings, although important under‐sampling artifacts were observed for iRads reconstructed using large beam spacings, especially for cRads. iRads of the anthropomorphic head phantom showed improved apparent contrast using cRads, especially to identify bony structures. Conclusions This work is the first investigation of image quality metrics such as spatial resolution and low contrast detectability for integrated mode cRads, with a full comparison with pRads. Enhanced image quality is obtained with cRads compared to pRads, although pRads still maintain high WET accuracy and deliver image quality within acceptable bounds.

AbstractBackgroundMotion of lung tumors during radiotherapy leads to decreased accuracy of the delivered dose distribution. This is especially true for proton radiotherapy due to the finite range of the proton beam. Methods for mitigating motion rely on knowing the position of the tumor during treatment.PurposeProton radiography uses the treatment beam, at an energy high enough to traverse the patient, to produce a radiograph. This work shows the first results of using an integrated‐mode proton radiography system to track the position of moving objects in an experimental phantom study; demonstrating the potential of using this method for measuring tumor motion.MethodsProton radiographs of an anthropomorphic lung phantom, with a motor‐driven tumor insert, were acquired approximately every 1 s, using tumor inserts of 10, 20, and 30 mm undergoing a known periodic motion. The proton radiography system used a monolithic scintillator block and digital cameras to capture the residual range of each pencil beam passing through the phantom. These ranges were then used to produce a water equivalent thickness map of the phantom. The centroid of the tumor insert in the radiographs was used to determine its position. This measured position was then compared to the known motion of the phantom to determine the accuracy.ResultsSubmillimeter accuracy on the measurement of the tumor insert was achieved when using a 30 mm tumor insert with a period of 24 s and was found to be improved for decreasing motion amplitudes with a mean absolute error (MAE) of 1.0, 0.9, and 0.7 mm for 20, 15, and 10 mm respectively. Using smaller tumor inserts reduced the accuracy with a MAE of 1.8 and 1.9 mm for a 20 and 10 mm insert respectively undergoing a periodic motion with an amplitude of 20 mm and a period of 24 s. Using a shorter period resulted in significant motion artifacts reducing the accuracy to a MAE of 2.2 mm for a 12 s period and 3.1 mm for a 6 s period for the 30 mm insert with an amplitude of 20 mm.ConclusionsThis work demonstrates that the position of a lung tumor insert in a realistic anthropomorphic phantom can be measured with high accuracy using proton radiographs. Results show that the accuracy of the position measurement is the highest for slower tumor motions due to a reduction in motion artifacts. This indicates that the primary obstacle to accurate measurement is the speed of the radiograph acquisition. Although the slower tumor motions used in this study are not clinically realistic, this work demonstrates the potential for using proton radiography for measuring tumor motion with an increased scanning speed that results in a decreased acquisition time.

Proton radiography is a promising development in proton therapy, and researchers are currently exploring optimal detector materials to construct proton radiography detector arrays. High-density glass scintillators may improve integrating-mode proton radiography detectors by increasing spatial resolution and decreasing detector thickness. We evaluated several new scintillators, activated with europium or terbium, with proton response measurements and Monte Carlo simulations, characterizing relative luminosity, ionization quenching, and proton radiograph spatial resolution. We applied a correction based on Birks’s analytical model for ionization quenching. The data demonstrate increased relative luminosity with increased activation element concentration, and higher relative luminosity for samples activated with europium. An increased glass density enables more compact detector geometries and higher spatial resolution. These findings suggest that a tungsten and gadolinium oxide-based glass activated with 4% europium is an ideal scintillator for testing in a full-size proton radiography detector.

Abstract Integrated-mode proton radiography leading to water equivalent thickness (WET) maps is an avenue of interest for motion management, patient positioning, and in vivo range verification. Radiographs can be obtained using a pencil beam scanning setup with a large 3D monolithic scintillator coupled with optical cameras. Established reconstruction methods either (1) involve a camera at the distal end of the scintillator, or (2) use a lateral view camera as a range telescope. Both approaches lead to limited image quality. The purpose of this work is to propose a third, novel reconstruction framework that exploits the 2D information provided by two lateral view cameras, to improve image quality achievable using lateral views. The three methods are first compared in a simulated Geant4 Monte Carlo framework using an extended cardiac torso (XCAT) phantom and a slanted edge. The proposed method with 2D lateral views is also compared with the range telescope approach using experimental data acquired with a plastic volumetric scintillator. Scanned phantoms include a Las Vegas (contrast), 9 tissue-substitute inserts (WET accuracy), and a paediatric head phantom. Resolution increases from 0.24 (distal) to 0.33 lp mm −1 (proposed method) on the simulated slanted edge phantom, and the mean absolute error on WET maps of the XCAT phantom is reduced from 3.4 to 2.7 mm with the same methods. Experimental data from the proposed 2D lateral views indicate a 36% increase in contrast relative to the range telescope method. High WET accuracy is obtained, with a mean absolute error of 0.4 mm over 9 inserts. Results are presented for various pencil beam spacing ranging from 2 to 6 mm. This work illustrates that high quality proton radiographs can be obtained with clinical beam settings and the proposed reconstruction framework with 2D lateral views, with potential applications in adaptive proton therapy.

Abstract The purpose of this study is to compare the image quality of an integrating proton radiography (PR) system, composed of a monolithic scintillator and two digital cameras, using integral lateral-dose and integral depth-dose image reconstruction techniques. Monte Carlo simulations were used to obtain the energy deposition in a 3D monolithic scintillator detector (30 × 30 × 30 cm 3 poly vinyl toluene organic scintillator) to create radiographs of various phantoms—a slanted aluminum cube for spatial resolution analysis and a Las Vegas phantom for contrast analysis. The light emission of the scintillator was corrected using Birks scintillation model. We compared two integrating PR methods and the expected results from an idealized proton tracking radiography system. Four different image reconstruction methods were utilized in this study: integral scintillation light projected from the beams-eye view, depth-dose based reconstruction methods both with and without optimization, and single particle tracking PR was used for reference data. Results showed that heterogeneity artifact due to medium-interface mismatch was identified from the Las Vegas phantom simulated in air. Spatial resolution was found to be highest for single-event reconstruction. Contrast levels, ranked from best to worst, were found to correspond to particle tracking, optimized depth-dose, depth-dose, and projection-based image reconstructions. The image quality of a monolithic scintillator integrating PR system was sufficient to warrant further exploration. These results show promise for potential clinical use as radiographic techniques for visualizing internal patient anatomy during proton radiotherapy.

Abstract Existing systems for proton beam dosimetry are limited in their ability to provide a complete, accurate, and detailed account of volumetric dose distribution. In this work, we describe the design and development of a portable, fast, and reusable liquid scintillator-based three-dimensional (3D) optical detection system for use in proton therapy. Our long-term goal is to use this system clinically for beam characterization, dosimetry, and quality assurance studies of discrete spot scanning proton beam systems. The system used a 20 × 20 × 20 cm 3 liquid scintillator volume. Three mutually orthogonal cameras surrounding this volume captured scintillation photons emitted in response to the proton beams. The cameras exhibited a mean spatial resolution of 0.21 mm over the complete detection volume and a temporal resolution of 11 ms. The system is shown to be capable of capturing all 94 beam energies delivered by a synchrotron and performing rapid beam range measurements with a mean accuracy of 0.073 ± 0.030 mm over all energies. The range measurement uncertainty for doses less than 1 cGy was found to be ±0.355 mm, indicating high precision for low dose detection. Finally, we demonstrated that using multiple cameras allowed for the precise locations of the delivered beams to be tracked in 3D. We conclude that this detector is capable of real-time and accurate tracking of dynamic spot beam deliveries in 3D. The high-resolution light profiles it generates will be useful for future 3D construction of dose maps.