Selection of the Accelerating Structure of a Small Linear Electron Accelerator for Radiotherapy
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Engineering↗
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Screw instability in a linear Hall accelerator.
Screw instability in linear Hall accelerator
ELECTRIC FIELD↗
Electron-Beam Corkscrew Motion in an Advanced Linear Induction Accelerator
Scorpius is a multipulse linear induction accelerator (LIA) under development for flash radiography. Because it has substantially more cells than present LIAs, higher magnetic focusing fields are needed to suppress beam breakup (BBU). Therefore, it is more susceptible to corkscrew motion of the beam, which also depends on beam energy spread and focusing magnet misalignments. For energy spread and alignment tolerances expected for Scorpius, a magnetic tune designed to suppress BBU is shown to produce corkscrew motion within the range that can be controlled through the use of steering dipoles on existing LIAs. Finally, a gap-voltage modulation scheme is shown to almost completely eliminate chromatic effects such as corkscrew.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Accelerator Science: Circular vs. Linear
Particle accelerator are scientific instruments that allow scientists to collide particles together at incredible energies to study the secrets of the universe. However, there are many manners in which particle accelerators can be constructed. In this video, Fermilab’s Dr. Don Lincoln explains the pros and cons of circular and linear accelerators.
43 PARTICLE ACCELERATORS↗
Development and initial operating characteristics of the 20 megawatt linear plasma accelerator facility
A 20-megawatt linear plasma accelerator facility, a steady flow, Faraday-type plasma accelerator facility for high velocity aerodynamic testing, was constructed, developed, and brought to an operational status. The accelerator has a 63.5-mm-square and 0.5-meter-long channel and utilizes nitrogen-seeded with 2 % mole fraction of cesium vapor. Modification of the original accelerator design characteristics and the improvements necessary to make the arc heater a suitable plasma source are described. The measured accelerator electrode current distribution and the electrode-wall potential distributions are given. The computed and the measured values are in good agreement. Measured pitot pressure indicates that an accelerator exit velocity of 9.2 km/sec, is obtained with 30 of the 36 electrode pairs powered and corresponds to a velocity increase to about 2 1/4 times the computed entrance velocity. The computed stagnation enthalpy at the accelerator exit is 92 MJ/kg, and the mass density corresponds to an altitude of about 58 km. The 92 MJ/kg stagnation enthalpy corresponds to a kinetic energy content at low temperature equivalent to a velocity of 13.6 km/sec.
Carter, A. F.↗
Spatial orientation of optokinetic nystagmus and ocular pursuit during orbital space flight
On Earth, eye velocity of horizontal optokinetic nystagmus (OKN) orients to gravito-inertial acceleration (GIA), the sum of linear accelerations acting on the head and body. We determined whether adaptation to micro-gravity altered this orientation and whether ocular pursuit exhibited similar properties. Eye movements of four astronauts were recorded with three-dimensional video-oculography. Optokinetic stimuli were stripes moving horizontally, vertically, and obliquely at 30 degrees/s. Ocular pursuit was produced by a spot moving horizontally or vertically at 20 degrees/s. Subjects were either stationary or were centrifuged during OKN with 1 or 0.5 g of interaural or dorsoventral centripetal linear acceleration. Average eye position during OKN (the beating field) moved into the quick-phase direction by 10 degrees during lateral and upward field movement in all conditions. The beating field did not shift up during downward OKN on Earth, but there was a strong upward movement of the beating field (9 degrees) during downward OKN in the absence of gravity; this likely represents an adaptation to the lack of a vertical 1-g bias in-flight. The horizontal OKN velocity axis tilted 9 degrees in the roll plane toward the GIA during interaural centrifugation, both on Earth and in space. During oblique OKN, the velocity vector tilted towards the GIA in the roll plane when there was a disparity between the direction of stripe motion and the GIA, but not when the two were aligned. In contrast, dorsoventral acceleration tilted the horizontal OKN velocity vector 6 degrees in pitch away from the GIA. Roll tilts of the horizontal OKN velocity vector toward the GIA during interaural centrifugation are consistent with the orientation properties of velocity storage, but pitch tilts away from the GIA when centrifuged while supine are not. We speculate that visual suppression during OKN may have caused the velocity vector to tilt away from the GIA during dorsoventral centrifugation. Vertical OKN and ocular pursuit did not exhibit orientation toward the GIA in any condition. Static full-body roll tilts and centrifugation generating an equivalent interaural acceleration produced the same tilts in the horizontal OKN velocity before and after flight. Thus, the magnitude of tilt in OKN velocity was dependent on the magnitude of interaural linear acceleration, rather than the tilt of the GIA with regard to the head. These results favor a 'filter' model of spatial orientation in which orienting eye movements are proportional to the magnitude of low frequency interaural linear acceleration, rather than models that postulate an internal representation of gravity as the basis for spatial orientation.
Flight Experiment↗
SU-F-T-491: Photon Beam Matching Analysis at Multiple Sites Up to Twelve Years Post Installation
Purpose: To determine if the photon beams associated with several models of accelerators are matched with ‘Golden Beam’ data (VGBD) to assess treatment planning modeling and delivery. Methods: Six accelerators’ photon beams were evaluated to determine if they matched the manufacturer’s (Varian Medical Systems, Inc.) VGBD. Additional direct comparisons of the 6X and 18X beams using the manufacturer’s specification of Basic and Fine beam matching were also performed. The Cseries accelerator models were 21 EX (3), IX (2), and a IX Trilogy, ranging from three to twelve years post installation. Computerized beam scanning was performed (IBA Blue Phantom 2) with 2 CC13 ion chambers in water at 100 cm SSD. Dmax (10 cm2 field size), percentage depth dose (6 cm2, 10 cm2, 20 cm2, and 30 cm2 field sizes) and beam uniformity (10 cm2, 30 cm2 and 40 cm2 field sizes) were evaluated. Results: When comparing the beams with VGBD using the ‘Basic’ matching criteria, all beams were within the specifications ( 1.5mm at dmax, 1% PDD, and 2% Profiles). When considering the “Fine” matching criteria ( 1.5mm at dmax, 0.5% PDD, and 2% Profiles), only three of six 6MV beams and two of six high energy (five 18MV & one 15MV) beams passed. Direct comparisons between accelerators using the Clinac IX (installed 2012) as the reference beam datasets resulted in all 6 MV and 18MV beams meeting both the “Basic” and “Fine” criterion with the exception of two accelerators. Conclusion: Linear accelerators installed up to nine years apart are capable of meeting the manufacturers beam matching criteria for “Basic” matching. Without any adjustments most beams, when evaluated, may meet the “Fine” match criteria. The use of a single dataset (VGBD or designated accelerator reference data) for treatment planning commissioning is acceptable and can provide quality treatment delivery.
61 RADIATION PROTECTION AND DOSIMETRY↗
Neural processing of gravito-inertial cues in humans. I. Influence of the semicircular canals following post-rotatory tilt
Sensory systems often provide ambiguous information. Integration of various sensory cues is required for the CNS to resolve sensory ambiguity and elicit appropriate responses. The vestibular system includes two types of sensors: the semicircular canals, which measure head rotation, and the otolith organs, which measure gravito-inertial force (GIF), the sum of gravitational force and inertial force due to linear acceleration. According to Einstein's equivalence principle, gravitational force is indistinguishable from inertial force due to linear acceleration. As a consequence, otolith measurements must be supplemented with other sensory information for the CNS to distinguish tilt from translation. The GIF resolution hypothesis states that the CNS estimates gravity and linear acceleration, so that the difference between estimates of gravity and linear acceleration matches the measured GIF. Both otolith and semicircular canal cues influence this estimation of gravity and linear acceleration. The GIF resolution hypothesis predicts that inaccurate estimates of both gravity and linear acceleration can occur due to central interactions of sensory cues. The existence of specific patterns of vestibuloocular reflexes (VOR) related to these inaccurate estimates can be used to test the GIF resolution hypothesis. To investigate this hypothesis, we measured eye movements during two different protocols. In one experiment, eight subjects were rotated at a constant velocity about an earth-vertical axis and then tilted 90 degrees in darkness to one of eight different evenly spaced final orientations, a so-called "dumping" protocol. Three speeds (200, 100, and 50 degrees /s) and two directions, clockwise (CW) and counterclockwise (CCW), of rotation were tested. In another experiment, four subjects were rotated at a constant velocity (200 degrees /s, CW and CCW) about an earth-horizontal axis and stopped in two different final orientations (nose-up and nose-down), a so-called "barbecue" protocol. The GIF resolution hypothesis predicts that post-rotatory horizontal VOR eye movements for both protocols should include an "induced" VOR component, compensatory to an interaural estimate of linear acceleration, even though no true interaural linear acceleration is present. The GIF resolution hypothesis accurately predicted VOR and induced VOR dependence on rotation direction, rotation speed, and head orientation. Alternative hypotheses stating that frequency segregation may discriminate tilt from translation or that the post-rotatory VOR time constant is dependent on head orientation with respect to the GIF direction did not predict the observed VOR for either experimental protocol.
Non-NASA Center↗
Design of the 20-megawatt linear plasma accelerator facility
Design of 20-megawatt linear plasma accelerator facility
Carter, A. F.↗
When and how to treat an IMRT patient on a second accelerator without replanning?
When a linear accelerator is unavailable for treatment, a clinical decision is imminent regarding whether a patient should be treated on a linear accelerator other than the machine the patient was scheduled on, or whether treatment should be postponed until the original Linac becomes available. This work investigates the feasibility of switching patients to different accelerators for intensity-modulated radiation therapy (IMRT). We have performed Monte Carlo simulations of photon beams from different Linac models and vendors. Prostate and head and neck (H&N) treatment plans for Siemens Primus, Primart, and Varian 21EX accelerators are studied in this work. Dose distributions for given plans are recalculated using different beam data with the same nominal energy from different Linacs. We have compared dose-volume histograms (DVHs) and the maximum, the minimum, and the mean doses to the target and critical structures because of switching accelerators. In the process of switching a treatment plan to a different accelerator, issues exist, including optimum penumbra compensation, dose distribution at the boundary of target and critical structures, and multileaf collimator (MLC) leaf-width effects, which need to be considered and verified with measurements. Our Monte Carlo simulation results confirm that, for the cases we tested, the dose received by 95% of the planning target volume differs by 0.2% to 1.5% between Siemens Primus and Varian 21EX Linacs. The discrepancy is within our clinical acceptance criteria of 3% for IMRT treatments. In making the final decision on whether to switch machines or not, the tumor control probabilities (TCPs) based on a linear-quadratic model are compared. Based on the analyses performed in this work, it is therapeutically more beneficial to switch a patient to a different machine than to postpone a treatment until the original machine is available, especially for fast-growing tumors such as H&N cancers.
62 RADIOLOGY AND NUCLEAR MEDICINE↗
SU-F-T-631: Linear Accelerator-Based Frameless Extracranial Radiosurgery for Treatment of Occipital Neuralgia for Non-Surgical Candidates
Purpose: Occipital neuralgia is a condition wherein pain is transmitted by the occipital nerves. Non-invasive therapies generally alleviate symptoms; however, persistent or recurring pain may require invasive procedures. Repeated invasive procedures upon failure are considered higher risk and are often contraindicated due to compounding inherent risk. SRS has not been explored as a treatment option largely due to the extracranial nature of the target (as opposed to the similar, more established trigeminal neuralgia), but advances in linear-accelerator frameless-based SRS now present an opportunity to evaluate the novel potential of this modality for this application. Methods: Patient presented with severe occipital pain following decompression and fusion of the cervical vertebrae with prior intervention attempted via radiofrequency ablation yielding temporary pain cessation. A 0.6 mm slice spacing CT was obtained for treatment planning, and a cervical spine oriented 1.0 mm slice spacing CT myelogram was obtained for the purpose of defining the targeted C2 occipital dorsal root ganglion (to receive 80 Gy to the isocenter) and spinal cord. Results: The spinal cord was most proximally 12.0 mm from the isocenter receiving a maximum dose of 3.36 Gy, and doses to 0.35 and 1.2 cc of 1.84 Gy and 0.79 Gy, respectively. The brain maximum dose was 2.29 Gy. The treatment was successfully performed with a NovalisTX (Varian) equipped with ExacTrac stereoscopic x-ray image guidance (BrainLAB). Treatment time was 59 minutes for 18,323 MUs. Imaging was performed prior to each arc delivery resulting in twenty-one imaging sessions (twelve requiring positional corrections with the remaining verified within tolerance). The average deviation magnitude requiring a positional or rotational correction was 0.96±0.25 mm, 0.8±0.41° while the average deviation magnitude deemed within tolerance was 0.41±0.12 mm, 0.57±0.28°. Conclusion: Linear accelerator-based frameless radiosurgery provides an accurate, non-invasive alternative for treating occipital neuralgia where an invasive procedure is contraindicated.
60 APPLIED LIFE SCIENCES↗
Acceleration of linear and logarithmic convergence
Eleven different methods for accelerating convergence of sequences and series have been tested and compared on a wide range of test problems, including both linearly and logarithmically convergent series, monotone and alternating series. All but one of these methods are already in the literature, and they include both linear and nonlinear methods. The only methods found to accelerate convergence across the board were the u and v transforms of Levin and the theta algorithm of Brezinski. The paper gives detailed comparisons of all the tested methods on the basis of number of correct digits in the answer as a function of number of terms of the series used. A theorem of Germain-Bonne states that methods of a certain form which are exact on geometric series will accelerate linear convergence. The theorem applies to theta sub 2, and we have extended it to apply to Levin's transforms. No corresponding theorem is known for logarithmic convergence, but u, v, and theta are exact on certain large classes of logarithmic series, and all tested methods lacking this property failed to accelerate some logarithmically convergent series.
Smith, D. A.↗
Heat transfer to insulator wall of linear MGD accelerator
Heat transfer to insulator wall of linear MGD accelerator attached to shock tube, comparing measurements with Hartmann boundary layer analysis
Cochran, R. A.↗
MO-FG-202-05: Identifying Treatment Planning System Errors in IROC-H Phantom Irradiations
Purpose: Treatment Planning System (TPS) errors can affect large numbers of cancer patients receiving radiation therapy. Using an independent recalculation system, the Imaging and Radiation Oncology Core-Houston (IROC-H) can identify institutions that have not sufficiently modelled their linear accelerators in their TPS model. Methods: Linear accelerator point measurement data from IROC-H’s site visits was aggregated and analyzed from over 30 linear accelerator models. Dosimetrically similar models were combined to create “classes”. The class data was used to construct customized beam models in an independent treatment dose verification system (TVS). Approximately 200 head and neck phantom plans from 2012 to 2015 were recalculated using this TVS. Comparison of plan accuracy was evaluated by comparing the measured dose to the institution’s TPS dose as well as the TVS dose. In cases where the TVS was more accurate than the institution by an average of >2%, the institution was identified as having a non-negligible TPS error. Results: Of the ∼200 recalculated plans, the average improvement using the TVS was ∼0.1%; i.e. the recalculation, on average, slightly outperformed the institution’s TPS. Of all the recalculated phantoms, 20% were identified as having a non-negligible TPS error. Fourteen plans failed current IROC-H criteria; the average TVS improvement of the failing plans was ∼3% and 57% were found to have non-negligible TPS errors. Conclusion: IROC-H has developed an independent recalculation system to identify institutions that have considerable TPS errors. A large number of institutions were found to have non-negligible TPS errors. Even institutions that passed IROC-H criteria could be identified as having a TPS error. Resolution of such errors would improve dose delivery for a large number of IROC-H phantoms and ultimately, patients.
60 APPLIED LIFE SCIENCES↗
Patterns of Care, Tolerability, and Safety of the First Cohort of Patients Treated on a Novel High-Field MR-Linac Within the MOMENTUM Study: Initial Results From a Prospective Multi-Institutional Registry
High-field magnetic resonance-linear accelerators (MR-Linacs), linear accelerators combined with a diagnostic magnetic resonance imaging (MRI) scanner and online adaptive workflow, potentially give rise to novel online anatomic and response adaptive radiation therapy paradigms. The first high-field (1.5T) MR-Linac received regulatory approval in late 2018, and little is known about clinical use, patient tolerability of daily high-field MRI, and toxicity of treatments. Herein we report the initial experience within the MOMENTUM Study (NCT04075305), a prospective international registry of the MR-Linac Consortium.
62 RADIOLOGY AND NUCLEAR MEDICINE↗
Free electron lasers driven by linear induction accelerators: High power radiation sources
The technology of Free Electron Lasers (FELs) and linear induction accelerators (LIAs) is addressed by outlining the following topics: fundamentals of FELs; basic concepts of linear induction accelerators; the Electron Laser Facility (a microwave FEL); PALADIN (an infrared FEL); magnetic switching; IMP; and future directions (relativistic klystrons). This presentation is represented by viewgraphs only.
Orzechowski, T. J.↗
Steering an intense relativistic electron beam in a linear induction accelerator
The best electron beam transport through a linear induction accelerator (LIA) is achieved when the beam is well centered on the magnetic axis of the focusing solenoids. Since the beam may be injected offset from, or at an angle to, the centerline, dipole magnets are usually provided as a means to steer the beam and center it. Steering may be accomplished by trial and error, but this is very time consuming, especially for accelerators with a low repetition rate for beam position measurements and dipole adjustments. With this in mind, this article presents a steering method requiring a minimal number of measurements and adjustments to position the beam at any desired location, including positioning on the centerline.
47 OTHER INSTRUMENTATION↗
Beam Breakup in a Solid-State Powered Linear Induction Accelerator
Scorpius is a new multi-pulse linear induction accelerator (LIA) for flash radiography of large-scale, explosively-driven hydrodynamic experiments. It has been proposed to use solid-state pulsed-power modules to provide the accelerating voltages for Scorpius. It is expected that this approach can provide four programmable 50-kV accelerating pulses to a single gap.
43 PARTICLE ACCELERATORS↗