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Consideration of HTS rapid-cycling magnet for staged muon acceleration

The HTS conductor hysteresis dominates magnet cable power loss but is independent of the magnetic field ramping rate. This makes the HTS conductor suitable to power the rapid-cycling accelerator magnet. We present a possible application of the HTS rapid-cycling magnet as outlined in [1,2] for the staged muon acceleration including the front-end Recirculating Linear Accelerator and the followed-up Rapid Cycling Synchrotrons delivering the muon beams to the Muon Collider.[1] H. Piekarz, S. Otten, A. Kario, H. ten Kate, “Rapid-cycling HTS magnet for muon acceleration”, US MC Inaugural Meeting, FERMILAB-POSTER-24-0219-AD, August 7-9, 2024[2] H. Piekarz, B. Claypool, S. Hays, M. Kufer, V. Shiltsev, “Record High Ramping Rates in HTS Based Supercond. Accelerator Magnet”, MT 27, IEEE Trans. on Applied Superccond, 32 (2022) 6, 4100404

Piekarz, Henryk [Fermilab]↗

Experimental and Numerical Studies on the Proposed Application of Hollow Electron Beam Collimation for the LHC at CERN

This thesis work was carried out in the framework of the U.S. LHC Accelerator Research Program (USLARP), a collaboration between the European Organization for Nuclear Research (CERN) and the U.S. Department of Energy. The first half of the work was completed at Fermilab (USA), the location of the Tevatron, a proton-antiproton collider and the second largest particle collider in the world. The second half was completed at CERN (Switzerland), the location of the largest proton collider in the world (Large Hadron Collider (LHC)). This thesis characterizes a Hollow Electron Beam (HEB) for possible usage at the LHC to enhance its collimation through Hollow Electron Beam Lenses (HEBLs). Collimation is a long established principle in high energy particle accelerators. Hollow Electron Beam Collimation (HEBC) aims to enhance current collimation systems by controlling diffusion of primary halo particles into the limiting aperture. It works on the principle of a transverse radial electric field that kicks the primary halo particles outwards upon each pass in a multi-pass system. The transverse field is produced by a HEB that is coaxially aligned with the accelerator beam, producing a negligible electric field in the center and a strong transverse electric field at amplitudes higher than the inner radius of the electron beam. Ideally, halo particles are affected without perturbation of the beam core. One of the main advantages of this system is to decrease the dependence on instantaneous loss spikes and beam jitter. A solid experimental basis of HEBC was accumulated at the Tevatron. The application of this technique at the LHC is now under investigation. The aim of this thesis is to present a preliminary report to support a future optimal conceptual design report. It characterizes the available hardware in order to facilitate the design of a Hollow Electron Gun (HEG) for the LHC, characterizes the effect on beam diffusion by determining the transverse electric fields of the electron beam and initiates 3D simulations in order to determine the effect of the HEBL on the beam core. Experiments were conducted in the Tevatron Electron Lens Test Stand (TELTS) at Fermi National Accelerator Lab (Fermilab) (USA) in the lower Linear Accelerator (LINAC) gallery.

43 PARTICLE ACCELERATORS↗

Experimental Particle Physics

The high energy physics group at the University of South Carolina, under the leadership of Profs. S.R. Mishra, R. Petti, M.V. Purohit, J.R. Wilson (co-PI's), and C. Rosenfeld (PI), engaged in studies in "Experimental Particle Physics." The group collaborated with similar groups at other universities and at national laboratories to conduct experimental studies of elementary particle properties. We utilized the particle accelerators at the Fermi National Accelerator Laboratory (Fermilab) in Illinois, the Stanford Linear Accelerator Center (SLAC) in California, and the European Center for Nuclear Research (CERN) in Switzerland. Mishra, Rosenfeld, and Petti worked predominantly on neutrino experiments. Experiments conducted in the last fifteen years that used cosmic rays and the core of the sun as a source of neutrinos showed conclusively that, contrary to the former conventional wisdom, the "flavor" of a neutrino is not immutable. A neutrino of flavor "e," "mu," or "tau," as determined from its provenance, may swap its identity with one of the other flavors -- in our jargon, they "oscillate." The oscillation phenomenon is extraordinarily difficult to study because neutrino interactions with our instruments are exceedingly rare -- they travel through the earth mostly unimpeded -- and because they must travel great distances before a substantial proportion have made the identity swap. Three of the experiments that we worked on, MINOS, NOvA, and LBNE utilize a beam of neutrinos from an accelerator at Fermilab to determine the parameters governing the oscillation. Two other experiments that we worked on, NOMAD and MIPP, provide measurements supportive of the oscillation experiments. Good measurements of the neutrino oscillation parameters may constitute a "low energy window" on related phenomena that are otherwise unobservable because they would occur only at energies way above the reach of conceivable accelerators. Purohit and Wilson participated in the BaBar experiment, which collected data at SLAC until 2008. They continued to analyze the voluminous BaBar data with an emphasis on precision tests of Quantum Chromodynamics and on properties of the "eta_B," a bottom quark paired in a meson with a strange quark. The ATLAS experiment became the principal research focus for Purohit. One of the world's largest pieces of scientific equipment, ATLAS observes particle collisions at the highest-energy particle accelerator ever built, the Large Hadron Collider (LHC) at CERN. Our efforts on ATLAS included participation in the commissioning, calibration, and installation of components called "CSCs". The unprecedented energy of 14 TeV enabled the ATLAS and CMS collaborations to declare discovery of the famous Higgs particle in 2012.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

SU-E-J-205: Monte Carlo Modeling of Ultrasound Probes for Real-Time Ultrasound Image-Guided Radiotherapy

Purpose: To quantify the effect of ultrasound (US) probe beam attenuation for radiation therapy delivered under real-time US image guidance by means of Monte Carlo (MC) simulations. Methods: MC models of two Philips US probes, an X6-1 matrix-array transducer and a C5-2 curved-array transducer, were built based on their CT images in the EGSnrc BEAMnrc and DOSXYZnrc codes. Due to the metal parts, the probes were scanned in a Tomotherapy machine with a 3.5 MV beam. Mass densities in the probes were assigned based on an electron density calibration phantom consisting of cylinders with mass densities between 0.2–8.0 g/cm{sup 3}. Beam attenuation due to the probes was measured in a solid water phantom for a 6 MV and 15 MV 15x15 cm{sup 2} beam delivered on a Varian Trilogy linear accelerator. The dose was measured with the PTW-729 ionization chamber array at two depths and compared to MC simulations. The extreme case beam attenuation expected in robotic US image guided radiotherapy for probes in upright position was quantified by means of MC simulations. Results: The 3.5 MV CT number to mass density calibration curve was found to be linear with R{sup 2} > 0.99. The maximum mass densities were 4.6 and 4.2 g/cm{sup 3} in the C5-2 and X6-1 probe, respectively. Gamma analysis of the simulated and measured doses revealed that over 98% of measurement points passed the 3%/3mm criteria for both probes and measurement depths. The extreme attenuation for probes in upright position was found to be 25% and 31% for the C5-2 and X6-1 probe, respectively, for both 6 and 15 MV beams at 10 cm depth. Conclusion: MC models of two US probes used for real-time image guidance during radiotherapy have been built. As a Result, radiotherapy treatment planning with the imaging probes in place can now be performed. J Schlosser is an employee of SoniTrack Systems, Inc. D Hristov has financial interest in SoniTrack Systems, Inc.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Initial experimental evidence of self-collimation of target-normal-sheath-accelerated proton beam in a stack of conducting foils

Phenomena consistent with self-collimation (or weak self-focusing) of laser target-normal-sheath-accelerated protons was experimentally observed for the first time, in a specially engineered structure (“lens”) consisting of a stack of 300 thin aluminum foils separated by 50 μm vacuum gaps. The experiments were carried out in a “passive environment,” i.e., no external fields applied, neutralization plasma or injection of secondary charged particles was imposed. Experiments were performed at the petawatt “PHELIX” laser user facility (E = 100 J, Δt = 400 fs, λ = 1062 nm) at the “Helmholtzzentrum für Schwerionenforschung–GSI” in Darmstadt, Germany. The observed rms beam spot reduction depends inversely on energy, with a focusing degree decreasing monotonically from 2 at 5.4 MeV to 1.5 at 18.7 MeV. The physics inside the lens is complex, resulting in a number of different mechanisms that can potentially affect the particle dynamics within the structure. We present a plausible simple interpretation of the experiment in which the combination of magnetic self-pinch forces generated by the beam current together with the simultaneous reduction of the repulsive electrostatic forces due to the foils are the dominant mechanisms responsible for the observed focusing/collimation. This focusing technique could be applied to a wide variety of space-charge dominated proton and heavy ion beams and impact fields and applications, such as HEDP science, inertial confinement fusion in both fast ignition and heavy ion fusion approaches, compact laser-driven injectors for a Linear Accelerator (LINAC) or synchrotron, medical therapy, materials processing, etc.

43 PARTICLE ACCELERATORS↗

SU-E-T-606: A Novel Integrated VMAT/IMRT Technique For the Treatment of Non-Small Cell Lung Cancer

Purpose: To investigate a novel Integrated VMAT/IMRT technique which combines volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) for non-small cell lung cancer (NSCLC). Methods: 2 partial arcs VMAT, 5-field IMRT and Integrated VMAT/IMRT plans were created for 17 patients with NSCLC. The Integrated VMAT/IMRT technique consisted of 2 partial VMAT arcs and 5 IMRT fields. The dose distribution of planning target volume (PTV) and organs at risk (OARs) for Integrated VMAT/IMRT was compared with IMRT and VMAT. The monitor units (MUs) and treatment delivery time were also evaluated. For each plan, a dry run was performed to assess the dosimetric accuracy with MatriXX from IBA. Results: Integrated VMAT/IMRT significantly improved the target conformity and homogeneity. The V30 of normal lung for Integrated plans was significantly lower than IMRT plans (8.4% vs 9.2%; p<0.05). The V5 and mean lung dose (MLD) of normal lung for Integrated plans were 9.8% and 4.6% lower than VMAT plans (p<0.05). The maximum dose of spinal cord for Integrated plans was 4.9 Gy lower than IMRT plans (p<0.05). The mean delivery time of IMRT, VMAT and Integrated plans was 280 s, 114 s, and 327 s, respectively. The mean MUs needed for IMRT, VMAT and Integrated plans were 933, 512, and 737, respectively. The gamma pass rates were beyond 90% at the 3%/3 mm criteria when the gantry angles were set to 0° for pretreatment verification. Conclusion: Integrated VMAT/IMRT technique significantly reduced V5, V10 and MLD of normal lung compared with VMAT, and the irradiated volume of the OARs receiving medium to high dose with fewer MUs compared with IMRT. Integrated VMAT/IMRT technique can be a feasible radiotherapy technique with better plan quality and accurately delivered on the linear accelerator. Ruijie Yang was funded by the grant project: National Natural Science Foundation of China (No. 81071237). Other authors have no competing interest for this work.

60 APPLIED LIFE SCIENCES↗

Photoluminescence of radiation-induced color centers in lithium fluoride thin films for advanced diagnostics of proton beams

Systematic irradiation of thermally evaporated 0.8 μm thick polycrystalline lithium fluoride films on glass was performed by proton beams of 3 and 7 MeV energies, produced by a linear accelerator, in a fluence range from 10{sup 11} to 10{sup 15} protons/cm{sup 2}. The visible photoluminescence spectra of radiation-induced F{sub 2} and F{sub 3}{sup +} laser active color centers, which possess almost overlapping absorption bands at about 450 nm, were measured under laser pumping at 458 nm. On the basis of simulations of the linear energy transfer with proton penetration depth in LiF, it was possible to obtain the behavior of the measured integrated photoluminescence intensity of proton irradiated LiF films as a function of the deposited dose. The photoluminescence signal is linearly dependent on the deposited dose in the interval from 10{sup 3} to about 10{sup 6 }Gy, independently from the used proton energies. This behavior is very encouraging for the development of advanced solid state radiation detectors based on optically transparent LiF thin films for proton beam diagnostics and two-dimensional dose mapping.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

SU-E-T-421: Feasibility Study of Volumetric Modulated Arc Therapy with Constant Dose Rate for Endometrial Cancer

Purpose: To investigate the feasibility, efficiency, and delivery accuracy of volumetric modulated arc therapy with constant dose rate (VMAT-CDR) for whole-pelvic radiotherapy (WPRT) of endometrial cancer. Methods: The nine-Field intensity-modulated radiotherapy (IMRT), VMAT with variable dose-rate (VMAT-VDR), and VMAT-CDR plans were created for 9 patients with endometrial cancer undergoing WPRT. The dose distribution of planning target volume (PTV), organs at risk (OARs), and normal tissue (NT) were compared. The monitor units (MUs) and treatment delivery time were also evaluated. For each VMAT-CDR plan, a dry Run was performed to assess the dosimetric accuracy with MatriXX from IBA. Results: Compared with IMRT, the VMAT-CDR plans delivered a slightly greater V20 of the bowel, bladder, pelvis bone, and NT, but significantly decreased the dose to the high-dose region of the rectum and pelvis bone. The MUs Decreased from 1105 with IMRT to 628 with VMAT-CDR. The delivery time also decreased from 9.5 to 3.2 minutes. The average gamma pass rate was 95.6% at the 3%/3 mm criteria with MatriXX pretreatment verification for 9 patients. Conclusion: VMAT-CDR can achieve comparable plan quality with significant shorter delivery time and smaller number of MUs compared with IMRT for patients with endometrial cancer undergoing WPRT. It can be accurately delivered and be an alternative to IMRT on the linear accelerator without VDR capability. This work is supported by the grant project, National Natural; Science Foundation of China (No. 81071237)

60 APPLIED LIFE SCIENCES↗

SU-E-J-16: A Review of the Magnitude of Patient Imaging Shifts in Relation to Departmental Policy Changes

Purpose: To evaluate how changes in imaging policy affect the magnitude of shifts applied to patients. Methods: In June 2012, the department's imaging policy was altered to require that any shifts derived from imaging throughout the course of treatment shall be considered systematic only after they were validated with two data points that are consistent in the same direction. Multiple additions and clarifications to the imaging policy were implemented throughout the course of the data collection, but they were mostly of administrative nature. Entered shifts were documented in MOSAIQ (Elekta AB) through the localization offset. The MOSAIQ database was queried to identify a possible trend. A total of 25,670 entries were analyzed, including four linear accelerators with a combination of MV planar, kV planar and kV three dimensional imaging. The monthly average of the magnitude of the vector was used. Plan relative offsets were excluded. During the evaluated period of time, one of the satellite facilities acquired and implemented Vision RT (AlignRT Inc). Results: After the new policy was implemented the shifts variance and standard deviation decreased. The decrease is linear with time elapsed. Vision RT implementation at one satellite facility reduced the number of overall shifts, specifically for breast patients. Conclusion: Changes in imaging policy have a significant effect on the magnitude of shifts applied to patients. Using two statistical points before applying a shift as persistent decreased the overall magnitude of the shifts applied to patients.

60 APPLIED LIFE SCIENCES↗

Thermal and epithermal neutron fluence rate gradient measurements by PADC detectors in LINAC radiotherapy treatments-field

LINAC VARIAN 2100 is where energetic electrons produce Bremsstrahlung radiation, with energies above the nucleon binding energy (E≈5.5MeV). This radiation induce (γ,n) and (e,e’n) reactions mainly in the natural tungsten target material (its total photoneutron cross section is about 4000 mb in a energy range from 9-17 MeV). These reactions may occur also in other components of the system (e.g. multi leaf collimator). During radiation treatment the human body may receive an additional dose inside and outside the treated volume produced by the mentioned nuclear reactions. We measured the neutron density at the treatment table using nuclear track detectors (PADC-NTD). These covered by a boron-converter are employed, including a cadmium filter, to determine the ratio between two groups of neutron energy, i.e. thermal and epithermal. The PADC-NTD detectors were exposed to the radiation field at the iso-center during regular operation of the accelerator. Neutron are determined indirectly by the converting reaction {sup 10}B(n,α){sup 7}Li the emerging charged particle leave their kinetic energy in the PADC forming a latent nuclear track, enlarged by chemical etching (6N, NaOH, 70°C). Track density provides information on the neutron density through calibration coefficient (∼1.6 10{sup 4} neutrons /track) obtained by a californium source. We report the estimation of the thermal and epithermal neutron field and its gradient for photoneutrons produced in radiotherapy treatments with 18 MV linear accelerators. It was obsered that photoneutron production have higher rate at the iso-center.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

SU-E-T-580: Comparison of Cervical Carcinoma IMRT Plans From Four Commercial Treatment Planning Systems (TPS)

Purpose: Different treatment planning systems (TPS) use different treatment optimization and leaf sequencing algorithms. This work compares cervical carcinoma IMRT plans optimized with four commercial TPSs to investigate the plan quality in terms of target conformity and delivery efficiency. Methods: Five cervical carcinoma cases were planned with the Corvus, Monaco, Pinnacle and Xio TPSs by experienced planners using appropriate optimization parameters and dose constraints to meet the clinical acceptance criteria. Plans were normalized for at least 95% of PTV to receive the prescription dose (Dp). Dose-volume histograms and isodose distributions were compared. Other quantities such as Dmin(the minimum dose received by 99% of GTV/PTV), Dmax(the maximum dose received by 1% of GTV/PTV), D100, D95, D90, V110%, V105%, V100% (the volume of GTV/PTV receiving 110%, 105%, 100% of Dp), conformity index(CI), homogeneity index (HI), the volume of receiving 40Gy and 50 Gy to rectum (V40,V50) ; the volume of receiving 30Gy and 50 Gy to bladder (V30,V50) were evaluated. Total segments and MUs were also compared. Results: While all plans meet target dose specifications and normal tissue constraints, the maximum GTVCI of Pinnacle plans was up to 0.74 and the minimum of Corvus plans was only 0.21, these four TPSs PTVCI had significant difference. The GTVHI and PTVHI of Pinnacle plans are all very low and show a very good dose distribution. Corvus plans received the higer dose of normal tissue. The Monaco plans require significantly less segments and MUs to deliver than the other plans. Conclusion: To deliver on a Varian linear-accelerator, the Pinnacle plans show a very good dose distribution. Corvus plans received the higer dose of normal tissue. The Monaco plans have faster beam delivery.

60 APPLIED LIFE SCIENCES↗

Dependence of diode sensitivity on the pulse rate of delivered radiation

Purpose: It has been reported that diode sensitivity decreases by as much as 2% when the average dose rate set at the accelerator console was decreased from 600 to 40 MU/min. No explanation was given for this effect in earlier publications. This work is a detailed investigation of this phenomenon: the change of diode sensitivity versus the rate of delivery of dose pulses in the milliseconds and seconds range. Methods: X-ray beams used in this work had nominal energies of 6 and 15 MV and were generated by linear accelerators. The average dose rate was varied from 25 to 600 MU/min, which corresponded to time between microsecond-long dose pulses of 60-2.7 ms, respectively. The dose-per-pulse, dpp, was changed by positioning the detector at different source-to-detector distance. A variety of diodes fabricated by a number of manufacturers were tested in this work. Also, diodes in three different MapCHECKs (Sun Nuclear, Melbourne, FL) were tested. Results: For all diodes tested, the diode sensitivity decreases as the average dose rate is decreased, which corresponds to an increase in the pulse period, the time between radiation pulses. A sensitivity decrease as large as 5% is observed for a 60-ms pulse period. The diode sensitivity versus the pulse period is modeled by an empirical exponential function. This function has a fitting parameter, t{sub eff}, defined as the effective lifetime. The values of t{sub eff} were found to be 1.0-14 s, among the various diodes. For all diodes tested, t{sub eff} decreases as the dpp decreases and is greater for 15 MV than for 6 MV x rays. The decrease in diode sensitivity after 20 s without radiation can be reversed by as few as 60 radiation pulses. Conclusions: A decrease in diode sensitivity occurs with a decrease in the average dose rate, which corresponds to an increase in the pulse period of radiation. The sensitivity decrease is modeled by an empirical exponential function that decreases with an effective lifetime, t{sub eff}, of 1.0-14 s. t{sub eff} varies widely for different diodes, dpp, and x-ray energy. It is hypothesized that the capture of excess minority carriers by charge traps, cause the observed decrease in diode sensitivity. Also, it is hypothesized that the slow reopening of these traps occurs in the hundreds of milliseconds to seconds range at ambient temperature and this underlies the slow decrease in the diode sensitivity. Calibration of a diode is best done at the average dose rate with which it will be used. This is easily accomplished for radiation deliveries in which the average dose rate is a constant. However, for a VMAT delivery the average dose rate is a variable. For measurements made under these conditions diodes can be calibrated with a median or average dose rate which splits the difference in diode sensitivity that is known to occur with changes in average dose rate.

60 APPLIED LIFE SCIENCES↗

Cobalt, Linac, or Other: What Is the Best Solution for Radiation Therapy in Developing Countries?

The international growth of cancer and lack of available treatment is en route to become a global crisis. With >60% of cancer patients needing radiation therapy at some point during their treatment course, the lack of available facilities and treatment programs worldwide is extremely problematic. The number of deaths from treatable cancers is projected to increase to 11.5 million deaths in 2030 because the international population is aging and growing. In this review, we present how best to answer the need for radiation therapy facilities from a technical standpoint. Specifically, we examine whether cobalt teletherapy machines or megavoltage linear accelerator machines are best equipped to handle the multitudes in need of radiation therapy treatment in the developing world.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

SU-E-T-309: Tangential Modulated Arc Therapy: A Novel Technique for the Treatment of Superficial Disease

Purpose: We propose a new type of treatment that employs a modulated and sliding tangential photon field to provide superior coverage of superficial targets when compared to other commonly employed methods while drastically reducing dose to the underlying sensitive structures often present in these cases. Methods: Modulated treatment plans were formulated for a set of three representative cases. The first was a revised treatment of a scalp sarcoma, while the second was a treatment of a right posterior chest wall sarcoma. For these cases, asymmetric jaw placement, angular limitations, and central isocenter placements were used to force the optimization algorithm into finding solutions with beamlines that were not perpendicular to the body surface. The final case targeted the chest wall of a breast cancer patient, in which standard treatments were compared to the use of modulated fields with multiple isocenters along the chest wall. Results: When compared with unrestricted modulated arcs, the tangential arc scalp treatment reduced the max and mean doses delivered to the brain by 33Gy (from 55 to 22Gy) and 6Gy (from 14Gy to 8Gy), respectively. In the right posterior chest wall case, the V10 in the ipsilateral lung was kept below 5% while retaining a Rx dose (45Gy) target coverage of over 97%. For the breast case, the modulated plan achieved reductions in high dose to the ipsilateral lung and heart by a factor of 2–3 when compared to classic laterally opposed tangents and reduced the V5 by 40% when compared to standard modulated arcs. Conclusion: Tangential modulated arc therapy has outperformed the conventional modalities of treatment for superficial lesions used in our clinic. We hope that with the advent of digitally controlled linear accelerators, we can uncover further benefits of this new technique and extend its applicability to a wider section of the patient population.

60 APPLIED LIFE SCIENCES↗

SU-F-T-319: The Impact of Radiation Beam Obliquity and Air Gap Thickness On Optically Stimulated Luminescent in Vivo Dosimetry for Radiation Therapy

Purpose: Optically-stimulated luminescent dosimeters (OSLDs) are increasingly utilized for in vivo dosimetry of complex radiation delivery techniques. Measured doses, however, underestimate planned doses for plans that utilize thermoplastic mask immobilization. The purpose of this work was to quantify the effect of beam obliquity and air gap span between the mask and backscatter material, on measured-to-planned OSLD dose agreement. Methods: A previously-used thermoplastic mask was cut, reheated, and flattened to form a 33 by 9 cm{sup 2} stage approximately 2 mm thick. Two OSLDs were placed on the stage on 5 cm of solid water, covered with 50 by 50 by 5 mm{sup 3} square of bolus, and scanned in the CT simulator. Plans were created with 10 by 10 cm{sup 2} open fields using 4, 6, 10, and 15 MV photon beams at 0°, 45°, and 90° incidence. The isocenter was placed between the OSLDs at 5 mm depth. Dose was calculated and averaged for two OSLDs. Artificial air gaps of 3, 5, 10, and 20 mm were introduced in the plan and dose was recalculated for each energy/angle/gap combination. The experimental setup was replicated on a linear accelerator and air gaps were introduced by “bridging” the thermoplastic stage across solid water plastic of varying thickness. Fields were delivered as planned. OSLDs were read 12–15 hours after irradiation. Results: Measured-toplanned percent differences were constant with increasing gap thickness for 0° and 45° beam angles. At 90° and 0 cm gap, planned dose underestimated measured dose by 10–23% for all energies. This discrepancy decreased linearly to 0% with a 20 mm gap. OSLD signal did not decrease more than 6% for any gap span and energy. Conclusion: With the exception of parallel beam incidence, beam obliquity and air gap thickness did not have a substantial effect on measured-to-planned dose agreement.

07 ISOTOPE AND RADIATION SOURCES↗

Initial Feasibility and Clinical Implementation of Daily MR-Guided Adaptive Head and Neck Cancer Radiation Therapy on a 1.5T MR-Linac System: Prospective R-IDEAL 2a/2b Systematic Clinical Evaluation of Technical Innovation

This prospective study is, to our knowledge, the first report of daily adaptive radiation therapy (ART) for head and neck cancer (HNC) using a 1.5T magnetic resonance imaging-linear accelerator (MR-linac) with particular focus on safety and feasibility and dosimetric results of an online rigid registration-based adapt to position (ATP) workflow.

62 RADIOLOGY AND NUCLEAR MEDICINE↗