Linear Accelerator System Research Articles

An MTCA-based LLRF prototype system of LINAC in Hefei Advanced light facility

The linear accelerator (LINAC) system currently under construction at the Hefei Advanced Light Facility (HALF) employs acceleration tubes to accelerate the electron beam to 2.2 GeV. The RF field within the acceleration tube is controlled by an MTCA.4-based low-level radio frequency (LLRF) prototype system to meet stringent phase stability requirements. This paper introduces the structure of the LLRF system, encompassing both its hardware and software components. The LLRF system uses the non-IQ-sampling method to suppress odd harmonics through digital filters, achieving a signal-to-noise ratio (SNR) of approximately 73 dB. Given that the power sources of most acceleration tubes operate in a saturated state, the radio-frequency (RF) control algorithm within the software is based on a separate amplitude open-loop and phase close-loop to maintain stable phase control during amplitude disturbances. The design of the RF control logic, including digital filters, the REF phase tracking method, the close-loop feedback controller, and the output pulse mode control algorithm, is detailed. Offline pulse-to-pulse test results demonstrate that it achieves a phase stability of 0.0176° rms with a solid-state amplifier, meeting the facility's requirements.

Two-fractionated stereotactic magnetic resonance-guided adaptive radiation therapy for patients with prostate cancer (SMART PRO trial): protocol for a confirmatory clinical trial

IntroductionIn an MRI-guided linear accelerator (MR-LINAC) system, the planned doses for organs at risk and for tumours are assessed by MR imaging and re-contouring at every treatment. This allows treatment...

Design and commissioning of the ISOL beamline at the RAON facility

The Rare isotope Accelerator complex for ON-line experiment (RAON) located at the Daejeon site of the Institute for Basic Science (IBS) is the first Radioactive Isotope (RI) beam facility in Korea. The RAON facility employs an in-flight method using RI beams produced using an Isotope Separator On-Line (ISOL) method to explore unstable nuclei in uncharted regions. A high-power ISOL facility is critical for providing the required high-quality, intense RI beams. The design, construction, and installation of the ISOL system have been completed, and the beam transport line commissioning with stable isotope beams has been conducted. As a result, all requirements, such as the transmission and the mass-resolving power, for the beamlines were satisfied, and the beams from the Target Ion Source (TIS) were transported to the end of the ISOL beamline, which is the entrance to the superconducting linear accelerator system. The ISOL beamline is now ready for the transport of RI beams.

AAPM Task Group 334: A guidance document to using radiotherapy immobilization devices and accessories in an MR environment.

Use of magnetic resonance (MR) imaging in radiation therapy has increased substantially in recent years as more radiotherapy centers are having MR simulators installed, requesting more time on clinical diagnostic MR systems, or even treating with combination MR linear accelerator (MR-linac) systems. With this increased use, to ensure the most accurate integration of images into radiotherapy (RT), RT immobilization devices and accessories must be able to be used safely in the MR environment and produce minimal perturbations. The determination of the safety profile and considerations often falls to the medical physicist or other support staff members who at a minimum should be a Level 2 personnel as per the ACR. The purpose of this guidance document will be to help guide the user in making determinations on MR Safety labeling (i.e., MR Safe, Conditional, or Unsafe) including standard testing, and verification of image quality, when using RT immobilization devices and accessories in an MR environment.

Convolutional LSTM model for cine image prediction of abdominal motion.

Objective.In this study, we tackle the challenge of latency in magnetic resonance linear accelerator (MR-Linac) systems, which compromises target coverage accuracy in gated real-time radiotherapy. Our focus is on enhancing motion prediction precision in abdominal organs to address this issue. We developed a convolutional long short-term memory (convLSTM) model, utilizing 2D cine magnetic resonance (cine-MR) imaging for this purpose.Approach.Our model, featuring a sequence-to-one architecture with six input frames and one output frame, employs structural similarity index measure (SSIM) as loss function. Data was gathered from 17 cine-MRI datasets using the Philips Ingenia MR-sim system and an Elekta Unity MR-Linac equivalent sequence, focusing on regions of interest (ROIs) like the stomach, liver, pancreas, and kidney. The datasets varied in duration from 1 to 10 min.Main results.The study comprised three main phases: hyperparameter optimization, individual training, and transfer learning with or without fine-tuning. Hyperparameters were initially optimized to construct the most effective model. Then, the model was individually applied to each dataset to predict images four frames ahead (1.24-3.28 s). We evaluated the model's performance using metrics such as SSIM, normalized mean square error, normalized correlation coefficient, and peak signal-to-noise ratio, specifically for ROIs with target motion. The average SSIM values achieved were 0.54, 0.64, 0.77, and 0.66 for the stomach, liver, kidney, and pancreas, respectively. In the transfer learning phase with fine-tuning, the model showed improved SSIM values of 0.69 for the liver and 0.78 for the kidney, compared to 0.64 and 0.37 without fine-tuning.Significance. The study's significant contribution is demonstrating the convLSTM model's ability to accurately predict motion for multiple abdominal organs using a Unity-equivalent MR sequence. This advancement is key in mitigating latency issues in MR-Linac radiotherapy, potentially improving the precision and effectiveness of real-time treatment for abdominal cancers.

A comprehensive investigation of the performance of a commercial scintillator system for applications in electron FLASH radiotherapy.

Dosimetry in ultra-high dose rate (UHDR) beamlines is significantly challenged by limitations in real-time monitoring and accurate measurement of beam output, beam parameters, and delivered doses using conventional radiation detectors, which exhibit dependencies in ultra-high dose-rate (UHDR) and high dose-per-pulse (DPP) beamline conditions. In this study, we characterized the response of the Exradin W2 plastic scintillator (Standard Imaging, Inc.), a water-equivalent detector that provides measurements with a time resolution of 100Hz, to determine its feasibility for use in UHDR electron beamlines. The W2 scintillator was exposed to an UHDR electron beam with different beam parameters by varying the pulse repetition frequency (PRF), pulse width (PW), and pulse amplitude settings of an electron UHDR linear accelerator system. The response of the W2 scintillator was evaluated as a function of the total integrated dose delivered, DPP, and mean and instantaneous dose rate. To account for detector radiation damage, the signal sensitivity (pC/Gy) of the W2 scintillator was measured and tracked as a function of dose history. The W2 scintillator demonstrated mean dose rate independence and linearity as a function of integrated dose and DPP for DPP ≤ 1.5Gy (R2>0.99) and PRF ≤ 90Hz. At DPP>1.5Gy, nonlinear behavior and signal saturation in the blue and green signals as a function of DPP, PRF, and integrated dose became apparent. In the absence of Cerenkov correction, the W2 scintillator exhibited PW dependence, even at DPP values<1.5Gy, with a difference of up to 31% and 54% in the measured blue and green signal for PWs ranging from 0.5 to 3.6 µs. The change in signal sensitivity of the W2 scintillator as a function of accumulated dose was approximately 4%/kGy and 0.3%/kGy for the measured blue and green signal responses, respectively, as a function of integrated dose history. The Exradin W2 scintillator can provide output measurements that are both dose rate independent and linear in response if the DPP is kept ≤1.5Gy (corresponding to a mean dose rate up to 290Gy/s in the used system), as long as proper calibration is performed to account for PW and changes in signal sensitivity as a function of accumulated dose. For DPP>1.5Gy, the W2 scintillator's response becomes nonlinear, likely due to limitations in the electrometer related to the high signal intensity.

Repeatability and reproducibility of prostate apparent diffusion coefficient values on a 1.5 T magnetic resonance linear accelerator

Background and PurposeIntegrated magnetic resonance linear accelerator (MR-Linac) systems offer potential for biologically based adaptive radiation therapy using apparent diffusion coefficient (ADC). Accurate tracking of longitudinal ADC changes is key to establishing ADC-driven dose adaptation. Here, we report repeatability and reproducibility of intraprostatic ADC using deformable image registration (DIR) to correct for inter-fraction prostate changes. Materials and MethodsThe study included within-fraction repeat ADC measurements for three consecutive fractions for 20 patients with prostate cancer treated on a 1.5 T MR-Linac. We deformably registered successive fraction T2-weighted images and applied the deformation vector field to corresponding ADC maps to align to fraction 2. We delineated gross tumour volume (GTV), peripheral zone (PZ) and prostate clinical target volume (CTV) regions-of-interest (ROIs) on T2-weighted MRI and copied to ADC maps. We computed intraclass correlation coefficients (ICC) and percent repeatability coefficient (%RC) to determine within-fraction repeatability and between-fraction reproducibility for individual voxels, mean and 10th percentile ADC values per ROI. ResultsThe ICC between repeats and fractions was excellent for mean and 10th percentile ADC in all ROIs (ICC > 0.86), and moderate repeatability and reproducibility existed for individual voxels (ICC > 0.542). Similarly, low %RC within-fraction (4.2–17.9 %) mean and 10th percentile ADC existed, with greater %RC between fractions (10.2–36.8 %). Higher %RC existed for individual voxel within-fraction (21.7–30.6 %) and between-fraction (32.1–34.5 %) ADC. ConclusionsResults suggest excellent ADC repeatability and reproducibility in clinically relevant ROIs using DIR to correct between-fraction anatomical changes. We established the precision of voxel-level ADC tracking for future biologically based adaptation implementation.

Synchronisation of linear accelerator for fruit irradiation with FPGA-based system

Linear accelerator (LINAC) has been broadly used in various applications in recent years. For instance, the LINAC can produce X-rays and high-energy electrons, which are utilised in radiation therapy for medical purposes and also applied to fruit steriliser for exportation. At present, the prototype of the 6 MeV LINAC for fruit irradiation by using a magnetron radio frequency (RF) source has been developed at Synchrotron Light Research Institute (SLRI) in Nakhon Ratchasima, Thailand. The LINAC consists of several key sub-systems such as pulsed modulators both for electron gun and magnetron used to power the LINAC, magnetic guidance and beam diagnostic systems. All these have been integrated and worked in proper function to reach a proper and stable RF acceleration. Therefore, a timing system is required with well synchronisation for precise transfer of the clock and reference trigs to each sub-system. Furthermore, the low-cost in-house synchronous equipment has been built here for domestic researchers when they would like to apply this system to their work in the future; consequently, the FPGA-based system for real-time synchronisation has been created to achieve the sequences of the LINAC timing system. In this design, the Zybo Z7 development board from the Xilinx Zynq-7000 family is chosen, which integrates the programmable logic (PL) and processing system (PS) in the same chip. The experimental results are presented by the LINAC operation with synchronisation between a precise time and various delay times on both pulsed modulators, which the synchronous technique with accurate time demonstrates excellent performances leading to stable X-ray radiation for fruit irradiation with synchronous operation between the electron gun and magnetron.

MRI-guided Pelvic Radiation Therapy: A Primer for Radiologists.

Radiation therapy (RT) is a core pillar of oncologic treatment, and half of all patients with cancer receive this therapy as a curative or palliative treatment. The recent integration of MRI into the RT workflow has led to the advent of MRI-guided RT (MRIgRT). Using MRI rather than CT has clear advantages for guiding RT to pelvic tumors, including superior soft-tissue contrast, improved organ motion visualization, and the potential to image tumor phenotypic characteristics to identify the most aggressive or treatment-resistant areas, which can be targeted with a more focal higher radiation dose. Radiologists should be familiar with the potential uses of MRI in planning pelvic RT; the various RT techniques used, such as brachytherapy and external beam RT; and the impact of MRIgRT on treatment paradigms. Current clinical experience with and the evidence base for MRIgRT in the settings of prostate, cervical, and bladder cancer are discussed, and examples of treated cases are illustrated. In addition, the benefits of MRIgRT, such as real-time online adaptation of RT (during treatment) and interfraction and/or intrafraction adaptation to organ motion, as well as how MRIgRT can decrease toxic effects and improve oncologic outcomes, are highlighted. MRIgRT is particularly beneficial for treating mobile pelvic structures, and real-time adaptive RT for tumors can be achieved by using advanced MRI-guided linear accelerator systems to spare organs at risk. Future opportunities for development of biologically driven adapted RT with use of functional MRI sequences and radiogenomic approaches also are outlined. ©RSNA, 2023 Quiz questions for this article are available in the supplemental material.

Optimization of target system for the production of 99Mo via 100Mo(γ,n)99Mo reaction

With an increase of stopping operation of nuclear reactors worldwide, the supply of medical 99Mo becomes difficult and thus many efforts have been made to find an alternative. A process based on an electron linear accelerator (linac) system and a100Mo target via the 100Mo (γ,n)99Mo reaction receives a lot of attention due to the relatively low level of co-produced impurities. This process has been recently developed at the Institute of Modern Physics (IMP) and the Monte Carlo simulation was used to optimize the target system before operating pilot irradiation experiments. First, tungsten and tantalum, as mostly used converter materials, were tested. The yield of 99Mo was evaluated with respect to the converter thickness and the electron beam energy by means of Geant4 simulations. Besides, the specific activity of 99Mo produced from one-stage approach (100Mo target without a converter) and two-stage approach (100Mo target with a converter) was compared when varying the testing conditions. The two-stage approach was selected for the experiment due to the higher specific activity of produced 99Mo at all tested conditions. A target consisting of a 10 mm thickness of the 100Mo tablets and a 2.4 mm thick Ta converter was irradiated for 40 h (50 MeV with 0.2 μA). The Geant4-calculated specific activity of generated 99Mo at the end of bombardment agreed well with the experimental value, which proved high level of accuracy of the Geant4 simulation. In future studies, the Geant4 simulation will be used to optimize the production process when using high power linac system.

Imaging Performance of the PET Scan on a Novel Ring Gantry-Based PET/CT Linear Accelerator System in the First-in-Human Study of Biology-Guided Radiotherapy

Biology-guided radiotherapy (BgRT) is a novel tracked dose delivery modality using real-time positron emission tomography (PET) to guide radiotherapy beamlets. The present study was performed with sequential cohorts of participants to evaluate the performance and safety of BgRT. Primary endpoints were previously reported. We hereby report on one of the secondary endpoints assessing a novel treatment planning machine with integrated dual kVCT/PET imaging ("novel device") performance in comparison to a third-party diagnostic PET/CT scan. This single-arm, open-label, prospective study included participants with at least 1 FDG-avid targetable primary or metastatic tumor (≥2cm and ≤5cm) in the lung or bone. PET imaging data were collected on the novel device and on a third-party diagnostic PET/CT performed in sequence once at the planning timepoint in Cohort I, and immediately before the last fraction among patients undergoing stereotactic radiotherapy in Cohort II. Three central read radiation oncologists (CRRO) provided an interpretation of the novel device PET scans which were compared to an agreement standard based on 3 central radiologists' review of the paired diagnostic PET/CT scan. Positive percent agreement for localization of the target tumor within the biology-tracking zone (BTZ) was the key metric because it reflects whether advancing patients to subsequent steps in the BgRT workflow based on the novel device's imaging was ultimately appropriate. In Cohort 1, 6 image comparisons were performed. The positive (%) agreement for the aggregate radiation oncologist review was 100% (5/5), reflecting that in all 5 cases where the aggregate radiation oncologists deemed the tumor to fall within the BTZ based upon the novel device PET images, the central radiologists came to the same conclusion upon review of the paired diagnostic PET/CT images. The overall (%) agreement for the aggregate radiation oncologist review was 83.3% (5/6): localization was not established on the novel device in 1 case, even though it was established on the diagnostic PET/CT. This would not pose risk in real world practice as BgRT candidacy would be aborted for tumors not visible on the novel device. In Cohort II, among the 7 image comparisons, there was 100% positive percent agreement between the aggregate CRRO and the agreement standard as the localization criteria was met in both scans for all 7 patients. This was concordant with a 100% overall percent agreement. This investigation demonstrated a 100% positive percent agreement between central review of this novel device images by radiation oncologists and central review of the accompanying third-party PET/CT images by radiologists. There were no cases where a positive localization by the aggregate CRRO was not confirmed by the third-party PET/CT standard, providing evidence against the likelihood of falsely positive localizations on the novel device that would inappropriately advance patients in the workflow.

Validation of Hypoxia Detection Sequences on the MR Linac

Magnetic resonance linear accelerator (MRL) systems permit acquisition of novel imaging at the time of radiotherapy. A validated MR hypoxia imaging biomarker could select patients for adaptive radiotherapy with hypoxia modification or dose escalation. The aims of this study were (1) to develop a protocol for quantitative hypoxia sensitive MRI (2) to validate these in prostate cancer (PCa) against pimonidazole-stained prostatectomy sections. Blood oxygen level dependent (BOLD), intravoxel incoherent motion (IVIM), oxygen-enhanced (OE) and dynamic contrast enhanced (DCE) MRI were used. Sequences were developed on a diagnostic 1.5 T MR (MRD) and MRL with healthy volunteers and PCa patients. The Hyprogen trial includes men with localized PCa scheduled for prostatectomy. Imaging is acquired twice prior to surgery and oral pimonidazole is taken 8-16 hours before surgery. Whole prostate (WP) and dominant prostatic lesion (DIL) were outlined on T2-weighted (T2W) images and a 'normal prostate' (NP) volume created by subtracting DIL from WP. Contours were applied to parametric maps from the quantitative MRI, with median and IQR extracted. Patient-specific 3D-printed prostate molds were created from WP volumes and used to guide prostate whole organ dissection. Three of 20 patients recruited to date. MRI data were acquired successfully. A personalized prostate mold was produced for each patient and facilitated dissection of the prostatectomy specimen in a matching plane to MRI to validate hypoxia detection of the MR protocol. Correlation with pimonidazole staining is underway. Imaging parameter median values for NP and DIL acquired on MRD and MRL for the first patient are shown (Table 1). The expected differences between NP and DIL for T1 and D are seen and median values for T2* are consistent with reported values in the literature. The MR hypoxia protocol can be acquired safely and is well-tolerated on the MRL. Once validated against pimonidazole staining adaptive radiotherapy protocols will be developed to use this information.

Technical Infrastructure for Clinical Translation of Electron FLASH

For safe clinical translation of electron FLASH, hardware tools for real-time beam control and software tools for treatment planning are necessary. The purpose of this study is to prototype high-throughput hardware for real-time beam control, along with accurate beam modeling of a modern clinical Linac configured to deliver FLASH dose-rates. For real-time beam current monitoring, a beam current transformer (BCT) was initially coupled to a fast digitizer and its linearity was established by varying dose per pulse. The radiation pulse width was modified, and this change was measured using the BCT. The BCT was then used to measure the variability of dose per pulse and pulse width due to a mistuned linear accelerator system. Next, the BCT was interfaced with a field programmable gate array (FPGA) which provides the ability for high-throughput and deterministic control of the Linac based on dose accumulation. For beam modeling, the program, TOol for PArticle Simulation (TOPAS), was used to obtain beam parameters by using Bayesian optimization of the beam energy, source size, angular, and energy spread via comparison of simulated and representative dose profiles. The beam model would then be employed to calculate 3D dose distribution in a CT scan of a 3D-printed anatomically realistic mouse phantom. The area under the current-time curve from the BCT exhibited excellent linearity (response = 12.80 nC/Gy) up to 2.5 Gy/Pulse (R2 = 0.99). The peak beam current for the electron FLASH beam was measured to be ∼10 mA for an instantaneous dose-rate of ∼5×105 Gy/s. The measured radiation pulse width agreed with the expected value (3.7 μs). The pulse width was then shortened and the measurement by the BCT indicated pulse widths of 1.8 μs and 0.5 μs corresponding to 0.7 Gy/pulse and 0.3 Gy/pulse, respectively. The beamline exhibited a ramp-up in dose per pulse and pulse width when using the automatic frequency controller (AFC). For the first pulse, the dose delivered was ∼0.1-0.3 Gy and the pulse width was 0.6 μs. The output stabilized to nominal values of dose and pulse width after 3-4 pulses. This ramp-up was mitigated by manually tuning the RF resonance with the AFC disabled, after which the BCT exhibited constant output and pulse width. The beam modeling work is in progress. We demonstrated that a BCT can provide real-time measurement of per-pulse output suitable as input for FLASH beam control based on dose accumulation. The next steps are to quantify the accuracy of the dose control mechanism with the FPGA-based hardware. Potential failure modes will be identified and mitigated in parallel with the development of the hardware. A 3D-printed mouse phantom has been constructed to facilitate beam modeling work for treatment planning (in progress). On completion of this work, it is expected that we will have key infrastructure elements needed to move towards an eventual FDA investigational device exemption for clinical trials.

Beamline and vacuum system design of HEBT for the A-FNS accelerator

The Advanced Fusion Neutron Source (A-FNS) project is progressing at the QST Rokkasho institute, Japan. The A-FNS will conduct fusion neutron irradiation tests of materials for a fusion demonstration reactor (DEMO reactor). For these tests a high intensity deuteron beam accelerator is used to produce a high neutron flux with the neutron energies around 14 MeV. The accelerator system accelerates a deuteron beam to a beam energy of 40 MeV, guides the beam towards a liquid lithium target and shapes the uniform beam at the Li target using a magnet system.Two of the main considerations for the A-FNS high energy beam transport (HEBT) design are protection of the Superconducting Radio-Frequency linear accelerator (SRF) and vacuum system design for the interface of the HEBT and the target system. For the A-FNS, back streaming of Li vapor from the windowless free surface of the Li target is a concern. In addition, widely different vacuum pressure must be maintained between the accelerator system and the target system. This paper shows the detailed design of the A-FNS HEBT section regarding these considerations.

Electron Energy Spectrometer for MIR-THz FEL Light Source at Chiang Mai University

The linear accelerator system of the PBP-CMU Electron Linac Laboratory has been designed with the aim of generating free-electron lasers (FELs) in the mid-infrared (MIR) and terahertz (THz) regions. The quality of the radiation is strongly dependent on the properties of the electron beam. Among the important beam parameters, the electron beam energy and energy spread are particularly important. To accurately measure the electron beam energy, the first dipole magnet in the bunch compressor system and the downstream screen station are employed as an energy spectrometer. The A Space Charge Tracking Algorithm (ASTRA) software is used for the design and optimization of this system. Simulation results demonstrate that the developed spectrometer is capable of accurately measuring the energy within the 5–25 MeV range. The screen station system is designed and constructed to have the ability to capture a beam size with a resolution of 0.1 mm per pixel. This resolution is achieved with a screen-to-camera distance of 1.2 m, which proves sufficient for precise energy measurement. The systematic error in energy measurement is found to be less than 10%, with a minimum energy spread of 0.4% achievable when the horizontal beam size remains below 3 mm.

Thoracic motion-compensated cone-beam computed tomography in under 20 seconds on a fast-rotating linac: A simulation study.

Rapid kV cone-beam computed tomography (CBCT) scans are achievable in under 20 s on select linear accelerator systems to generate volumetric images in three dimensions (3D). Daily pre-treatment four-dimensional CBCT (4DCBCT) is recommended in image-guided lung radiotherapy to mitigate the detrimental effects of respiratory motion on treatment quality. To demonstrate the potential for thoracic 4DCBCT reconstruction using projection data that was simulated using a clinical rapid 3DCBCT acquisition protocol. We simulated conventional (1320 projections over 4 min) and rapid (491 projections over 16.6 s) CBCT acquisitions using 4D computed tomography (CT) volumes of 14 lung cancer patients. Conventional acquisition data were reconstructed using the 4D Feldkamp-Davis-Kress (FDK) algorithm. Rapid acquisition data were reconstructed using 3DFDK, 4DFDK, and Motion-Compensated FDK (MCFDK). Image quality was evaluated using Contrast-to-Noise Ratio (CNR), Tissue Interface Width (TIW), Root-Mean-Square Error (RMSE), and Structural SIMilarity (SSIM). The conventional acquisition 4DFDK reconstructions had median phase averaged CNR, TIW, RMSE, and SSIM of 2.96, 8.02mm, 83.5, and 0.54, respectively. The rapid acquisition 3DFDK reconstructions had median CNR, TIW, RMSE, and SSIM of 2.99, 13.6mm, 112, and 0.44 respectively. The rapid acquisition MCFDK reconstructions had median phase averaged CNR, TIW, RMSE, and SSIM of 2.98, 10.2mm, 103, and 0.46, respectively. Rapid acquisition 4DFDK reconstruction quality was insufficient for any practical use due to sparse angular projection sampling. Results suggest that 4D motion-compensated reconstruction of rapid acquisition thoracic CBCT data are feasible with image quality approaching conventional acquisition CBCT data reconstructed using standard 4DFDK.

Development of a compact linear accelerator to generate ultrahigh dose rate high-energy X-rays for FLASH radiotherapy applications.

In recent years, the FLASH effect, in which ultrahigh dose rate (UHDR) radiotherapy (RT) can significantly reduce toxicity to normal tissue while maintaining antitumor efficacy, has been verified in many studies and even applied in human clinical cases. This work evaluates whether a room-temperature radio-frequency (RF) linear accelerator (linac) system can produce UHDR high-energy X-rays exceeding a dose rate of 40Gy/s at a clinical source-surface distance (SSD), exploring the possibility of a compact and economical clinical FLASH RT machine suitable for most hospital treatmentrooms. A 1.65m long S-band backward-traveling-wave (BTW) electron linac was developed to generate high-current electron beams, supplied by a commercial klystron-based power source. A tungsten-copper electron-to-photon conversion target for UHDR X-rays was designed and optimized with Monte Carlo (MC) simulations using Geant4 and thermal finite element analysis (FEA) simulations using ANSYS. EBT3 and EBT-XD radiochromic films, which were calibrated with a clinical machine Varian VitalBeam, were used for absolute dose measurements. A PTW ionization chamber detector was used to measure the relative total dose and a plane-parallel ionization chamber detector was used to measure the relative normalized dose of each pulse. The BTW linac generated 300-mA-pulse-current 11 MeV electron beams with 29kW mean beam power, and the conversion target could sustain this high beam power within a maximum irradiation duration of 0.75 s. The mean energy of the produced X-rays was 1.66 MeV in the MC simulation. The measured flat-filter-free (FFF) maximum mean dose rate of the room-temperature linac exceeded 80Gy/s at an SSD of 50cm and 45Gy/s at an SSD of 67.9cm, both at a 2.1cm depth of the water phantom. The FFF radiation fields at 50cm and 67.9cm SSD at a 2.1cm depth of the water phantom showed Gaussian-like distributions with 14.3 and 20cm full-width at half-maximum (FWHM) values, respectively. This work demonstrated the feasibility of UHDR X-rays produced by a room-temperature RF linac, and explored the further optimization of system stability. It shows that a simple and compact UHDR X-ray solution can be facilitated for both FLASH-RT scientific research and clinical applications.

Patterns of utilization and clinical adoption of 0.35 Tesla MR-guided radiation therapy in the United States – Understanding the transition to adaptive, ultra-hypofractionated treatments

Purpose/ObjectiveMagnetic resonance-guided radiation therapy (MRgRT) utilization is rapidly expanding worldwide, driven by advanced capabilities including continuous intrafraction visualization, automatic triggered beam delivery, and on-table adaptive replanning (oART). Our objective was to describe patterns of 0.35Tesla(T)-MRgRT (MRIdian) utilization in the United States (US) among early adopters of this novel technology. Materials/MethodsAnonymized administrative data from all US MRIdian treatment systems were extracted for patients completing treatment from 2014 to 2020. Detailed treatment information was available for all MRIdian linear accelerator (linac) systems and some cobalt systems. ResultsSeventeen systems at 16 centers delivered 5736 courses and 36,389 fractions (fraction details unavailable for 1223 cobalt courses), of which 21.1% were adapted. Ultra-hypofractionation (UHfx) (1–5 fractions) was used in 70.3% of all courses. At least one adaptive fraction was used for 38.5% of courses (average 1.7 adapted fractions/course), with higher oART use in UHfx dose schedules (47.7% of courses, average 1.9 adapted fractions per course). The most commonly treated organ sites were pancreas (20.7%), liver (16.5%), prostate (12.5%), breast (11.5%), and lung (9.4%). Temporal trends show a compounded annual growth rate (CAGR) of 59.6% in treatment courses delivered, with a dramatic increase in use of UHfx to 84.9% of courses in 2020 and similar increase in use of oART to 51.0% of courses. ConclusionsThis is the first comprehensive study reporting patterns of utilization among early adopters of MRIdian in the US. Intrafraction MR image-guidance, advanced motion management, and increasing adoption of adaptive radiation therapy has led to a substantial transition to ultra-hypofractionated regimens. 0.35 T-MRgRT has been predominantly used to treat abdominal and pelvic tumors with increasing use of on-table adaptive replanning, which represents a paradigm shift in radiation therapy.

Carbon Footprint of Clinical Photon Therapy: Initial Estimates

<h3>Purpose/Objective(s)</h3> There is an increasing concern about rising carbon dioxide (CO2) levels and its hazardous impact on human health and climate change. Radiation oncology uses high energy machines; however, their carbon footprint is not well understood. This study estimates the energy utilization of photon therapy, and estimates corresponding carbon footprint. Additionally, the study evaluates possible ways to offset CO2 with planting new trees. <h3>Materials/Methods</h3> Patients treated between 07/2020 and 06/2021 using a technology company's medical linear accelerator system at one of our satellite locations were evaluated. A technology company data sheet was used to estimate power draw of the system. BeamOn mode was specified as 39 kVA, Ready mode as 15 kVA, On mode as 11 kVA, and Standby mode as 7 kVA; these were converted to kW. This is likely the worst-case power use scenario, with power factor typically 0.75 – 1.0. Patients treated were reviewed for dose, number of fractions, number of fields, and duration of beam during each fraction. Field delivery durations were obtained from the EMR, and kWh power consumption was estimated per fraction and per treatment course. EPA web calculator was used to convert power consumption to tons of CO2, and the number of new trees required to potentially offset it. <h3>Results</h3> There were 176 patients treated for 191 treatment courses. Total of 4,517 fractions were delivered (average 23.6 fractions per treatment course). Total BeamOn time was 710,606 seconds (197.4 hours), average 157 seconds per fraction. Total BeamOn power consumption would be 7,698 kWh, or ∼1.7 kWh per fraction and ∼40 kWh per treatment course. Annual power consumption including standby mode would be ∼81,250 kWh; BeamOn consumption would account for 9.5% of total machine power usage. The corresponding carbon footprint for BeamOn time would be 5.46 metric tonnes of CO2, or 29 kg of CO2 per patient course. Carbon footprint for the annual machine power consumption would be 57.6 tonnes of CO2. Attributed footprint would thus be 57.6 / 191 = 301 kg CO2 per patient treatment course. The carbon offset required for patient BeamOn time would be 90 new trees planted (average 0.5 trees per patient course). The corresponding offset required to operate the medical linear accelerator system for a year would be 952 new trees planted. The attributed carbon offset per treatment course would thus be 5 new trees planted. <h3>Conclusion</h3> Carbon footprint of the technology company's medical linear accelerator system is likely to be ∼30 kg of CO2 equivalent for direct BeamOn treatment course energy used, ∼300 kg of CO2 equivalent per overall patient energy expenditure, and ∼5.5 metric tonnes of CO2 in total to operate per year. The corresponding number of new trees to be planted to offset the power use would be 0.5 new trees, 5 new trees, and 952 new trees respectively. We are working on measuring the actual power draw during various treatment modes, which could be 5-15% lower than the kVA specification, and calculating the actual carbon footprint of clinical radiation therapy for our next study.

Comparing patient acceptability of MR-guided radiotherapy to conventional CBCT on two Elekta systems: a questionnaire-based survey

AbstractBackground and Purpose:The magnetic resonance linear accelerator system (MR Linac) is a novel piece of radiotherapy (RT) equipment allowing the routine application of daily MR-guided treatment adaptation. The hardware design required for such technical capabilities and the increased complexity of the treatment workflow entails a notable departure from cone beam computed tomography (CBCT)-based RT. Patient tolerability of treatment is paramount to RT practice where high compliance is required. Presented is a comparative analysis of how such modality specific characteristics may ultimately impact the patient experience of treatment.Materials and Methods:Forty patients undergoing RT for prostate cancer (PCa) on either the MR Linac (n= 20) or a CBCT-based linac (n= 20) were provided with a validated patient reported outcomes measures (PROM’s) questionnaire at fraction 1 and fraction 20. The 18-item questionnaire provided patient responses recorded using a 4-point Likert scale, 0 denoting a response of ‘Not at all’, 1 ‘Slightly’, 2 ‘Moderately’ and 3 signifying ‘Very’. The analysis provided insight into both comparisons between modalities at singular time points (fractions 1 and 20), as well as a temporal analysis within a single modality, denoting changing patient experience.Results:Patients generally found the MR Linac treatment couch more comfortable, however, found the increase in treatment duration harder to tolerate. Responses for all items remained stable between first and last fraction across both cohorts, indicating minimal temporal variation within a single modality. None of the responses were statistically significant at the 0·01 level.Conclusion:Whether radiotherapy for PCa is delivered on a CBCT linac or the MR Linac, there is little difference in patient experience with minimal experiential variation within a single modality.