Background and purposeDiffusion weighted imaging (DWI) allows for the interrogation of tissue cellularity, which is a surrogate for cellular proliferation. Previous attempts to incorporate DWI into the workflow of a 0.35 T MR-linac (MRL) have lacked quantitative accuracy. In this study, accuracy, repeatability, and geometric precision of apparent diffusion coefficient (ADC) maps produced using an echo planar imaging (EPI)-based DWI protocol on the MRL system is illustrated, and in vivo potential for longitudinal patient imaging is demonstrated. Materials and methodsAccuracy and repeatability were assessed by measuring ADC values in a diffusion phantom at three timepoints and comparing to reference ADC values. System-dependent geometric distortion was quantified by measuring the distance between 93 pairs of phantom features on ADC maps acquired on a 0.35 T MRL and a 3.0 T diagnostic scanner and comparing to spatially precise CT images. Additionally, for five sarcoma patients receiving radiotherapy on the MRL, same-day in vivo ADC maps were acquired on both systems, one of which at multiple timepoints. ResultsPhantom ADC quantification was accurate on the 0.35 T MRL with significant discrepancies only seen at high ADC. Average geometric distortions were 0.35 (±0.02) mm and 0.85 (±0.02) mm in the central slice and 0.66 (±0.04) mm and 2.14 (±0.07) mm at 5.4 cm off-center for the MRL and diagnostic system, respectively. In the sarcoma patients, a mean pretreatment ADC of 910x10-6 (±100x10-6) mm2/s was measured on the MRL. ConclusionsThe acquisition of accurate, repeatable, and geometrically precise ADC maps is possible at 0.35 T with an EPI approach.

Magnetic resonance (MR) image guidance may facilitate safe ultrahypofractionated radiation dose escalation for inoperable pancreatic ductal adenocarcinoma. We conducted a prospective study evaluating the safety of 5-fraction Stereotactic MR-guided on-table Adaptive Radiation Therapy (SMART) for locally advanced (LAPC) and borderline resectable pancreatic cancer (BRPC). Patients with LAPC or BRPC were eligible for this multi-institutional, single-arm, phase 2 trial after ≥3 months of systemic therapy without evidence of distant progression. Fifty gray in 5 fractions was prescribed on a 0.35T MR-guided radiation delivery system. The primary endpoint was acute grade ≥3 gastrointestinal (GI) toxicity definitely attributed to SMART. One hundred thirty-six patients (LAPC 56.6%, BRPC 43.4%) were enrolled between January 2019 and January 2022. Mean age was 65.7 (36-85) years. Head of pancreas lesions were most common (66.9%). Induction chemotherapy mostly consisted of (modified)FOLFIRINOX (65.4%) or gemcitabine/nab-paclitaxel (16.9%). Mean CA19-9 after induction chemotherapy and before SMART was 71.7 U/mL (0-468). On-table adaptive replanning was performed for 93.1% of all delivered fractions. Median follow-up from diagnosis and SMART was 16.4 and 8.8 months, respectively. The incidence of acute grade ≥3 GI toxicity possibly or probably attributed to SMART was 8.8%, including 2 postoperative deaths that were possibly related to SMART in patients who had surgery. There was no acute grade ≥3 GI toxicity definitely related to SMART. One-year overall survival from SMART was 65.0%. The primary endpoint of this study was met with no acute grade ≥3 GI toxicity definitely attributed to ablative 5-fraction SMART. Although it is unclear whether SMART contributed to postoperative toxicity, we recommend caution when pursuing surgery, especially with vascular resection after SMART. Additional follow-up is ongoing to evaluate late toxicity, quality of life, and long-term efficacy.

This paper presents the design, prototype engineering, and initial testing of a relatively low-voltage, high aspect ratio electron gun for versatile pulsed power applications. The sheet beam, diode electron gun is designed for (23–25) kV voltage and 6 µA/V1.5, as is required for x-band klystron, as a part of a compact Klylac delivering electron beam energy in the MeV range. The gun prototype was engineered, built, and tested using a Scandinova M1-0.5 modulator. Beam loss on the anode aperture is evaluated from transient waveforms and equivalent circuit modeling.

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.

To develop an independent log file-based intensity-modulated radiation therapy (IMRT) quality assurance (QA) tool for the 0.35T magnetic resonance-linac (MR-linac) and investigate the ability of various IMRT plan complexity metrics to predict the QA results. Complexity metrics related to tissue heterogeneity were also introduced. The tool for particle simulation (TOPAS) Monte Carlo code was utilized with a previously validated linac head model. A cohort of 29 treatment plans was selected for IMRT QA using the developed QA tool and the vendor-supplied adaptive QA (AQA) tool. For 27 independent patient cases, various IMRT plan complexity metrics were calculated to assess the deliverability of these plans. A correlation between the gamma pass rates (GPRs) from the AQA results and calculated IMRT complexity metrics was determined using the Pearson correlation coefficients. Tissue heterogeneity complexity metrics were calculated based on the gradient of the Hounsfield units. The median and interquartile range for the TOPAS GPRs (3%/3mm criteria) were 97.24% and 3.75%, respectively, and were 99.54% and 0.36% for the AQA tool, respectively. The computational time for TOPAS ranged from 4 to 8h to achieve a statistical uncertainty of <1.5%, whereas the AQA tool had an average calculation time of a few minutes. Of the 23 calculated IMRT plan complexity metrics, the AQA GPRs had correlations with 7 out of 23 of the calculated metrics. Strong correlations (|r|>0.7) were found between the GPRs and the heterogeneity complexity metrics introduced in this work. An independent MC and log file-based IMRT QA tool was successfully developed and can be clinically deployed for offline QA. The complexity metrics will supplement QA reports and provide information regarding plan complexity.

BackgroundMagnetic resonance-guided radiotherapy (MRgRT) utilization is rapidly expanding, driven by advanced capabilities including better soft tissue imaging, continuous intrafraction target visualization, automatic triggered beam delivery, and the availability of on-table adaptive replanning. Our objective was to describe patterns of 0.35 Tesla (T)-MRgRT utilization in Europe and Asia among early adopters of this novel technology.MethodsAnonymized administrative data from all 0.35T-MRgRT treatment systems in Europe and Asia were extracted for patients who completed treatment from 2015 to 2020. Detailed treatment information was analyzed for all MR-linear accelerators (linac) and -cobalt systems.ResultsFrom 2015 through the end of 2020, there were 5796 completed treatment courses delivered in 46,389 individual fractions. 23.5% of fractions were adapted. Ultra-hypofractionated (UHfx) dose schedules (1–5 fractions) were delivered for 63.5% of courses, with 57.8% of UHfx fractions adapted on-table. The most commonly treated tumor types were prostate (23.5%), liver (14.5%), lung (12.3%), pancreas (11.2%), and breast (8.0%), with increasing compound annual growth rates (CAGRs) in numbers of courses from 2015 through 2020 (pancreas: 157.1%; prostate: 120.9%; lung: 136.0%; liver: 134.2%).ConclusionsThis is the first comprehensive study reporting patterns of utilization among early adopters of a 0.35T-MRgRT system in Europe and Asia. Intrafraction MR image-guidance, advanced motion management, and increasing adoption of on-table adaptive RT have accelerated a transition to UHfx regimens. MRgRT has been predominantly used to treat tumors in the upper abdomen, pelvis and lungs, and increasingly with adaptive replanning, which is a radical departure from legacy radiotherapy practices.

Safe delivery of hypofractionated radiotherapy requires high levels of accuracy due to the high doses of radiation delivered per fraction. Magnetic resonance guided radiotherapy (MRgRT) represents a new treatment paradigm which allows improved visualisation of targets and organs at risk, alongside the capability to adapt the treatment plan in real time prior to treatment delivery. There are challenges to delivering hypofractionated radiotherapy with conventional image-guided radiotherapy (IGRT) techniques and MRgRT may help to improve accuracy in radiation delivery in a number of clinical and anatomical scenarios.Specifically, there is an emerging role of MRgRT in delivering stereotactic body radiotherapy (SBRT) for locally advanced pancreatic cancer (LAPC) due to the superior soft tissue contrast provided by Magnetic Resonance Imaging combined with the ability to accommodate variation in anatomical appearances during treatment delivery. Reported data on the use of MRgRT in LAPC and it's role in enabling dose escalation are discussed in this article.There are further potential benefits to the use of MRgRT, for example the use of functional imaging during treatment delivery and generation of synthetic computed tomography, which have previously been impractical or unachievable. The overall aim of this article is to demonstrate the utility of MRgRT in facilitating safe delivery of hypofractionated radiotherapy and to highlight ways in which it may help to overcome challenges posed by current IGRT techniques.

When patients start their cancer journey, there can be increased anxiety about new procedures and processes in addition to their recent diagnosis.1 One of these potential anxiety-inducing experiences is a treatment plan that includes MR-guided radiation where breath holding is necessary. Lack of optimal patient preparation for such therapy can result in delays on the treatment unit itself which can increase patient and staff dissatisfaction.

The main focus of the recommended spatial accuracy tests for the multi‐leaf collimators (MLC) is calibration of the leaf position along the movement direction and overall alignment to the radiation isocenter. No explicit attention was typically paid to the alignment of the leaves from the opposing banks in the direction orthogonal to movement. This paper is a case study demonstrating that verification of such alignment at the time of acceptance testing is prudent. The original standard MLC (SMLC) on an MRIdian MRI‐guided linac (ViewRay Inc., Mountain View, CA, USA) was upgraded to a high‐speed MLC (HSMLC), which is supposed to be mechanically identical to the SMLC except for the higher drive screw pitch. The results of the end‐to‐end IMRT tests demonstrated unacceptable dosimetric results exemplified by an average and maximum ion chamber (IC) point dose error in the high‐dose low‐gradient region of 2.5 ± 1.4% and 4.6%, respectively. Before the upgrade, those values were 0.3 ± 0.7% and 0.9%, respectively. An exhaustive analysis of possible failure modes eventually zeroed in on the average misalignment of about 1 mm in the Y (along the couch) direction between the right and left upper MLC banks. The MLC was replaced, reducing the Y‐direction misalignment to 0.4 mm. As a result, the average and maximum IC dose‐errors became acceptable 1.0 ± 0.7% and 1.6%, respectively. Simple film and/or chamber array tests during acceptance testing can easily detect Y‐direction misalignments between opposing leaves banks measuring a fraction of a mm at isocenter. Left undetected, such misalignment can cause nontrivial dosimetric consequences.