Quantification of Magnetic Field Compatibility of Subcomponents in an MRI-Guided Radiation Therapy System

MR-PVHEE: A novel magnetic resonance-guided parallel-beam very high-energy electron radiotherapy system

Objective. With future advances in magnetic resonance imaging-guided radiation therapy, small photon beams are expected to be included regularly in clinical treatments. This study provides physical insights on detector dose-response to multiple megavoltage photon beam sizes coupled to magnetic fields and determines optimal orientations for measurements. Approach. Monte Carlo simulations determine small-cavity detector (solid-state: PTW60012 and PTW60019, ionization chambers: PTW31010, PTW31021, and PTW31022) dose-responses in water to an Elekta Unity 7 MV FFF photon beam. Investigations are performed for field widths between 0.25 and 10 cm in four detector axis orientations with respect to the 1.5 T magnetic field and the photon beam. The magnetic field effect on the overall perturbation factor (P MC) accounting for the extracameral components, atomic composition, and density is quantified in each orientation. The density (P ρ ) and volume averaging (P vol) perturbation factors and quality correction factors ( kQB,QfB,f ) accounting for the magnetic field are also calculated in each orientation. Main results. Results show that P vol remains the most significant perturbation both with and without magnetic fields. In most cases, the magnetic field effect on P vol is 1% or less. The magnetic field effect on P ρ is more significant on ionization chambers than on solid-state detectors. This effect increases up to 1.564 ± 0.001 with decreasing field size for chambers. On the contrary, the magnetic field effect on the extracameral perturbation factor is higher on solid-state detectors than on ionization chambers. For chambers, the magnetic field effect on P MC is only significant for field widths <1 cm, while, for solid-state detectors, this effect exhibits different trends with orientation, indicating that the beam incident angle and geometry play a crucial role. Significance. Solid-state detectors’ dose-response is strongly affected by the magnetic field in all orientations. The magnetic field impact on ionization chamber response increases with decreasing field size. In general, ionization chambers yield kQB,QfB,f closer to unity, especially in orientations where the chamber axis is parallel to the magnetic field.

BackgroundAccurate small‐field (SF) dosimetry is essential for Magnetic Resonance‐guided Radiation Therapy (MRgRT), particularly for stereotactic treatments. The static magnetic field (B‐field) alters detector response, necessitating detector‐specific correction factors kB,Qf,f to ensure dosimetric accuracy. While numerous studies have reported such factors, the published data shows considerable variability, creating uncertainty for clinical practice.PurposeThis study was undertaken to (1) systematically review the literature on SF photon dosimetry in MRgRT and (2) perform a meta‐analysis to pool quantitative data and investigate the sources of this variability.MethodsA systematic search of four databases was conducted following PRISMA 2020 guidelines. A qualitative synthesis was performed on all included studies (n = 86). A random‐effects meta‐analysis was performed on studies providing correction factors (beam quality correction factor, kmsr; field output correction factor, kclin) or uncorrected output factors (FOF) for fields ≤ 4 × 4 cm2 with associated uncertainties (n = 13 studies, 441 data points). Heterogeneity (I2) was quantified, and subgroup analyses (B‐field, detector model, and orientation) and meta‐regression (field size) were used to investigate its sources.ResultsThe qualitative review identified key themes in foundational physics, detector‐specific characteristics, and clinical quality assurance. The meta‐analysis revealed extremely high heterogeneity (I2 > 92%) across all pooled data, confirming that a single global‐average correction factor is statistically inappropriate. Subgroup analyses demonstrated that this heterogeneity is systematically driven by three main factors: magnetic field strength (e.g., 0.35 T vs. 1.5 T), specific detector model (with significant variations observed even among models of the same type), and detector orientation relative to the beam and B‐field. Meta‐regression showed that correction factors kmsr and kclin demonstrated no statistically significant dependence on field size in this SF regime, which suggests that the theoretical variation expected due to beam property changes is statistically masked by the high heterogeneity (I2 > 92) observed in the literature; conversely, FOFs showed a strong, statistically significant field‐size dependence.ConclusionThe findings confirm that detector response in MRgRT is a complex function of the measurement setup. A single correction factor is insufficient for clinical use. Dosimetric accuracy in MRgRT requires the use of correction factors that are specific to the MR‐Linac's B‐field strength, the exact detector model, and the measurement orientation. This analysis provides pooled, stratified data to support clinical physicists and inform the development of future dosimetry protocols.

Objective: Two benefits of MR-guided radiotherapy (MRgRT) are the ability to track target structures while treatment is being delivered and the ability to adapt plans daily for some lesions based on changing anatomy. These unique capacities come at two costs: increased capital for acquisition and greatly decreased workflow. An adaptive gated stereotactic body radiotherapy (MRgART) treatment routinely takes ~90 min to perform and requires the presence of both a physician and a physicist. This may significantly limit daily capacity. We previously described how “simple cases” were necessary for proton facilities to allow for debt management. In this manuscript, we seek to determine the optimal scheduling of different MRgRT plans to recoup capital costs. Materials/Methods: We assumed an MR-linac (MRL) was completely scheduled with patients over workdays of varying duration. Treatment times and reimbursement data from our facility for varying complexities of patients were extrapolated for varying numbers treated daily. We then derived the number of adaptive and non-adaptive patients required daily to optimize the schedules. HOPPS data were used to model reimbursement. Results: A single MRL treating 14 non-gated, non-adaptive IMRT patients over an 8 h workday would take about 4.8 years to cover initial acquisition and installation costs. However, such patients may be more quickly and efficiently treated with a conventional linear accelerator, while MRgART cases may only be treated with an MRL. By treating four of these daily, that same MRL room would cover costs in 2.4 years. Personnel, maintenance costs, and profit further complicate any business case for treating non-adaptive patients or for extending hours. Conclusions: In our previously published paper discussing proton therapy, we noted that debt is not variable with capacity; this remains true with MRgRT. Different from protons, a clinically optimal case load of adaptive patients provides an optimal business case as well. This requires a large patient cadre to ensure continuing throughput. As improvements in MRgRT are brought to the clinic, shorter adaptive and non-adaptive treatment times will help improve the timeframe to recoup costs but will require even more appropriate patients.

A Prospective Single-Arm Phase 2 Study of Stereotactic Magnetic Resonance Guided Adaptive Radiation Therapy for Prostate Cancer: Early Toxicity Results

Background: Stereotactic body radiotherapy (SBRT) for prostate cancer delivers high doses in few fractions but poses challenges in sparing adjacent organs at risk (OARs), particularly the intra-prostate urethra, bladder, rectum and penile bulb. Magnetic resonance-guided radiotherapy (MRgRT) using MR-Linac offers superior soft-tissue visualization and daily adaptive planning, potentially reducing OAR dose while maintaining target coverage. This study aimed to compare dose-volume parameters among MR-Linac (ML), volumetric modulated arc therapy (VMAT), and Tomotherapy (HT) plans for prostate SBRT. Methods: Thirty patients with localized prostate cancer were retrospectively analyzed. For each patient, three plans were generated: ML, VMAT and HT, using identical prescription and planning objectives. Dose-volume histogram (DVH) metrics were evaluated for clinical target volume (CTV), planning target volume (PTV), and OARs. Statistical comparisons were performed using non-parametric Friedman's Test with post hoc Bonferroni test, with significance set at a p < 0.05. Results: CTV coverage was comparable across all modalities. ML achieved significantly higher PTV Dmin and near-maximum doses compared to VMAT and HT. Notably, ML provided substantial urethral sparing, reducing Dmax and Dmean by approximately 3.3 Gy compared to both VMAT and HT (p < 0.001). Rectal dose metrics were also lower with ML, while bladder and penile bulb doses showed minor increases (<3.5 Gy), considered clinically negligible. Femoral head doses were reduced in ML plans. Conclusions: MR-Linac planning for prostate SBRT offers meaningful dosimetric advantages, particularly in intra-prostate urethra urethral dose reduction, without compromising target coverage. These findings support incorporating MR-guided adaptive workflows into SBRT protocols to enhance OAR protection and potentially reduce treatment-related toxicity.

To modify and evaluate an automatic American College of Radiology (ACR) phantom analysis toolbox for ACR quality assurance (QA) on a low-field MR-guided radiotherapy system (ViewRay). An open-source toolbox was modified for ACR QA of a 0.35T MRI system (ViewRay MRIdian). A total of 17 ACR datasets were evaluated, including 10 datasets acquired from different systems across the world, and seven datasets acquired at our center between 2014 and 2020. All required ACR tests, geometric accuracy (GA), high-contrast spatial resolution (HCSR), slice thickness accuracy (ST), slice position accuracy (SP), percent integral uniformity (PIU), percentage signal ghosting (PSG), and low-contrast object detectability (LCOD), were assessed manually and using the toolbox automatically. Measurements between manual and automatic analysis were compared. Precision, recall, and accuracy were calculated, where the manual results were used as the ground truth. The software took less than 2min to complete all seven tests, which usually requires 40min or more if analyzed manually. Overall, the automatic measurement was consistent with the manual result. The absolute differences between the two measurements were 0.72±0.66 (mm), 0.01±0.03, 0.50±0.60 (mm), 0.39±0.41 (mm), 1.01±1.00 (%), 0.0016±0.0019, and 2.79±2.29 for the seven tests. The precision of the automatic toolbox was 100% for all tests, indicating that a test would 100% pass a manual analysis if it had passed the automatic analysis. Recall and accuracy were≥96% for GA, HCSR, SP, PIU, PSG, and LCOD tests, and 91% for the ST test. An automatic ACR QA tool was adopted and evaluated for the low-field MR-guided radiotherapy (MRgRT) system. Overall, the toolbox provided comparable results as manual analysis, and reduced the processing time from over 40min to <2min. This toolbox holds the potential to be widely adopted either as a second check tool or partially replace human measurement for MRgRT programs using the same system.

In recent years, magnetic resonance imaging (MRI)-guided radiotherapy (RT) has experienced a notable increase in utilization due to technological advancements that leverage MRI's superior soft-tissue contrast and its non-invasive, non-ionizing imaging mechanism. Integrating MRI scanners with linac systems comes with several technical challenges, including the complex interactions between the linac components and the magnetic field of the MRI scanner. This study presents a comprehensive in silico finite element method (FEM)-based model for a proximity-type MRI-guided RT system consisting of an MRI-on-rail and a C-arm linac. The methodology enables the precise characterization of the MRI magnet's fringe field both in free space and when interacting with the ferromagnetic structure of the linac system. A comprehensive in silico FEM-based model was developed to simulate the MRI magnet and linac configuration. The magnet coil configuration was generated using linear programming based on the manufacturer's specifications for the 5 G line. The linac structure was modeled from technical schematics and coupled with the simulated magnet to construct the simulation environment. Fringe field measurements were conducted in a controlled environment to validate the simulation results. The measurements were taken at various distances from the MRI isocenter and in different directions to assess the spatial distribution of the fringe field. Simulations showed good agreement with experimental measurements, with a maximum difference of 1 G observed between simulated and measured fringe fields within the 3 to 5 m range from the MR isocenter, consistent with the 1 G Hall probe measurement tolerance. The linac's ferromagnetic structure significantly perturbed the magnetic fringe field, locally increasing field values to 40 G from an initial range of 0-23 G, and inducing local field differences of up to 30 G at its closest proximity to the magnet. Conversely, a local decrease of up to 2 G was observed near the linac isocenter. Furthermore, the room environment influenced the fringe field's spatial distribution, evidenced by deviations of approximately 6 and 4.2 G from the Espree reference field at 2.5 and 3 m, respectively. Despite these environmental effects, the overall agreement between simulations and experimental values, including measurements at the linac head (maximum difference less than 2 G), was highly satisfactory, confirming axial field symmetry and minimal impact from room layout variations. This study developed and validated a comprehensive FEM-based simulation methodology to accurately characterize magnetic field interactions in a proximity-type MRI-guided RT system. The methodology mapped the MRI magnet's fringe field in both free space and as perturbed by the linac's ferromagnetic structure, with experimental data also supporting these findings. This robust framework offers a reliable tool for guiding engineering activities and defining safety bounds for system design.

Cardiac applications in radiation therapy are rapidly expanding including magnetic resonance guided radiation therapy (MRgRT) for real-time gating for targeting and avoidance near the heart or treating ventricular tachycardia (VT). This work describes the development and implementation of a novel multi-modality and magnetic resonance (MR)-compatible cardiac phantom. The patient-informed 3D model was derived from manual contouring of a contrast-enhanced Coronary Computed Tomography Angiography scan, exported as a Stereolithography model, then post-processed to simulate female heart with an average volume. The model was 3D-printed using Elastic50A to provide MR contrast to water background. Two rigid acrylic modules containing cardiac structures were designed and assembled, retrofitting to an MR-safe programmable motor to supply cardiac and respiratory motion in superior-inferior directions. One module contained a cavity for an ion chamber (IC), and the other was equipped with multiple interchangeable cavities for plastic scintillation detectors (PSDs). Images were acquired on a 0.35 T MR-linac for validation of phantom geometry, motion, and simulated online treatment planning and delivery. Three motion profiles were prescribed: patient-derived cardiac (sine waveform, 4.3mm peak-to-peak, 60 beats/min), respiratory (cos4 waveform, 30mm peak-to-peak, 12 breaths/min), and a superposition of cardiac (sine waveform, 4mm peak-to-peak, 70 beats/min) and respiratory (cos4 waveform, 24mm peak-to-peak, 12 breaths/min). The amplitude of the motion profiles was evaluated from sagittal cine images at eight frames/s with a resolution of 2.4mm × 2.4mm. Gated dosimetry experiments were performed using the two module configurations for calculating dose relative to stationary. A CT-based VT treatment plan was delivered twice under cone-beam CT guidance and cumulative stationary doses to multi-point PSDs were evaluated. No artifacts were observed on any images acquired during phantom operation. Phantom excursions measured 49.3±25.8%/66.9±14.0%, 97.0±2.2%/96.4±1.7%, and 90.4±4.8%/89.3±3.5% of prescription for cardiac, respiratory, and cardio-respiratory motion profiles for the 2-chamber (PSD) and 12-substructure (IC) phantom modules respectively. In the gated experiments, the cumulative dose was<2% from expected using the IC module. Real-time dose measured for the PSDs at 10Hz acquisition rate demonstrated the ability to detect the dosimetric consequences of cardiac, respiratory, and cardio-respiratory motion when sampling of different locations during a single delivery, and the stability of our phantom dosimetric results over repeated cycles for the high dose and high gradient regions. For the VT delivery, high dose PSD was<1% from expected (5-6 cGy deviation of 5.9Gy/fraction) and high gradient/low dose regions had deviations<3.6% (6.3 cGy less than expected 1.73Gy/fraction). A novel multi-modality modular heart phantom was designed, constructed, and used for gated radiotherapy experiments on a 0.35 T MR-linac. Our phantom was capable of mimicking cardiac, cardio-respiratory, and respiratory motion while performing dosimetric evaluations of gated procedures using IC and PSD configurations. Time-resolved PSDs with small sensitive volumes appear promising for low-amplitude/high-frequency motion and multi-point data acquisition for advanced dosimetric capabilities. Illustrating VT planning and delivery further expands our phantom to address the unmet needs of cardiac applications in radiotherapy.

MR-Guided Radiation Therapy With Concurrent Gemcitabine/Nab-Paclitaxel Chemotherapy in Inoperable Pancreatic Cancer: A TITE-CRM Phase I Trial

Quality of Life and Toxicity in Patients With Pancreatic Ductal Adenocarcinoma Treated With Online Adaptive Stereotactic Magnetic Resonance Guided Radiation Therapy.

The First Report of Using Low-Field MRI-Guided Radiation Therapy in a Patient With a Cochlear Implant

Modern radiation therapy (RT) includes conformal therapy, intensity modulated radiation therapy, proton therapy, image-guided radiation therapy, adaptive radiation therapy (ART), and recently magnetic-resonance guided radiation therapy and, upcoming, 4π radiation therapy. These techniques show the continual increase in complexity of radiation therapy techniques which, coupled with a stagnant medical physics workforce, means that the amount and complexity of work per physicist has increased and is likely to increase in the foreseeable future. Three distinct challenges can be identified that need to be addressed. First, is the recent commercial development of automated multi-metastases stereotactic radiosurgery (SRS) techniques. These techniques plan and irradiate a number (up to approximately 20) brain lesions in one treatment session, typically employing one isocenter. The spatial accuracy specifications for SRS imply that attention to the angular accuracy is more critical for these treatments than conventional SRS or other treatment methods. In parallel, our and other groups are developing 4π techniques, which is a proposed method for optimizing both beam angles and intensity fluence to provide x-ray based dose distributions with unparalleled compactness and conformality. One cost to deliver these dose distributions is the added requirement to not only rotate the couch but also shift the couch to increase the number of available beam angles. These two techniques will require efficient and quantitative dose distribution measurements of relatively large volumes for, at least, end to end testing of multi-metastases and 4π treatments. Finally, magnetic resonance guided radiation therapy has led to a resurgence in the development of on-table ART, which requires that the medical physicist compare two calculated treatment plans and efficiently and effectively determine if differences between those treatment plans are clinically significant. Modifying and improving existing tools will be critical to the safe and effective on-table ART.

Impact of the implementation of a Contouring Atlas on Therapeutic Radiographer inter-observer contour variation in prostate radiotherapy

Magnetic Resonance-guided radiotherapy (MRgRT) marks the beginning of a new era. MR is a versatile and suitable imaging modality for radiotherapy, as it enables direct visualization of the tumor and the surrounding organs at risk. Moreover, MRgRT provides real-time imaging to characterize and eventually track anatomical motion. Nevertheless, the successful translation of new technologies into clinical practice remains challenging. To date, the initial availability of next-generation hybrid MR-linac (MRL) systems is still limited and therefore, the focus of the present preview was on the initial applicability in current clinical practice and on future perspectives of this new technology for different treatment sites.MRgRT can be considered a groundbreaking new technology that is capable of creating new perspectives towards an individualized, patient-oriented planning and treatment approach, especially due to the ability to use daily online adaptation strategies. Furthermore, MRL systems overcome the limitations of conventional image-guided radiotherapy, especially in soft tissue, where target and organs at risk need accurate definition. Nevertheless, some concerns remain regarding the additional time needed to re-optimize dose distributions online, the reliability of the gating and tracking procedures and the interpretation of functional MR imaging markers and their potential changes during the course of treatment. Due to its continuous technological improvement and rapid clinical large-scale application in several anatomical settings, further studies may confirm the potential disruptive role of MRgRT in the evolving oncological environment.