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The past decade has seen the increasing integration of magnetic resonance (MR) imaging into radiation therapy (RT). This growth can be contributed to multiple factors, including hardware and software advances that have allowed the acquisition of high-resolution volumetric data of RT patients in their treatment position (also known as MR simulation) and the development of methods to image and quantify tissue function and response to therapy. More recently, the advent of MR-guided radiation therapy (MRgRT) - achieved through the integration of MR imaging systems and linear accelerators - has further accelerated this trend. As MR imaging in RT techniques and technologies, such as MRgRT, gain regulatory approval worldwide, these systems will begin to propagate beyond tertiary care academic medical centers and into more community-based health systems and hospitals, creating new opportunities to provide advanced treatment options to a broader patient population. Accompanying these opportunities are unique challenges related to their adaptation, adoption, and use including modification of hardware and software to meet the unique and distinct demands of MR imaging in RT, the need for standardization of imaging techniques and protocols, education of the broader RT community (particularly in regards to MR safety) as well as the need to continue and support research, and development in this space. In response to this, an ad hoc committee of the American Association of Physicists in Medicine (AAPM) was formed to identify the unmet needs, roadblocks, and opportunities within this space. The purpose of this document is to report on the major findings and recommendations identified. Importantly, the provided recommendations represent the consensus opinions of the committee's membership, which were submitted in the committee's report to the AAPM Board of Directors. In addition, AAPM ad hoc committee reports differ from AAPM task group reports in that ad hoc committee reports are neither reviewed nor ultimately approved by the committee's parent groups, including at the council and executive committee level. Thus, the recommendations given in this summary should not be construed as being endorsed by or official recommendations from the AAPM.

Purpose To describe and characterize daily machine quality assurance (QA) for an MR-guided radiotherapy (MRgRT) linac system, in addition to reporting a longitudinal assessment of the dosimetric and mechanical stability over a 7-month period of clinical operation. Methods Quality assurance procedures were developed to evaluate MR imaging/radiation isocenter, imaging and patient handling system, and linear accelerator stability. A longitudinal assessment was characterized for safety interlocks, laser and imaging isocenter coincidence, imaging and radiation (RT) isocentricity, radiation dose rate and output, couch motion, and MLC positioning. A cylindrical water phantom and an MR-compatible A1SL detector were utilized. MR and RT isocentricity and MLC positional accuracy was quantified through dose measured with a 0.40 cm2 x 0.83 cm2 field at each cardinal angle. The relationship between detector response to MR/RT isocentricity and MLC positioning was established through introducing known errors in phantom position. Results Correlation was found between detector response and introduced positional error (N = 27) with coefficients of determination of 0.9996 (IEC-X), 0.9967 (IEC-Y), 0.9968 (IEC-Z) in each respective shift direction. The relationship between dose (DoseMR/RT+MLC ) and the vector magnitude of MLC and MR/RT positional error (Errormag ) was calculated to be a nonlinear response and resembled a quadratic function: DoseMR/RT+MLC [%] = -0.0253 Errormag [mm]2 - 0.0195 Errormag [mm]. For the temporal assessment (N = 7 months), safety interlocks were functional. Laser coincidence to MR was within ±2.0 mm (99.6%) and ±1.0 mm (86.8%) over the 7-month assessment. IGRT position-reposition shifts were within ±2.0 mm (99.4%) and ±1.0 mm (92.4%). Output was within ±3% (99.4%). Mean MLC and MR/RT isocenter accuracy was 1.6 mm, averaged across cardinal angles for the 7-month period. Conclusions The linac and IGRT accuracy of an MR-guided radiotherapy system has been validated and monitored over seven months for daily QA. Longitudinal assessment demonstrated a drift in dose rate, but temporal assessment of output, MLC position, and isocentricity has been stable.

MR‐guided radiotherapy requires strong imaging spatial integrity to deliver high quality plans and provide accurate dose calculation. The MRI system, however, can be compromised by the integrated linear accelerator (Linac), resulting in inaccurate imaging isocenter position and geometric distortion. Dependence on gantry position further complicates the correction of distortions. This work presents a new clinical application of a commercial phantom and software system that quantifies isocenter alignment and geometric distortion, as well as providing a deformation vector field (DVF). A large distortion phantom and a smaller grid phantom were imaged at multiple gantry angles from 0 to 330° on a 0.35 T integrated MR‐Linac. The software package was used to assess geometric distortion and generate DVFs to correct distortions within the phantom volume. The DVFs were applied to the grid phantom with resampling software then evaluated using structural similarity index measure (SSIM). Scans were also performed with a ferromagnetic clip near the phantom to investigate the correction of more severe artifacts. The mean magnitude isocenter shift was 0.67 mm, ranging from 0.25 to 1.04 mm across all angles. The DVF had a mean component value of 0.27 ± 0.02, 0.24 ± 0.01, and 0.19 ± 0.01 mm in the right‐left (RL), anterior‐posterior (AP), and superior‐inferior (SI) directions. The ferromagnetic clip increased isocenter position error from 1.98 mm to 2.20 mm and increased mean DVF component values in the RL and AP directions. The resampled grid phantom had an increased SSIM for all gantry angles compared to original images, increasing from 0.26 ± 0.001 to 0.70 ± 0.004. Through this clinical assessment, we were able to correct geometric distortion and isocenter shift related to gantry position on a 0.35 T MR‐Linac using the distortion phantom and software package. This provides encouragement that it could be used for quality assurance and clinically to correct systematic distortion caused by imaging at different gantry angles.

Objective Adaptive radiation therapy (ART) is an integral part of MR-guided RT (MRgRT), requiring a new RT plan for each treatment fraction and resulting in a significant increase in patient-specific quality assurance (PSQA). This study investigates the possibility of using treatment log-file for automated PSQA. Method All treatment plans were delivered in 1.5T Unity MR-Linac (Elekta). A Unity compatible version of LinacView (Standard Imaging) was commissioned to automatically monitor and analyze the log-files. A total of 220 fields were delivered and measured by ArcCheck® -MR (Sun Nuclear) and LinacView. Thirty incorrectly matched fields were also delivered to check for error detection sensitivity. The gamma analysis, γ, with 3%, 3 mm criteria was used in both ArcCheck® -MR and LinacView. Additionally, the gantry angle, jaws, and multileaf collimators (MLC) positions reported in the log-file were compared with plan positions using TG-142 criteria. Result The γ (3%, 3 mm) for the 190 plans were found to be between the range of 72.5%-100.0% and 95.4%-100.0% for ArcCheck® -MR and LinacVeiw, respectively. All the delivered gantry angle and jaws were found to be within 0.2° and 2 mm. MLCs that were outside the guard leaves or under the diaphragms were found to have more than 1.0 mm discrepancy. This was attributed to the linac internal override for these MLCs and had no dosimetric impact. Excluding these discrepancies, all MLC positions were found to be within 1.0 mm. The γ (3%, 3 mm) for the 30 incorrectly matched fields were found to be 3.9%-84.8% and 0.1%-64.4% for ArcCheck® -MR and LinacVeiw, respectively. Conclusion Significant ranked correlation demonstrates the automated log-file analysis can be used for PSQA and expedite the ART workflow. Ongoing PSQA will be compared with log-file analysis to investigate the longer term reproducibility and correlation.

PurposeWe report preliminary dosimetric data concerning the use of 1.5-T MR-guided daily-adaptive radiotherapy for abdomino-pelvic lymph-nodal oligometastases. We aimed to assess the impact of this technology on mitigating daily variations for both target coverage and organs-at-risk (OARs) sparing. MethodsA total of 150 sessions for 30 oligometastases in 23 patients were analyzed. All patients were treated with MR-guided stereotactic body radiotherapy (SBRT) for a total dose of 35Gy in 5 fractions. For each fraction, a quantitative analysis was performed for PTV volume, V35Gy and Dmean. Similarly, for OARs we assessed daily variations of volume, Dmean, Dmax. Any potential statistically significant change between baseline planning and daily-adaptive sessions was assessed using the Wilcoxon signed-rank test, assuming a p-valueResultsAverage baseline PTV, bowel, bladder and single intestinal loop volumes were respectively 8.9cc (range, 0.7-41.2cc), 1176cc (119-3654 cc), 95cc (39.7-202.9 cc), 18.3cc (9.1-37.7 cc). No significant volume variations were detected for PTV (p=0.21) bowel (p=0.36), bladder (p=0.47), except for single intestinal loops, which resulted smaller (p=0.026). Average baseline V35Gy and Dmean for PTV were respectively 85.6% (72-98.8%) and 35.6 Gy (34.6-36.1 Gy). We recorded a slightly positive trend in favor of daily-adaptive strategy vs baseline planning for improved target coverage, although not reaching statistical significance. (p=0.11 and p=0.18 for PTV-V35Gy and PTV-Dmean). Concerning OARs, a significant difference was observed in favor of daily-adapted treatments in terms of single intestinal loop Dmax [23.05 Gy (13.2-26.9 Gy) at baseline vs 20.5 Gy (12.1-24 Gy); p-value=0.0377] and Dmean [14.4 Gy (6.5-18 Gy) at baseline vs 13.0 Gy (6.7-17.6 Gy); p-value=0.0003].Specifically for bladder, the average Dmax was 18.6 Gy (0.4-34.3 Gy) at baseline vs 18.3 Gy (0.7-34.3 Gy) for a p-value=0.28; the average Dmean was 7.0 Gy (0.2-16.6 Gy) at baseline vs 6.98 Gy (0.2-16.4 Gy) for a p-value=0.66. Concerning the bowel, no differences in terms of Dmean [4.78 Gy (1.3-10.9 Gy) vs 5.6 Gy (1.4-10.5 Gy); p-value=0.23] were observed between after daily-adapted sessions. A statistically significant difference was observed for bowel Dmax [26.4 Gy (7.7-34 Gy) vs 25.8 Gy (7.8-33.1 Gy); p-value=0.0086].ConclusionsDaily-adaptive MR-guided SBRT reported a significantly improved single intestinal loop sparing for lymph-nodal oligometastases. Also bowel Dmax was significantly reduced with daily-adaptive strategy. A minor advantage was also reported in terms of PTV coverage, although not statistically significant.

Simple Summary Geometric variations of patients receiving stereotactic body radiotherapy (SBRT) after radical prostatectomy were evaluated in this study. With specific bladder filling and rectal emptying patient instructions, CTV shape and volume remain relatively stable and dose coverage is well maintained whilst maintaining OAR dose constraints in most treatment fractions. However, target coverage and OAR doses can be further improved by MR-guided online adaptive planning to account for interfractional geometric variations. Abstract Purpose: To evaluate geometric variations of patients receiving stereotactic body radiotherapy (SBRT) after radical prostatectomy and the dosimetric benefits of stereotactic MRI guided adaptive radiotherapy (SMART) to compensate for these variations. Materials/Methods: The CTV and OAR were contoured on 55 MRI setup scans of 11 patients treated with an MR-LINAC and enrolled in a phase II trial of post-prostatectomy SBRT. All patients followed institutional bladder and rectum preparation protocols and received five fractions of 6−6.8 Gy to the prostate bed. Interfractional changes in volume were calculated and shape deformation was quantified by the Dice similar coefficient (DSC). Changes in CTV-V95%, bladder and rectum maximum dose, V32.5Gy and V27.5Gy were predicted by recalculating the initial plan on daily MRI. SMART was retrospectively simulated if the predicted dose exceeded pre-set criteria. Results: The CTV volume and shape remained stable with a median volumetric change of 3.0% (IQR −3.0% to 11.5%) and DSC of 0.83 (IQR 0.79 to 0.88). Relatively large volumetric changes in bladder (median −24.5%, IQR −34.6% to 14.5%) and rectum (median 5.4%, IQR − 9.7% to 20.7%) were observed while shape changes were moderate (median DSC of 0.79 and 0.73, respectively). The median CTV-V95% was 98.4% (IQR 94.9% to 99.6%) for the predicted doses. However, SMART would have been deemed beneficial for 78.2% of the 55 fractions based on target undercoverage (16.4%), exceeding OAR constraints (50.9%), or both (10.9%). Simulated SMART improved the dosimetry and met dosimetric criteria in all fractions. Moderate correlations were observed between the CTV-V95% and target DSC (R2 = 0.73) and bladder mean dose versus volumetric changes (R2 = 0.61). Conclusions: Interfractional dosimetric variations resulting from anatomic deformation are commonly encountered with post-prostatectomy RT and can be mitigated with SMART.

MRI is the standard modality to assess anatomy and response to treatment in brain and spine tumors given its superb anatomic soft tissue contrast (e.g., T1 and T2) and numerous additional intrinsic contrast mechanisms that can be used to investigate physiology (e.g., diffusion, perfusion, spectroscopy). As such, hybrid MRI and radiotherapy (RT) devices hold unique promise for Magnetic Resonance guided Radiation Therapy (MRgRT). In the brain, MRgRT provides daily visualizations of evolving tumors that are not seen with cone beam CT guidance and cannot be fully characterized with occasional standalone MRI scans. Significant evolving anatomic changes during radiotherapy can be observed in patients with glioblastoma during the 6-week fractionated MRIgRT course. In this review, a case of rapidly changing symptomatic tumor is demonstrated for possible therapy adaptation. For stereotactic body RT of the spine, MRgRT acquires clear isotropic images of tumor in relation to spinal cord, cerebral spinal fluid, and nearby moving organs at risk such as bowel. This visualization allows for setup reassurance and the possibility of adaptive radiotherapy based on anatomy in difficult cases. A review of the literature for MR relaxometry, diffusion, perfusion, and spectroscopy during RT is also presented. These techniques are known to correlate with physiologic changes in the tumor such as cellularity, necrosis, and metabolism, and serve as early biomarkers of chemotherapy and RT response correlating with patient survival. While physiologic tumor investigations during RT have been limited by the feasibility and cost of obtaining frequent standalone MRIs, MRIgRT systems have enabled daily and widespread physiologic measurements. We demonstrate an example case of a poorly responding tumor on the 0.35 T MRIgRT system with relaxometry and diffusion measured several times per week. Future studies must elucidate which changes in MR-based physiologic metrics and at which timepoints best predict patient outcomes. This will lead to early treatment intensification for tumors identified to have the worst physiologic responses during RT in efforts to improve glioblastoma survival.

Simple Summary Stereotactic body radiotherapy is used in the treatment of liver tumors. However, adjacent organs at risk (OAR) frequently limit the applicable dose to the target volume. The introduction of hybrid magnetic resonance imaging (MRI)-guided radiotherapy systems may allow dose escalation strategies with better OAR sparing due to improved soft tissue visualization, adaptive treatment planning and real-time motion management. Here we report the feasibility and early results of online adaptive MR-guided radiotherapy of primary and secondary liver tumors in eleven patients. The treatment was feasible and successfully completed in all patients. After a median follow-up of five months, no local failure occurred and no ≥ grade 2 toxicity was observed. The technique should be compared to conventional SBRT in further studies to assess the advantages of the technique. Abstract Purpose: To assess the feasibility and early results of online adaptive MR-guided radiotherapy (oMRgRT) of liver tumors. Methods: We retrospectively examined consecutive patients with primary or secondary liver lesions treated at our institution using a 0.35T hybrid MR-Linac (Viewray Inc., Mountain View, CA, USA). Online-adaptive treatment planning was used to account for interfractional anatomical changes, and real-time intrafractional motion management using online 2D cine MRI was performed using a respiratory gating approach. Treatment response and toxicity were assessed during follow-up. Results: Eleven patients and a total of 15 lesions were evaluated. Histologies included cholangiocarcinomas and metastases of neuroendocrine tumors, colorectal carcinomas, sarcomas and a gastrointestinal stroma tumor. The median BED10 of the PTV prescription doses was 84.4 Gy (range 59.5–112.5 Gy) applied in 3–5 fractions and the mean GTV BED10 was in median 147.9 Gy (range 71.7–200.5 Gy). Online plan adaptation was performed in 98% of fractions. The median overall treatment duration was 53 min. The treatment was feasible and successfully completed in all patients. After a median follow-up of five months, no local failure occurred and no ≥ grade two toxicity was observed. OMRgRT resulted in better PTV coverage and fewer OAR constraint violations. Conclusion: Early results of MR-linac based oMRgRT for the primary and secondary liver tumors are promising. The treatment was feasible in all cases and well tolerated with minimal toxicity. The technique should be compared to conventional SBRT in further studies to assess the advantages of the technique.

Radiotherapy treatments need adequate quality control (QC) to ensure a correct delivery of the prescribed dose to the target area. One of the most extended safety nets for treatments in conventional radiotherapy machines is in-vivo EPID dosimetry, which uses the dose acquired by an Electronic Portal Imaging Device (EPID) during treatment to accurately reconstruct the dose as it was delivered to the patient.We developed a method to validate radiotherapy treatments delivered on a novel system: the Unity MR-Linac. This machine, which combines a radiation source (linac) and an imaging device (MRI), will help to irradiate tumors more accurately by means of a new range of techniques only available thanks to the image guidance of the MRI during irradiation. The verification of such treatments can be performed by using images of the delivered beam captured by an EPID situated opposite to the radiation source, behind the cryostat of the MRI scanner. This project focuses on the adaptation of an already existing algorithm used with conventional linacs to the new physics and design characteristics of the Unity MR-linac. The main challenge for this adaptation is the presence of the MRI scanner between the patient and the EPID, acting as a secondary source of scatter and as an attenuation medium for the beam.

Novel magnetic-resonance-guided radiotherapy (MRgRT) permits real-time soft-tissue visualization, respiratory-gated delivery with minimal safety margins, and time-consuming daily plan re-optimisation. We report on early clinical outcomes of MRgRT and routine plan re-optimization for large primary renal cell cancer (RCC). Thirty-six patients were treated with MRgRT in 40 Gy/5 fractions. Prior to each fraction, re-contouring of tumor and normal organs on a pretreatment MR-scan allowed daily plan re-optimization. Treatment-induced toxicity and radiological responses were scored, which was followed by an offline analysis to evaluate the need for such daily re-optimization in 180 fractions. Mean age and tumor diameter were 78.1 years and 5.6 cm, respectively. All patients completed MRgRT with an average fraction duration of 45 min. Local control (LC) and overall survival rates at one year were 95.2% and 91.2%. No grade &ge, 3 toxicity was reported. Plans without re-optimization met institutional radiotherapy constraints in 83.9% of 180 fractions. Thus, daily plan re-optimization was required for only a minority of patients, who can be identified upfront by a higher volume of normal organs receiving 25 Gy in baseline plans. In conclusion, stereotactic MRgRT for large primary RCC showed low toxicity and high LC, while daily plan re-optimization was required only in a minority of patients.

Radiotherapy components of an magnetic resonnace‐guided radiotherapy (MRgRT) system can alter the magnetic fields, causing spatial distortion and image deformation, altering imaging and radiation isocenter coincidence and the accuracy of dose calculations. This work presents a characterization of radiotherapy component impact on MR imaging quality in terms of imaging isocenter variation and spatial integrity changes on a 0.35T MRgRT system, pre‐ and postupgrade of the system. The impact of gantry position, MLC field size, and treatment table power state on imaging isocenter and spatial integrity were investigated. A spatial integrity phantom was used for all tests. Images were acquired for gantry angles 0–330° at 30° increments to assess the impact of gantry position. For MLC and table power state tests all images were acquired at the home gantry position (330°). MLC field sizes ranged from 1.66 to 27.4 cm edge length square fields. Imaging isocenter shift caused by gantry position was reduced from 1.7 mm at gantry 150° preupgrade to 0.9 mm at gantry 120° postupgrade. Maximum spatial integrity errors were 0.5 mm or less pre‐ and postupgrade for all gantry angles, MLC field sizes, and treatment table power states. However, when the treatment table was powered on, there was significant reduction in SNR. This study showed that gantry position can impact imaging isocenter, but spatial integrity errors were not dependent on gantry position, MLC field size, or treatment table power state. Significant isocenter variation, while reduced postupgrade, is cause for further investigation.

Purpose Real‐time magnetic resonance guided radiation therapy (MRgRT) uses 2D cine imaging for target tracking. This work evaluates the percent image uniformity (PIU) and spatial integrity of cine images in the presence of multileaf collimator (MLC) and gantry motion in order to simulate sliding window and volumetric modulated arc therapy (VMAT) conditions. Methods Percent image uniformity and spatial integrity of cine images were measured (1) during MLC motion, (2) as a function of static gantry position, and (3) during gantry rotation. PIU was calculated according to the ACR MRI Quality Control Manual. Spatial integrity was evaluated by measuring the geometric distortion of 16 measured marker positions (10 cm or 15.225 cm from isocenter). Results The PIU of cine images did not vary by more than 1% from static linac conditions during MLC motion and did not vary by more than 3% during gantry rotation. Banding artifacts were present during gantry rotation. The geometric distortion in the cine images was less than 0.88 mm for all points measured throughout MLC motion. For all static gantry positions, the geometric distortion was less than 0.88 mm at 10 cm from isocenter and less than 1.4 mm at 15.225 cm from isocenter. During gantry rotation, the geometric distortion remained less than 0.92 mm at 10 cm from isocenter and less than 1.60 mm at 15.225 cm from isocenter. Conclusion During MLC motion, cine images maintained adequate PIU, and the geometric distortion of points within 15.225 cm from isocenter was less than the 1 mm threshold necessary for real‐time target tracking and gating. During gantry rotation, PIU was negatively affected by banding artifacts, and spatial integrity was only maintained within 10 cm from isocenter. Future work should investigate the effects imaging artifacts have on real‐time target tracking during MRgRT.

Highlights • The case report of a patient affected by locally advanced cervical MRgRT is described. • MRgRT appears to be feasible for cervical cancer and may improve treatment quality. • MRgRT insights are discussed focusing on adaptive response and toxicity monitoring.

Background and Purpose: Inverse treatment planning for lung cancer can be challenging since density heterogeneities may appear inside the planning target volume (PTV). One method to improve the quality of intensity modulation is the override of low density tissues inside the PTV during plan optimization. For magnetic resonance-guided radiation therapy (MRgRT), where the influence of the magnetic field on secondary electrons is sensitive to the tissue density, the reliability of density overrides has not yet been proven. This work, therefore, gains a first insight into density override strategies for MRgRT. Material and methods: Monte Carlo-based treatment plans for five lung cancer patients were generated based on free-breathing CTs and two density override strategies. Different magnetic field configurations were considered with their effect being accounted for during optimization. Optimized plans were forward calculated to 4D-CTs and accumulated for the comparison of planned and expected delivered dose. Results: For MRgRT, density overrides led to a discrepancy between the delivered and planned dose. The tumor volume coverage deteriorated for perpendicular magnetic fields of 1.5 T to 93.6% (D98%). For inline fields a maximal increase of 2.2% was found for the mean dose. In terms of organs at risk, a maximal sparing of 0.6 Gy and 0.9 Gy was observed for lung and heart, respectively. Conclusions: In this work, first results on the effect of density overrides on treatment planning for MRgRT are presented. It was observed that the underestimation of magnetic field effects in overridden densities during treatment planning resulted in an altered delivered dose, depending on the field strength and orientation. Keywords: Density overrides, Treatment planning, IMRT, Monte Carlo, MRgRT, MR-linac, Magnetic field, Lung cancer, Dose delivery, Tumor motion

Accuracy in radiotherapy is of vital importance for individual patient treatment and for clinical trials on which patient treatments. There is a continuous demand for improvement of dosimetry methods and adaption of dosimetry (primary) standards for new applications, including the introduction of new treatment modalities (e.g. FFF) and reference conditions (e.g. magnetic fields in MRgRT). For primary standard dosimetry laboratories (PSDLs), it is generally not feasible to acquire such modalities. Therefore, it is strongly desired that Dw measurements with primary standards can be performed on-site in clinical beams for new treatment modalities. In this study, a transportable absorbed dose to water primary standard, a water calorimeter, was developed for operation on-site in various beam geometries including MRI-incorporated treatment machines. The relative combined standard uncertainty (k = 1) for Dw measurements with the new standard in 60Co and MV-photons was 0.37 % (k = 1). Calibrations were carried out with a standard uncertainty of 0.42 % and kQ-factors are determined with a standard uncertainty of 0.40 %. An international key comparison with the BIPM (BIPM.RI(I)-K6) resulted in registration of related calibration measurement capabilities in the BIPM key comparison database (KCDB). This provided the required evidence for international acceptance of the water calorimeter as the Dutch absorbed dose to water primary standard for accelerator photon beams, ensuring acceptance of its measurement results through the CIPM Mutual Recognition Arrangement, CIPM MRA. Measurements with the new primary standard were performed in FFF-cFF beam pairs at 6 MV and 10 MV of an Elekta Versa HD for a selection of three different Farmer-type ion chambers. Based on these measurements and with the equipment described, it was concluded that the kQ factors provided by current codes of practice can be applied for flattening filter free beams without additional correction. However, an ion chamber volume averaging correction must be applied. Commissioning of the calorimeter in the 7 MV-photon beam of a pre-clinical MRI-linac in a 1.5 T magnetic field showed that the calorimeter corrections are either independent of magnetic field or can be determined in a magnetic field. The uncertainty for measurement of Dw with a water calorimeter in a 1.5 T magnetic field was estimated to be the same as under conventional reference conditions. Therefore, the VSL water calorimeter can be applied as a primary standard for Dw in magnetic fields and is currently the only primary standard operable in a magnetic field that provides direct access to the international traceability framework. For the first time ion chamber correction factors, kQ and kB, were directly measured in an MRI-linac. It was concluded that calculated and measured results in existing literature agree with the new values. The values in this study are currently the only available measured values for kQ and kB in an MRI-linac that are directly linked to the international traceability framework for the quantity absorbed dose to water, Dw. The new primary standard and its application stimulates the development of new measurement methods and contributes to a safe introduction of future radiation techniques.

Accuracy in radiotherapy is of vital importance for individual patient treatment and for clinical trials on which patient treatments. There is a continuous demand for improvement of dosimetry methods and adaption of dosimetry (primary) standards for new applications, including the introduction of new treatment modalities (e.g. FFF) and reference conditions (e.g. magnetic fields in MRgRT). For primary standard dosimetry laboratories (PSDLs), it is generally not feasible to acquire such modalities. Therefore, it is strongly desired that Dw measurements with primary standards can be performed on-site in clinical beams for new treatment modalities. In this study, a transportable absorbed dose to water primary standard, a water calorimeter, was developed for operation on-site in various beam geometries including MRI-incorporated treatment machines. The relative combined standard uncertainty (k = 1) for Dw measurements with the new standard in 60Co and MV-photons was 0.37 % (k = 1). Calibrations were carried out with a standard uncertainty of 0.42 % and kQ-factors are determined with a standard uncertainty of 0.40 %. An international key comparison with the BIPM (BIPM.RI(I)-K6) resulted in registration of related calibration measurement capabilities in the BIPM key comparison database (KCDB). This provided the required evidence for international acceptance of the water calorimeter as the Dutch absorbed dose to water primary standard for accelerator photon beams, ensuring acceptance of its measurement results through the CIPM Mutual Recognition Arrangement, CIPM MRA. Measurements with the new primary standard were performed in FFF-cFF beam pairs at 6 MV and 10 MV of an Elekta Versa HD for a selection of three different Farmer-type ion chambers. Based on these measurements and with the equipment described, it was concluded that the kQ factors provided by current codes of practice can be applied for flattening filter free beams without additional correction. However, an ion chamber volume averaging correction must be applied. Commissioning of the calorimeter in the 7 MV-photon beam of a pre-clinical MRI-linac in a 1.5 T magnetic field showed that the calorimeter corrections are either independent of magnetic field or can be determined in a magnetic field. The uncertainty for measurement of Dw with a water calorimeter in a 1.5 T magnetic field was estimated to be the same as under conventional reference conditions. Therefore, the VSL water calorimeter can be applied as a primary standard for Dw in magnetic fields and is currently the only primary standard operable in a magnetic field that provides direct access to the international traceability framework. For the first time ion chamber correction factors, kQ and kB, were directly measured in an MRI-linac. It was concluded that calculated and measured results in existing literature agree with the new values. The values in this study are currently the only available measured values for kQ and kB in an MRI-linac that are directly linked to the international traceability framework for the quantity absorbed dose to water, Dw. The new primary standard and its application stimulates the development of new measurement methods and contributes to a safe introduction of future radiation techniques.

New hybrid radiotherapy treatment systems combining an MRI scanner with a source of ionizing radiation are being introduced in the clinic. The strong magnetic fields of MRI considerably affect radiation dose distributions, especially at tissue-air interfaces due to the electron return effect (ERE). Experimental investigation of the ERE within a sub-millimeter thick surface layer is still highly challenging. In the present work, we examine and quantify the magnetic field induced perturbations of dose distributions within a 0.5 mm layer surrounding millimeter-size air cavities by applying electron paramagnetic resonance imaging (EPRI). Air-filled fused quartz tubes (inner diameter 3 or 4 mm) mimic small air cavities and serve as model systems. The tubes were irradiated inside a PMMA phantom by a 6 MV photon beam. The irradiations were performed in the presence or absence of a transverse, magnetic field providing a magnetic field strength of 1.0 Tesla. The spatial distributions of radiation induced paramagnetic defects in the quartz tubes were subsequently determined by applying field-swept echo-detected EPRI and were then converted to relative dose distributions. The transverse magnetic field leads to considerable local dose enhancements and reductions (up to 35%) with respect to the mean dose within the quartz tubes. The experimentally determined dose distributions are in good quantitative agreement with Monte Carlo radiation transport simulations. The results of this work demonstrate the feasibility of field-swept echo-detected EPRI to measure magnetic field induced perturbations of dose distributions within a sub-millimeter thick surface layer at the dosimeter-air interface. ispartof: PHYSICS IN MEDICINE AND BIOLOGY vol:64 issue:17 ispartof: location:England status: published

Purpose: In this work, the fabrication, operation, and evaluation of a probe-format graphite calorimeter — herein referred to as Aerrow — as an absolute clinical dosimeter of high-energy photon beams while in the presence of a B = 1.5 T magnetic field is described. Comparable to a cylindrical ionization chamber (IC) in terms of utility and usability, Aerrow has been developed for the purpose of accurately measuring absorbed dose to water in the clinic with a minimum disruption to the existing clinical workflow. To our knowledge, this is the first reported application of graphite calorimetry to magnetic resonance imaging (MRI)-guided radiotherapy. Methods: Based on a previously numerically optimized and experimentally validated design, an Aerrow prototype capable of isothermal operation was constructed in-house. Graphite-to-water dose conversions as well as magnetic field perturbation factors were calculated using Monte Carlo, while heat transfer and mass impurity corrections and uncertainties were assessed analytically. Reference dose measurements were performed in the absence and presence of a B = 1.5 T magnetic field using Aerrow in the 7 MV FFF photon beam of an Elekta MRI-linac and were directly compared to the results obtained using two calibrated reference-class IC types. The feasibility of performing solid phantom-based dosimetry with Aerrow and the possible influence of clearance gaps is also investigated by performing reference-type dosimetry measurements for multiple rotational positions of the detector and comparing the results to those obtained in water. Results: In the absence of the B-field, as well as in the parallel orientation while in the presence of the B-field, the absorbed dose to water measured using Aerrow was found to agree within combined uncertainties with those derived from TG-51 using calibrated reference-class ICs. Statistically significant differences on the order of (2–4)%, however, were observed when measuring absorbed dose to water using the ICs in the perpendicular orientation in the presence of the B-field. Aerrow had a peak-to-peak response of about 0.5% when rotated within the solid phantom regardless of whether the B-field was present or not. Conclusions: This work describes the successful use of Aerrow as a straightforward means of measuring absolute dose to water for large high-energy photon fields in the presence of a 1.5 T B-field to a greater accuracy than currently achievable with ICs. The detector-phantom air gap does not appear to significantly influence the response of Aerrow in absolute terms, nor does it contribute to its rotational dependence. This work suggests that the accurate use of solid phantoms for absolute point dose measurement is possible with Aerrow.

The output of MRI-integrated photon therapy (MRgXT) devices is measured in terms of absorbed dose to water, D w. Traditionally this is done with reference type ion chambers calibrated in a beam quality Q 0 without magnetic field. To correct the ion chamber response for the application in the magnetic field, a factor needs to be applied that corrects for both beam quality Q and the presence of the magnetic field B, k Q,B. This can be expressed as the product of k Q, without magnetic field, and ion chamber magnetic field correction, k B. k B depends on the magnetic field strength and its direction, the direction of the beam and the orientation and type of the ion chamber. In this study, for the first time, both k Q and k B were measured directly for six waterproof ion chambers (3 × PTW 30013 and 3 × IBA FC65-G) in a pre-clinical 7 MV MRI-linac at 0 T and at 1.5 T. Measurements were done with the only available primary standard built for this purpose, a water calorimeter. Resulting k Q factors for PTW and IBA chambers were 0.985(5) and 0.990(4), respectively. k B factors were measured with the chambers in antiparallel direction to the magnetic field (|| 180°), and perpendicular direction (⊥ −90°). k B|| and k B⊥ for the PTW chambers were 0.985(6) and 0.963(4), respectively and for IBA chambers 0.995(4) and 0.956(4). Agreement with the available literature values was shown, partly caused by the relatively large standard deviation (SD) in those values. The values in this study are currently the only available measured values for k Q and k B in an MRI-linac that are directly linked to the international traceability framework for the quantity absorbed dose to water, D w.

Purpose With the increasing use of MR‐guided radiation therapy (MRgRT), it becomes important to understand and explore accuracy of medical dosimeters in the presence of magnetic field. The purpose of this work is to characterize metal‐oxide‐semiconductor field‐effect transistors (MOSFETs) in MRgRT systems at 0.345 T magnetic field strength. Methods A MOSFET dosimetry system, developed by Best Medical Canada for in‐vivo patient dosimetry, was used to study various commissioning tests performed on a MRgRT system, MRIdian® Linac. We characterized the MOSFET dosimeter with different cable lengths by determining its calibration factor, monitor unit linearity, angular dependence, field size dependence, percentage depth dose (PDD) variation, output factor change, and intensity modulated radiation therapy quality assurance (IMRT QA) verification for several plans. MOSFET results were analyzed and compared with commissioning data and Monte Carlo calculations. Results MOSFET measurements were not found to be affected by the presence of 0.345 T magnetic field. Calibration factors were similar for different cable length dosimeters either placed at the parallel or perpendicular direction to the magnetic field, with variations of less than 2%. The detector showed good linearity (R2 = 0.999) for 100–600 MUs range. Output factor measurements were consistent with ionization chamber data within 2.2%. MOSFET PDD measurements were found to be within 1% for 1–15 cm depth range in comparison to ionization chamber. MOSFET normalized angular response matched thermoluminescent detector (TLD) response within 5.5%. The IMRT QA verification data for the MRgRT linac showed that the percentage difference between ionization chamber and MOSFET was 0.91%, 2.05%, and 2.63%, respectively for liver, spine, and mediastinum. Conclusion MOSFET dosimeters are not affected by the 0.345 T magnetic field in MRgRT system. They showed physics parameters and performance comparable to TLD and ionization chamber; thus, they constitute an alternative to TLD for real‐time in‐vivo dosimetry in MRgRT procedures.

MRI guided radiotherapy devices are currently in clinical use. Detector responses are affected by the magnetic field and need to be characterized in terms of absorbed dose to water, D w, against primary standards under these conditions. The aim of this study was to commission a water calorimeter, accepted as the Dutch national standard for D w in MV photons and to validate its claimed standard uncertainty of 0.37% in the 7 MV photon beam of a pre-clinical MRI-linac in a 1.5 T magnetic field. To evaluate the primary standard on a fundamental basis, realisation of D w at 1.5 T was evaluated parameter by parameter. A thermodynamic description was given to demonstrate potential temperature effects due to the magneto-caloric effect (MCE). Methods were developed for measurement of depth, variation in detector distance and beam output in the bore of the MRI-linac. This resulted in D w measurements with a magnetic field of 1.5 T and, after ramp-down, without magnetic field. It was shown that the measurement of ΔT w and calorimeter corrections are either independent of or can be determined in a magnetic field. The chemical heat defect, h, was considered zero within its stated uncertainty, as for 0 T. Evaluation of the MCE and measurements done during magnet ramp-down, indicated no changes in the specific heat capacity of water. However, variations of the applied monitor system increased the uncertainty on beam output normalization. This study confirmed that the uncertainty for measurement of D w with a water calorimeter in a 1.5 T magnetic field is estimated to be the same as under conventional reference conditions. The VSL water calorimeter can be applied as a primary standard for D w in magnetic fields and is currently the only primary standard operable in a magnetic field that provides direct access to the international traceability framework.

Magnetic resonance-guided radiation therapy (MRgRT) not only allows for superior soft-tissue setup and online MR-guidance during delivery but also for inter-fractional plan re-optimization or adaptation. This plan adaptation involves repeat MR imaging, organs at risk (OARs) re-contouring, plan prediction (i.e., recalculating the baseline plan on the anatomy of that moment), plan re-optimization, and plan quality assurance. In contrast, intrafractional plan adaptation cannot be simply performed by pausing delivery at any given moment, adjusting contours, and re-optimization because of the complex and composite nature of deformable dose accumulation. To overcome this limitation, we applied a practical workaround by partitioning treatment fractions, each with half the original fraction dose. In between successive deliveries, the patient remained in the treatment position and all steps of the initial plan adaptation were repeated. Thus, this second re-optimization served as an intrafractional plan adaptation at 50% of the total delivery. The practical feasibility of this partitioning approach was evaluated in a patient treated with MRgRT for locally advanced pancreatic cancer (LAPC). MRgRT was delivered in 40Gy in 10 fractions, with two fractions scheduled successively on each treatment day. The contoured gross tumor volume (GTV) was expanded by 3 mm, excluding parts of the OARs within this expansion to derive the planning target volume for daily re-optimization (PTVOPT). The baseline GTVV95% achieved in this patient was 80.0% to adhere to the high-dose constraints for the duodenum, stomach, and bowel (V33 Gy

Purpose Magnetic resonance imaging-guided radiation therapy (MRgRT) requires patient positioning within the MR bore and prolonged MR imaging during delivery, both of which are new in radiation oncology. Patient tolerance of MRgRT was prospectively evaluated using patient-reported outcome questionnaires (PRO-Q). Methods Our MRgRT procedure involves daily high-resolution MR scanning, limited re-contouring, daily plan re-optimization, quality assurance (QA), and gated delivery. Patients with claustrophobia are excluded. Mean fraction duration was 45 and 60 minutes for stereotactic treatments during free-breathing and breath-hold, respectively. Patient-controlled video-feedback was used for breath-hold delivery. PRO-Qs collected in the first 150 patients treated included questions on MR-related complaints and also evaluated aspects of active participation. Results Almost one-third of patients (29%) scored at least one PRO-Q item on MR-related complaints as 'moderate' or 'very much', with noise, feeling cold, and paresthesia being the most frequently scored in this way. Considerable anxiety was reported by 5%, but no medication was required for this in any patient. Patient participation in video feedback for breath-hold delivery was appreciated by the majority of patients, all of whom completed the procedure. Only 5% of patients considered treatment duration to be unacceptably long. Conclusion Despite the lengthy MRgRT procedure, outcomes of PRO-Q indicate that it was well-tolerated by patients.

In this case presentation, we describe the challenges of performing magnetic resonance-guided radiation therapy (MRgRT) with plan adaptation in a patient with advanced endometriosis, in whom several prior therapeutic attempts were unsuccessful and extensive pelvic irradiation was regarded as being too toxic. Treatment was delivered in two sessions, first for the seemingly only active right ovary, and at a later stage for the left ovary. Some logistical problems were encountered during the preparation of the first treatment, which were subsequently optimized for the second treatment by using transvaginal ultrasound to determine the optimum time point for simulation and delivery. Using breath-hold gated delivery and plan adaptation, radiation dose to the bowel could be minimized, resulting in good tolerance of treatment. Because of the need to simulate and deliver in a brief optimal time span for visibility of the follicles in the ovaries, a single fraction dose of 8 Gy was used in our patient. Hormonal outcome after her second treatment is still pending. In conclusion, MRgRT with plan adaptation is feasible for the occasional patient with refractory endometriosis. Simulation and delivery needs to be synchronized with the menstrual cycle, ensuring that the Graafian follicles allow the ovaries to be visible on magnetic resonance imaging (MRI). Because the ovaries are only visible on T2-weighted MRI for a very brief period of time, we suggest that it is preferable to use single fraction radiotherapy with a brief interval between simulation imaging and delivery.

Adaptive treatment planning using a device that integrates magnetic resonance imaging (MRI) and a radiation therapy unit is the key development in radiation oncology to further improve targeted dose delivery and avoid complications, since MRI provides excellent soft-tissue visualization. The designs of these modalities aim to produce simultaneous and unimpeded operation of an MRI and a medical linear accelerator. However, the presence of a magnetic field influences the response of radiation detectors due to the Lorentz force acting on the secondary electrons. The objective of this project was to determine the response of three recently developed MR-compatible ionization chambers in magnetic fields of different field strengths and field orientations. The response of the MR-compatible Exradin ionization chambers was investigated both by means of experiments and simulations. For the magnetic field perpendicular to photon beam and ion chamber axis, both experiments and simulations could be conducted, and the experimental measurements showed good agreement with the simulation results. In this orientation, the measured chamber response showed a significant increase with magnetic flux density. Simulations only were carried out for the other two orientations: the magnetic field parallel to the chamber axis as well as parallel to the beam. There was very little change in chamber response for all magnetic flux densities in both orientations. Overall, the chamber response in a magnetic field could be modelled well using EGSnrc Monte-Carlo simulations within measurement accuracy., Gydymo planavimas integruojant magnetinio rezonanso vaizdinimą (MRI) į spindulinės terapijos įrenginį yra svarbus žingsnis spindulinėje terapijoje, siekiant pagerinti į taikinį (auglį) suvedamos dozės tikslumą ir išvengti kompolikacijų, nes MRI pasižymi puikia minkštųjų audinių vizualizacija. Kuriant tokią įrangą, siekiama, kad magnetinio rezonanso vaizdinimo įrenginys ir medicininis greitintuvas funcijonuotų vienu metu netrukdydami vienas kitam atlikti priskirtų funkcijų. Tačiau magnetinis laukas daro įtaką detektoriais registruojamam signalui, nes pasireiškia antrinius elektronus veikianti Lorenco jėga. Šio projekto tikslas buvo nustatyti magnetinio lauko (integruotas MRI įrenginys) poveikį signalams, registruojamiems naudojant tris skirtingas naujai sukurtas jonizacines kameras, atsižvelgiant į magnetinio lauko stiprumą ir orientaciją kamerų atžvilgiu. Suderintų su MRI Exradin tipo jonizacinių kamerų registravimo efektyvumas buvo tiriamas tiek eksperimentiškai, tiek matematinio modeliavimo būdu. Atvejis, kai magnetinis laukas yra statmenas fotonų srautui ir jonizacinės kameros ašiai buvo tiriamas abiem minėtais metodais. Eksperimentiniai rezultatai gerai sutapo su modeliavimo rezultais, ir buvo nustatyta, kad jonizacinės kameros jautrumas didėjo, didėjant magnetinio lauko stiprumui. Kitiems dviems atvejams tirti - kai magnetinis laukas orientuotas lygiagrečiai jonizacinės kameros ašiai ir kai laukas orirntuotas lygiagrečiai fotonų spinduliui,- buvo pritaikytas tik matematinis modeliavimas. Esant tokioms magnetinio lauko orientacijoms, buvo stebimas tik labai nedidelis jautrio pokytis, kintant magnetinio lauko stiprumui. Darbe parodyta, kad magnetinio lauko įtaka jonizacine kamera registruojamam signalui gali būti sėkmingai nustatoma atliekant Monte Karlo modeliavimą naudojant EGSnrc programinį paketą, pateikiant modeliavimo rezultatą tokiu pat tikslumu, kaip ir matavimo rezultatas.

The out-of-field surface dose contribution due to backscattered or ejected electrons, focused by the magnetic field, is evaluated in this work. This electron streaming effect (ESE) can contribute to out-of-field skin doses in orthogonal magnetic resonance guided radiation therapy machines. Using the EGSnrc Monte Carlo package, a phantom is set-up along the central axis of an incident 10 10 cm2 7 MV FFF photon beam. The phantom exit or entry surface is inclined with respect to the magnetic field, and an out-of-field water panel is positioned 10 cm away from, and centered on, the isocenter. The doses from streaming backscattered or ejected electrons, for either a 0.35 T or 1.5 T magnetic field, are evaluated in the out-of-field water panel for surface inclines of 10, 30, and 45°. The magnetic field focuses electrons emitted from the inclined phantom. Dose distributions at the surface of the out-of-field water panel are sharper in the 1.5 T magnetic field as compared to 0.35 T. The maximum doses for the 0.35 T simulations are 23.2%, 37.8%, and 39.0% for the respective 10, 30, and 45° simulations. For 1.5 T, for the same angles, the maximum values are 17.1%, 29.8%, and 35.8%. Dose values drop to below 2% within the first 1 cm of the out-of-field water phantom. The phantom thickness is an important variable in the magnitude of the ESE dose. The ESE can produce large out-of-field skin doses and must be a consideration in treatment planning in the MRgRT work-flow. Treatments often include multiple beams which will serve to spread out the effect, and many beams, such as anterior–posterior, will reduce the skin dose due to the ESE. A 1 cm thick shielding of either a bolus placed on the patient or mounted on the present RF coils would greatly reduce the ESE dose contributions. Further exploration of the capabilities of treatment planning systems to screen for this effect is required.