BACKGROUND:Radiotherapy volumes for patients with glioblastoma have remained unchanged for decades and result in large volumes of irradiated brain. We aimed to show the safety of a small-margin, MRI-guided adaptive radiotherapy approach for glioblastoma. METHODS:This single-arm, phase 2 trial was done at Sunnybrook Health Sciences Centre, Toronto, ON, Canada, and included patients with pathologically confirmed glioblastoma and planned for concurrent daily chemoradiotherapy up to 60 Gy in 30 daily fractions over 6 weeks (long course) or 40 Gy in 15 daily fractions over 3 weeks (short course). Eligible patients were adults (≥18 years) with an expected life expectancy greater than 12 weeks and an Eastern Cooperative Oncology performance status less than or equal to 2. A 5 mm clinical target volume (CTV) was applied, with the allowance of associated T2-weighted fluid-attenuated inversion recovery hyperintense regions in a personalised way at the discretion of the treating physician. All patients were treated with 1·5 T MRI-guided linear accelerator (MR-Linac) incorporating weekly gadolinium-enhanced online adaptive fractions. The primary outcome was the risk of marginal failure powered for non-inferiority with a margin of 10% compared with a historical control. All patients were included in the primary and safety analyses. This study is registered with ClinicalTrials.gov, NCT04726397. FINDINGS:Between April 28, 2021, and May 12, 2023, 109 patients were assessed for eligibility and 98 were enrolled and received treatment on the UNITED protocol with 59 [60%] patients receiving long-course radiotherapy and 39 [40%] receiving short-course radiotherapy. Median follow-up was 14·2 months (IQR 8·9-19·7). 53 (54%) of 98 patients were male and 45 (46%) were female. The observed risk of marginal failure was 4% (95% CI 0-8). The most common grade 3-4 adverse events were lymphopenia (eight [11%] of 72 patients without baseline lymphopenia) and thrombocytopenia (four [4%] of 98 patients). Serious adverse events occurred in one (1%) patient due to febrile neutropenia and there were no treatment-related deaths. INTERPRETATION:MRI-guided adaptation for glioblastoma enables margin de-escalation and resulted in a low rate of marginal failure. A randomised trial comparing this technique to a standard large-margin radiotherapy approach will establish whether toxicity and quality-of-life improvements can be realised. FUNDING:None.

Purpose/Objective(s) The SUNSET phase I trial investigated the maximum tolerated dose for ultracentral (UC) lung tumors treated with stereotactic body radiotherapy (SBRT). Here we report a detailed spatial and dosimetric secondary analysis of the treatment plan and assess relationships between doses to targets, organs-at-risk (OARs) and clinical outcomes. Materials/Methods Five Canadian institutions enrolled patients with UC primary lung cancer, cT1-3N0M0, and all received SBRT to a dose of 60 Gy in 8 daily fractions. Maximum dose (Dmax) was limited to 120% of prescription. Targets were delineated as internal target volume (ITV) (combining gross tumor volume (GTV) across respiratory phases) expanded by 5 mm to form planning target volume (PTV). All OARs including great vessels and bilateral proximal bronchial tree up to segmental bronchus were contoured at baseline. To evaluate OAR and target doses, planning datasets and treatment plans were imported into a central repository. Descriptive statistics were generated for baseline characteristics and dosimetry. Univariable logistic and Cox proportional hazards regression modelling were performed to identify significant dosimetric predictors for related grade ≥ 2 adverse events (CTCAE v4.0), overall survival (OS) and local control (LC). Results All 30 enrolled patients (13 males, 17 females) were included in this sub-study. At median follow-up of 36.5 months, 9 patients (30.0%) experienced grade 2 adverse events and 1 patient (3.3%) each with grade 3(dyspnea) and 5(infection) attributed to treatment. Median ITV and PTV sizes were 11.3 cc (IQR: 6.2-26.8) and 34.4 cc (IQR: 21.9-62.2), respectively. PTV overlapped with at least one OAR for all patients, most commonly the proximal bronchial tree (PBT) or trachea (27/30). Additionally, a second overlapping structure was identified in 26/30 patients, most commonly the pulmonary artery (14/26) and esophagus (9/26). The mean overlap of first and second OAR with PTV were 0.95 cc (range = 0-4.2) and 0.7 cc (range = 0-4.7), respectively. The mean Dmax was 69.4 Gy (range = 64-72.5 Gy); all were within the PTV. Mean (± SD) PTV D98 was 56.1 ± 7.8 Gy, while D0.1 cc of PBT, esophagus and pulmonary artery were 53.1 ± 12.6 Gy, 26.5 ± 10.5 Gy and 57 ± 8.6 Gy, respectively. On analysis, combined overlap volume (cc) of the two primary overlapping OARs with PTV was associated with statistically significant inferior LC (hazard ratio [HR] per 1 cc increase: 2.86, P = 0.012); however, overlap volume was not associated with an increase in grade ≥ 2 adverse events (odds ratio [OR] per 1 cc increase: 1.17, P = 0.49). There was no association between OAR doses (D1 cc/D0.1 cc) with toxicity. PTV under-coverage (D98) was not associated with worse LC (HR per 5 Gy increase: 1.54, P = 0.68). Conclusion Within the dose constraints used in the trial, there was no relationship identified between OAR doses and toxicity. Local control decreased with increasing overlap of PTV with OARs, however, this was not associated with dosimetric under-coverage of the target.

BackgroundThe radiotherapy process relies on several metrics in determining a notion of "distance" from one three-dimensional region-of-interest (ROI) to another. The majority are symmetric (or commutative) and do not contain information pertaining to directionality. Growth versus regression, for example, is not inherently distinguished by these metrics.PurposeThe purpose of this work was to formalize a unidirectional distance metric, motivated by radiotherapy margin concepts, which we term the migration distance. Informally, the migration distance from ROI X to Y is the minimum isotropic expansion of X such that Y is completely encompassed by the expansion. If Y is contained within X, the migration distance is negative with magnitude equal to the maximum isotropic contraction of X such that Y remains contained within contraction. The metric is demonstrated by quantifying glioblastoma interfraction target changes.MethodsAn explicit mathematical formulation of the migration distance is presented and contrasted with the related Hausdorff distance. The results are demonstrated for the gross tumor volume (GTV) dynamics of a glioblastoma cohort consisting of 111 patients that underwent standard chemoradiotherapy with offline MR imaging at planning, fraction 10, fraction 20, and 1-month post radiotherapy.ResultsThe mean +/- SD of the GTV migration distance relative to planning was 5.9 +/- 3.9 mm at fraction 10, 6.2 +/- 4.4 mm at fraction 20, and 7.9 +/- 7.1 mm at 1-month post radiotherapy. The maximum GTV migration distance across all patients at the same timepoints was 20.4, 20.7, and 45.5 mm, respectively.ConclusionsWe have proposed and demonstrated a unidirectional distance metric. The migration distance may have applications in the quantification of anatomical changes, planning target volume designs, and dosimetric radiotherapy plan assessment.

BackgroundMagnetic resonance (MR)-guided radiation therapy provides capabilities to utilize high-resolution and real-time MR imaging before and during treatment, which is critical for adaptive radiotherapy. This emerging modality has been promptly adopted in the clinic settings in advance of adaptations to reference dosimetry formalism that are needed to account for the presence of strong magnetic fields. In particular, the influence of magnetic field on the uncertainty of parameters in the reference dosimetry equation needs to be determined in order to fully characterize the uncertainty budget for reference dosimetry in MR-guided radiation therapy systems. PurposeTo identify and quantify key sources of uncertainty in the reference dosimetry of external high energy radiotherapy beams in the presence of a strong magnetic field. MethodsIn the absence of a formalized Task Group report for reference dosimetry in MR-integrated linacs, the currently suggested formalism follows the TG-51 protocol with the addition of a quality conversion factor k(BQ) accounting for the effects of the magnetic field on ionization chamber response. In this work, we quantify various sources of uncertainty that impact each of the parameters in the formalism, and evaluate their overall contribution to the final dose. Measurements are done in a 1.5 T MR-Linac (Unity, Elekta AB, Stockholm, Sweden) which integrates a 1.5 T Philips MR scanner and a 7 MVFFF linac. The responses of several reference-class small volume ionization chambers (Exradin:A1SL, IBA:CC13, PTW:Semiflex-3D) and Farmer type ionization chambers (Exradin:A19, IBA:FC65-G) were evaluated throughout this process. Long-term reproducibility and stability of beam quality, TPR1020, was also measured with an in-house built phantom. ResultsRelative to the conventional external high energy linacs, the uncertainty on overall reference dose in MR-linac is more significantly affected by the chamber setup: A translational displacement along y-axis of +/- 3 mm results in dose variation of < |0.20| +/- 0.02% (k = 1), while rotation of +/- 5 degrees in horizontal and vertical parallel planes relative to relative to the direction of magnetic field, did not exceed variation of xy-plane (horizontal) rotations (< |0.44| +/- 0.02% (k = 1)) than for yz-plane (vertical) rotations (< ||0.28| +/- 0.02% (k = 1)), which we associate with the gradient of k(B,Q) as a function of chamber orientation with respect to direction of the B-0-field. Uncertainty in P-ion (for two depths), P-pol (with various sub-studies including effects of cable length, cable looping in the MRgRT bore, connector type in magnetic environment), and P-rp were determined. Combined conversion factor k(Q)x k(B,Q) was provided for two reference depths at four cardinal angle orientations. Over a two-year period, beam quality was quite stable with TPR1020 being 0.669 +/- 0.01%. The actual magnitude of TPR1020 was measured using identical equipment and compared between two different Elekta Unity MR-Linacs with results agreeing to within 0.21%. ConclusionIn this work, the uncertainty of a number of parameters influencing reference dosimetry was quantified. The results of this work can be used to identify best practice guidelines for reference dosimetry in the presence of magnetic fields, and to evaluate an uncertainty budget for future reference dosimetry protocols for MR-linac.

Purpose: Magnetic resonance image-guided radiotherapy for intracranial indications is a promising advance; however, uncertainties remain for both target localization after translation-only MR setup and intrafraction motion. This investigation quantified these uncertainties and developed a population-based planning target volume (PTV) model to explore target and organ-at-risk (OAR) volumetric coverage tradeoffs.Methods: Sixty-six patients, 49 with a primary brain tumor and 17 with a post-surgical resection cavity, treated on a 1.5T-based MR-linac across 1329 fractions were included. At each fraction, patients were setup by translation-only fusion of the online T1 MRI to the planning image. Each fusion was independently repeated offline ac-counting for rotations. The six degree-of-freedom difference between fusions was applied to transform the planning CTV at each fraction (CTVfx). A PTV model parameterized by volumetric CTVfx coverage, proportion of fractions, and proportion of patients was developed. Intrafraction motion was quantified in a 412 fraction subset as the fusion difference between post-and pre-irradiation T1 MRIs.Results: For the left-right/anterior-posterior/superior-inferior axes, mean +/- SD of the rotational fusion differ-ences were 0.1 +/- 0.8/0.1 +/- 0.8/-0.2 +/- 0.9 degrees. Covering 98 % of the CTVfx in 95 % of fractions in 95 % of patients required a 3 mm PTV margin. Margin reduction decreased PTV-OAR overlap; for example, the proportion of optic chiasm overlapped by the PTV was reduced up to 23.5 % by margin reduction from 4 mm to 3 mm.Conclusions: An evidence-based PTV model was developed for brain cancer patients treated on the MR-linac. Informed by this model, we have clinically adopted a 3 mm PTV margin for conventionally fractionated intra-cranial patients.

Background: The static magnetic field present in magnetic resonance (MR)-guided radiotherapy systems can influence dose deposition and charged particle collection in air-filled ionization chambers. Thus, accurately quantifying the effect of the magnetic field on ionization chamber response is critical for output calibration. Formalisms for reference dosimetry in a magnetic field have been proposed, whereby a magnetic field quality conversion factor k(B,Q) is defined to account for the combined effects of the magnetic field on the radiation detector. Determination of k(B,Q) in the literature has focused on Monte Carlo simulation studies, with experimental validation limited to only a few ionization chamber models.Purpose: The purpose of this study is to experimentally measure k(B,Q) for 11 ionization chamber models in two commercially available MR-guided radiotherapy systems: Elekta Unity and ViewRay MRIdian.Methods: Eleven ionization chamber models were characterized in this study: Exradin A12, A12S, A28, and A26, PTW T31010, T31021, and T31022, and IBA FC23-C, CC25, CC13, and CC08. The experimental method to measure k(B,Q) utilized cross-calibration against a reference Exradin A1SL chamber. Absorbed dose to water was measured for the reference A1SL chamber positioned parallel to the magnetic field with its centroid placed at the machine isocenter at a depth of 10 cm in water for a 10 x 10 cm(2) field size at that depth. Output was subsequently measured with the test chamber at the same point of measurement. k(B,Q) for the test chamber was computed as the ratio of reference dose to test chamber output, with this procedure repeated for each chamber in each MR-guided radiotherapy system. For the high-field 1.5 T Elekta Unity system, the dependence of k(B,Q) on the chamber orientation relative to the magnetic field was quantified by rotating the chamber about the machine isocenter.Results: Measured k(B,Q) values for our test dataset of ionization chamber models ranged from 0.991 to 1.002, and 0.995 to 1.004 for the Elekta Unity and ViewRay MRIdian, respectively, with k(B,Q) tending to increase as the chamber sensitive volume increased. Measured k(B,Q) values largely agreed within uncertainty to published Monte Carlo simulation data and available experimental data. k(B,Q) deviation from unity was minimized for ionization chamber orientation parallel or antiparallel to the magnetic field, with increased deviations observed at perpendicular orientations. Overall (k = 1) uncertainty in the experimental determination of the magnetic field quality conversion factor, k(B,Q) was 0.71% and 0.72% for the Elekta Unity and ViewRay MRIdian systems, respectively.Conclusions: For a high-field MR-linac, the characterization of ionization chamber performance as angular orientation varied relative to the magnetic field confirmed that the ideal orientation for output calibration is parallel. For most of these chamber models, this study represents the first experimental characterization of chamber performance in clinical MR-linac beams. This is a critical step toward accurate output calibration for MR-guided radiotherapy systems and the measured k(B,Q) values will be an important reference data source for forthcoming MR-linac reference dosimetry protocols.

Introduction: Magnetic resonance imaging-linear accelerator radiotherapy is an innovative technology that requires special consideration for secondary electron interactions within the magnetic field, which can alter dose deposition at air–tissue interfaces. As part of ongoing quality assurance and quality improvement of new radiotherapy technologies, the purpose of this study was to evaluate skin dose modelled from the treatment planning systems of a magnetic resonance imaging-linear accelerator and a conventional linear accelerator, and then correlate with in vivo measurements of delivered skin dose from each linear accelerator. Methods: In this prospective cohort study, 37 consecutive glioma patients had treatment planning completed and approved prior to radiotherapy initiation using commercial treatment planning systems: a Monte Carlo-based algorithm for magnetic resonance imaging-linear accelerator or a convolution-based algorithm for conventional linear accelerator. In vivo skin dose was measured using an optically stimulated luminescent dosimeter. Results: Monte Carlo-based magnetic resonance imaging-linear accelerator plans and convolution-based conventional linear accelerator plans had similar dosimetric parameters for target volumes and organs-at-risk. However, magnetic resonance imaging-linear accelerator plans had 1.52 Gy higher mean dose to air cavities ( P < .0001) and 1.10 Gy higher mean dose to skin ( P < .0001). In vivo skin dose was 14.5% greater for magnetic resonance imaging-linear accelerator treatments ( P = .0027), and was more accurately predicted by Monte Carlo-based calculation ( ρ = 0.95, P < .0001) versus convolution-based ( ρ = 0.80, P = .0096). Conclusion: This is the first prospective dosimetric comparison of glioma patients clinically treated on both magnetic resonance imaging-linear accelerator and conventional linear accelerator. Our findings suggest that skin doses were significantly greater with magnetic resonance imaging-linear accelerator plans but correlated better with in vivo measurements of actual skin dose from delivered treatments. Future magnetic resonance imaging-linear accelerator planning processes are being designed to account for skin dosimetry and treatment delivery.

PurposeThis study reports the workflow and initial clinical experience of high grade glioma (HGG) radiotherapy on the 1.5 T MR-Linac (MRL), with a focus on the temporal variations of the tumor and feasibility of multi-parametric image (mpMRI) acquisition during routine treatment workflow.Materials and methodsTen HGG patients treated with radiation within the first year of the MRL’s clinical operation, between October 2019 and August 2020, were identified from a prospective database. Workflow timings were recorded and online adaptive plans were generated using the Adapt-To-Position (ATP) workflow. Temporal variation within the FLAIR hyperintense region (FHR) was assessed by the relative FHR volumes (n = 281 contours) and migration distances (maximum linear displacement of the volume). Research mpMRIs were acquired on the MRL during radiation and changes in selected functional parameters were investigated within the FHR.ResultsAll patients completed radiotherapy to a median dose of 60 Gy (range, 54-60 Gy) in 30 fractions (range, 30-33), receiving a total of 287 fractions on the MRL. The mean in-room time per fraction with or without post-beam research imaging was 42.9 minutes (range, 25.0–69.0 minutes) and 37.3 minutes (range, 24.0–51.0 minutes), respectively. Three patients (30%) required re-planning between fractions 9 to 12 due to progression of tumor and/or edema identified on daily MRL imaging. At the 10, 20, and 30-day post-first fraction time points 3, 3, and 4 patients, respectively, had a FHR volume that changed by at least 20% relative to the first fraction. Research mpMRIs were successfully acquired on the MRL. The median apparent diffusion coefficient (ADC) within the FHR and the volumes of FLAIR were significantly correlated when data from all patients and time points were pooled (R=0.68, p<.001).ConclusionWe report the first clinical series of HGG patients treated with radiotherapy on the MRL. The ATP workflow and treatment times were clinically acceptable, and daily online MRL imaging triggered adaptive re-planning for selected patients. Acquisition of mpMRIs was feasible on the MRL during routine treatment workflow. Prospective clinical outcomes data is anticipated from the ongoing UNITED phase 2 trial to further refine the role of MR-guided adaptive radiotherapy.

Purpose: Cardiac toxicity is a major concern for left-sided breast cancer patients receiving radiation therapy (RT). Different breath-hold techniques may be used to reduce cardiac dose including voluntary deep inspiration breath hold (vDIBH), the Elekta Active Breathing Coordinator (ABC), and the AlignRT system from VisionRT. The purpose of this study is to evaluate the differences in heart position and mean heart dose for these three techniques during RT of left-sided breast cancer patients. Methods: In this prospective study, 55 left-sided breast cancer patients receiving post-operative RT were randomly assigned to vDIBH, ABC or AlignRT. Patients were set up daily to tattoos and cone-beam computed tomography (CBCT) imaging was performed weekly to verify patient position. During weekly CBCT, patients were matched to chestwall and all shifts were applied prior to treatment. Weekly CBCTs were retrospectively assessed in the Pinnacle Treatment Planning System. The delivered mean heart dose was calculated by adjusting the heart contour from the planning CT scan to match the heart position observed on each weekly CBCT. The mean cardiac dose was re-calculated using the planned treatment fields and the adjusted heart contours for patients receiving any of the three breath-hold techniques. Three-dimensional cardiac displacement between the planned (CT) and treated (CBCT) heart positions was obtained from the changes in coordinates of the heart centroids. Abstract Results: A total of 7 patients received a dose of 5000 cGy in 25 fractions, 45 patients received 4256 cGy in 16 fractions and 3 patients received 4005 cGy in 15 fractions. Sixteen of these patients received RT with VBH, 19 patients were treated with ABC and 20 patients were treated with AlignRT. The change in mean cardiac dose between planning CT and treatment CBCT was 2.9% (vDIBH), 2.0% (ABC) and 6.6% (AlignRT). The median standard deviations for change in interfractional heart dose on treatment were 11.0 cGy (vDIBH), 13.6 cGy (ABC) and 16.1 cGy (AlignRT). The mean cardiac displacement between planning CT and treatment CBCT were 0.37 cm (vDIBH), 0.35 cm (ABC), and 0.35 cm (AlignRT). All cardiac displacements were found to be statistically non-significant in comparing the three breath-hold techniques (p < 0.36). All differences in the dosimetric data were also found to be statistically non-significant in comparing the three breath-hold techniques (p < 1.35). Conclusion: This study demonstrates that the differences in heart position on treatment are negligible for vDIBH, ABC and AlignRT, and that the cardiac dose sparing is equivalent for these three breath-hold techniques.

The value and uncertainty of conversion factor for A1SL ionization chamber obtained directly via water calorimetry has been revised and updated [1]. This update impacts Table II in our 2020 paper [2] and the value of uncertainty for and total uncertainty must be changed to provide the corrected Table II below. Figures 7 and 8 were also impacted, as used in cross-calibration was used to generate these figures. The updated figures and the caption to Figure 7 are provided below. The update also impacts Table IV of the paper. The updated Table IV is shown below. The update impacts the Results section of the abstract: "We measured the absolute magnitude of the magnetic field correction factor for the Exradin-A19, A1SL, IBA FC65-G and CC13 to be 0.938 ± 1.13%, 0.968 ± 0.99%, 0.950 ± 1.13% and 0.975 ± 1.13%, respectively." should be corrected to: "We measured the absolute magnitude of the magnetic field correction factor for the Exradin A19, A1SL, IBA FC65-G and CC13 to be 0.930 ± 1.24%, 0.960 ± 1.17%, 0.942 ± 1.24% and 0.967 ± 1.24%, respectively." The change also impacts the Result section of the paper in the third paragraph. "We have thus renormalized the entire response distribution of A1SL in Figure 7 at 270° to the experimentally measured of 0.985 ± 0.83%." should be corrected to: "We have thus renormalized the entire response distribution of A1SL in Figure 7 at 270° to the experimentally measured of 0.977 ± 0.98%." and "In this 180-degree orientation, which is the preferred setup for reference dosimetry by several institutions including ours, the was measured to be 0.980 ± 1.13%, 0.996 ± 1.13% and 0.990 ± 1.13%, for the Exradin A19, IBA FC65-G, and CC13, respectively." should be corrected to "In this 180-degree orientation, which is the preferred setup for reference dosimetry by several institutions including ours, the was measured to be 0.980 ± 1.24%, 0.988 ± 1.24% and 0.982 ± 1.24%, for the Exradin A19, IBA FC65-G, and CC13, respectively." The change impacts the Discussion section. "As such, the agreement between Malkov and Rogers' Monte Carlo simulation and our experimental results for A19 is <0.53% for FV and less than 0.42% for RV for all orientations, as shown in Table IV and Figure 8(b). For the A1SL chamber, Table IV shows a partial agreement between our results and the Monte Carlo work of Malkov and Rogers. The agreement is relatively good, -0.41 %, at 0° for both FV and RV, while it becomes significantly different for 90° (−0.1% for FV vs −1.45% for RV) and 270° (−0.61% for FV vs +0.41% for RV). Table IV only gives a snapshot of a few cardinal orientations. It is only through a full comparison of the curves, as we have provided in Figure 8(a) that one can notice that from 0° to 180°, the data almost overlaps the distribution for FV, while from 180° to 360°, the experimental data is in better agreement with RV." should be corrected to: "As such, the agreement between Malkov and Rogers' Monte Carlo simulation and our experimental results for A19 is <0.76% for FV and less than 0.68% for RV for all orientations, as shown in Table IV and Figure 8(b). Within uncertainty, our results also suggest a larger absolute value of conversion factor for A19. For the A1SL chamber, Table IV shows a partial agreement between our results and the Monte Carlo work of Malkov and Rogers. The agreement is relatively good, about 0.4%, at 0° for both FV and RV, while it becomes significantly different for 90° (0.7% for FV vs −0.65% for RV) and 270° (0.2% for FV vs 1.21% for RV). Table IV only gives a snapshot of a few cardinal orientations. It is only through a full comparison of the curves, as we have provided in Figure 8(a) that one can notice that from 0° to 180°, the data tracks between two Monte Carlo datasets and more congruent with RV, while from 180° to 360°, the experimental data are in better agreement with FV." The sixth and seventh paragraph of Discussion section are also impacted: "At parallel orientation, a difference of 1.61% was observed when comparing for IBA FC65-G to Monte Carlo study performed by Malkov and Rogers [9]." "Similarly, comparing the magnitude of our measured correction factor for the same chamber at 180° angle against the experimental data by de Prez et al. [17] has shown a disagreement of 1.1%, although for the 90° orientation, the agreement was about 0.42%." "Finally, the result for the CC13 chamber agrees within 0.6% with the only available Monte Carlo study for this chamber [9]." should be corrected to: "At parallel orientation, a difference of 0.85% was observed when comparing for IBA FC65-G to Monte Carlo study performed by Malkov and Rogers [9]." "Similarly, comparing the magnitude of our measured correction factor for the same chamber at 180° angle against the experimental data by de Prez et al. [17] has shown a disagreement of 0.26%, although for the 90° orientation, the agreement was about 0.46%." "Finally, the result for the CC13 chamber agrees well within 0.16% with the only available Monte Carlo study for this chamber [9]."

C. Tseng: Advisory Board; Sanofi. H. Chen: Employee; North York General Hospital. J. Stewart: None. A. Lau: None. R. Chan: None. L.S. Lawrence: Student affiliated with University of Toronto; Sunnybrook Research Institute. M. Campbell: None. S.D. Myrehaug: Advisory Board; Novartis AG.H. Soliman: None. Z.A. Husain: Independent Contractor; RadOncQuestions LLC. Research Grant; Merck. Travel Expenses; Elekta; NIH Head and neck PULA task force. J. Detsky: None. P. Maralani: None. B.M. Keller: None. M.E. Ruschin: Patent/License Fees/Copyright; Elekta AB.A. Sahgal: Research Grant; Elekta.

Over the last few years, magnetic resonance image‐guided radiotherapy systems have been introduced into the clinic, allowing for daily online plan adaption. While quality assurance (QA) is similar to conventional radiotherapy systems, there is a need to introduce or modify measurement techniques. As yet, there is no consensus guidance on the QA equipment and test requirements for such systems. Therefore, this report provides an overview of QA equipment and techniques for mechanical, dosimetric, and imaging performance of such systems and recommendation of the QA procedures, particularly for a 1.5T MR‐linac device. An overview of the system design and considerations for QA measurements, particularly the effect of the machine geometry and magnetic field on the radiation beam measurements is given. The effect of the magnetic field on measurement equipment and methods is reviewed to provide a foundation for interpreting measurement results and devising appropriate methods. And lastly, a consensus overview of recommended QA, appropriate methods, and tolerances is provided based on conventional QA protocols. The aim of this consensus work was to provide a foundation for QA protocols, comparative studies of system performance, and for future development of QA protocols and measurement methods.

Magnetic Resonance Imaging (MRI)-Linear Accelerator (MR-Linac) radiotherapy is an innovative technology that requires special consideration for secondary electron interactions within the magnetic field, which can alter dose deposition at air-tissue interfaces. Thirty-seven consecutive glioma patients had treatment planning completed and approved prior to radiotherapy initiation using commercial treatment planning systems (TPS): a Monte Carlo-based or convolution-based TPS for MR-Linac or Cone Beam CT (CBCT)-guided Linac, respectively. In vivo skin dose was measured using an Optically Stimulated Luminescent Dosimeter (OSLD) and correlated with TPS skin dose. We found that Monte Carlo-based MR-Linac plans and convolution-based CBCT-Linac plans had similar dosimetric parameters for target volumes and organs-at-risk. However, MR-Linac plans had 1.52 Gy higher mean dose to air cavities (p<0.0001) and 1.10 Gy higher mean dose to skin (p<0.0001). In vivo skin dose was 14.5% greater for MR-Linac (p=0.0027), and were more accurately predicted by Monte Carlo-based calculation (ρ=0.95, p<0.0001) vs. convolution-based (ρ=0.80, p=0.0096). This is the first prospective dosimetric comparison of glioma patients clinically treated on both MR-Linac and CBCT-guided Linac. Skin doses were significantly greater with MR-Linac and correlated with in vivo measurements. Future MR-Linac planning processes are being designed to account for skin dosimetry and treatment delivery.

In this prospective study of clinically treated glioma patients on both MRL and conventional Linac, the dosimetric impact of the magnetic field was minimal for the target and standard OARs. However, higher doses to skin and air cavities were observed. In vivo correlation of dose to skin was more accurately predicted with Monaco. Future MRL planning processes are being designed to account for skin dosimetry and treatment delivery.

Abstract Background Magnetic Resonance Imaging (MRI)-Linear Accelerator (MR-Linac) radiotherapy requires special consideration for secondary electron interactions within the magnetic field, which can alter dose deposition at air-tissue interfaces. Methods Thirty-seven consecutive glioma patients treated during their radiotherapy course with at least one fraction delivered on MR-Linac or Cone Beam CT (CBCT)-guided Linac, were analyzed. Treatment planning for both systems were completed prior to radiotherapy initiation and approved for clinical delivery using commercial treatment planning systems (TPS): a Monte Carlo calculation-based or convolution calculation-based TPS for MR-Linac or CBCT-Linac, respectively. Dosimetric parameters for planning target volume (PTV), organs-at-risk (OARs), and air-tissue interface were compared. In vivo skin dose during a single fraction of MR-Linac and CBCT-Linac treatment was measured using an Optically Stimulated Luminescent Dosimeter (OSLD) and correlated with TPS skin dose. Results Monte Carlo-based MR-Linac plans and convolution-based CBCT-Linac plans exhibited minimal differences in PTV and OAR parameters. However, MR-Linac plans had greater doses within tissues surrounding air cavities (1.52 Gy higher mean Dmean, p < 0.0001) and skin (1.10 Gy higher mean Dmean, p < 0.0001). In vivo OSLD skin readings were 14.5% greater for MR-Linac treatments (p = 0.0027), and were more accurately predicted by Monte Carlo-based calculation (ρ = 0.95, p < 0.0001) vs. convolution-based (ρ = 0.80, p = 0.0096). Conclusions The magnetic field’s dosimetric impact was minimal for PTV and OARs in glioma as compared to standard CBCT-Linac treatment plans. However, skin doses were significantly greater with the MR-Linac and correlated with in vivo measurements. Future MR-Linac planning processes are being designed to account for skin dosimetry and treatment delivery.