Magnetic Resonance Guided Accelerated Partial Breast Irradiation - Single Institution Experience Using ViewRay Technology

Background: In the recent years PBI has emerged as an alternative to WBI. This results in less normal tissue irradiated, less morbidity and late complications, greater patient convenience. The comparative efficacy and toxicity profiles of PBI compared with WBI have shown similar ipsilateral breast tumor recurrence rates and reduced acute toxicities. With the advent of dedicated magnetic resonance-guided radiation therapy systems such as the MR-Linac there is potential for further improvement in the delivery of PBI and increased safety. We present our experience of MR-guided PBI in early-stage breast cancer (08/2017 -01/2021) and compare to patients treated with brachytherapy (1/2010 - 8/2015). Objectives: 1) Assess the efficacy and toxicity of adjuvant PBI using MR-GRT in early-stage breast cancer. 2) Compare to outcomes of HDR brachytherapy patients 3) Determine situations where online adaptive RT may be beneficial. Methods: Fifty patients treated with PBI (MR-GRT) and 29 treated with HDR brachytherapy (Contoura) were evaluated. Criteria for PBI included: unifocal tumors. Results: Acute reactions were minimal with skin reactions mild to none and limited to the lumpectomy site (RTOG score 0-1). Late effects were localized mild skin hyper-pigmentation. One patient had a rib fracture. There are no recurrences to date, with a median follow up of 12 months. None of the patients qualified for online adaptive planning, but end-inhale breath hold was utilized for left-sided tumors for cardiac sparing. In the brachytherapy group one had a local recurrence (at 3 y) and 12/29 had significant persistent seroma/thickening/retraction at the treated site. Conclusions: PBI using MR-guided radiation therapy is a feasible, well tolerated regimen for early-stage breast cancer with a favorable acute and late toxicity profile and excellent cosmetic result. The follow-up is presently too short for recurrence evaluation, but so far no relapses were seen at a median follow up of 12 months. When compared with brachytherapy the acute and late morbidity are significantly lower and the cosmetic result superior

Clinical utility of Gafchromic film in an MRI-guided linear accelerator

BACKGROUND: The purpose of this study is to comprehensively evaluate the suitability of Gafchromic EBT3 and EBT-XD film for dosimetric quality assurance in 0.35 T MR-guided radiotherapy. METHODS: A 0.35 T magnetic field strength was utilized to evaluate magnetic field effects on EBT3 and EBT-XD Gafchromic films by studying the effect of film exposure time within the magnetic field using two timing sequences and film not exposed to MR, the effect of magnetic field exposure on the crystalline structure of the film, and the effect of orientation of the film with respect to the bore within the magnetic field. The orientation of the monomer crystal was qualitatively evaluated using scanning electron microscopy (SEM) compared to unirradiated film. Additionally, dosimetric impact was evaluated through measurements of a series of open field irradiations (0.83 × 0.83-cm(2) to 19.92 × 19.92-cm(2)) and patient specific quality assurance measurements. Open fields were compared to planned dose and an independent dosimeter. Film dosimetry was applied to twenty conventional and twenty stereotactic body radiotherapy (SBRT) patient specific quality assurance cases. RESULTS: No visual changes in crystal orientation were observed in any evaluated SEM images nor were any optical density differences observed between films irradiated inside or outside the magnetic field for both EBT3 and EBT-XD film. At small field sizes, the average difference along dose profiles measured in film compared to the same points measured using an independent dosimeter and to predicted treatment planning system values was 1.23% and 1.56%, respectively. For large field sizes, the average differences were 1.91% and 1.21%, respectively. In open field tests, the average gamma pass rates were 99.8% and 97.2%, for 3%/3 mm and 3%/1 mm, respectively. The median (interquartile range) 3%/3 mm gamma pass rates in conventional QA cases were 98.4% (96.3 to 99.2%), and 3%/1 mm in SBRT QA cases were 95.8% (95.0 to 97.3%). CONCLUSIONS: MR exposure at 0.35 T had negligible effects on EBT3 and EBT-XD Gafchromic film. Dosimetric film results were comparable to planned dose, ion chamber and diode measurements

Adaptive Radiation Therapy (ART) Strategies and Technical Considerations: A State of the ART Review From NRG Oncology

The integration of adaptive radiation therapy (ART), or modifying the treatment plan during the treatment course, is becoming more widely available in clinical practice. ART offers strong potential for minimizing treatment-related toxicity while escalating or de-escalating target doses based on the dose to organs at risk. Yet, ART workflows add complexity into the radiation therapy planning and delivery process that may introduce additional uncertainties. This work sought to review presently available ART workflows and technological considerations such as image quality, deformable image registration, and dose accumulation. Quality assurance considerations for ART components and minimum recommendations are described. Personnel and workflow efficiency recommendations are provided, as is a summary of currently available clinical evidence supporting the implementation of ART. Finally, to guide future clinical trial protocols, an example ART physician directive and a physics template following standard NRG Oncology protocol is provided

Characteristics of a novel treatment system for linear accelerator–based stereotactic radiosurgery

The purpose of this study is to characterize the dosimetric properties and accuracy of a novel treatment platform (Edge radiosurgery system) for localizing and treating patients with frameless, image-guided stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Initial measurements of various components of the system, such as a comprehensive assessment of the dosimetric properties of the flattening filter-free (FFF) beams for both high definition (HD120) MLC and conical cone-based treatment, positioning accuracy and beam attenuation of a six degree of freedom (6DoF) couch, treatment head leakage test, and integrated end-to-end accuracy tests, have been performed. The end-to-end test of the system was performed by CT imaging a phantom and registering hidden targets on the treatment couch to determine the localization accuracy of the optical surface monitoring system (OSMS), cone-beam CT (CBCT), and MV imaging systems, as well as the radiation isocenter targeting accuracy. The deviations between the percent depth-dose curves acquired on the new linac-based system (Edge), and the previously published machine with FFF beams (TrueBeam) beyond Dmax were within 1.0% for both energies. The maximum deviation of output factors between the Edge and TrueBeam was 1.6%. The optimized dosimetric leaf gap values, which were fitted using Eclipse dose calculations and measurements based on representative spine radiosurgery plans, were 0.700 mm and 1.000 mm, respectively. For the conical cones, 6X FFF has sharper penumbra ranging from 1.2–1.8 mm (80%-20%) and 1.9–3.8 mm (90%-10%) relative to 10X FFF, which has 1.2–2.2mm and 2.3–5.1mm, respectively. The relative attenuation measurements of the couch for PA, PA (rails-in), oblique, oblique (rails-out), oblique (rails-in) were: -2.0%, -2.5%, -15.6%, -2.5%, -5.0% for 6X FFF and -1.4%, -1.5%, -12.2%, -2.5%, -5.0% for 10X FFF, respectively, with a slight decrease in attenuation versus field size. The systematic deviation between the OSMS and CBCT was -0.4 ± 0.2 mm, 0.1± 0.3mm, and 0.0 ± 0.1 mm in the vertical, longitudinal, and lateral directions. The mean values and standard deviations of the average deviation and maximum deviation of the daily Winston-Lutz tests over three months are 0.20 ± 0.03 mm and 0.66 ± 0.18 mm, respectively. Initial testing of this novel system demonstrates the technology to be highly accurate and suitable for frameless, linac-based SRS and SBRT treatment

Integration of MR guided linear accelerator for treatment of multiple brain metastases with single-isocenter using stereotactic radiosurgery.

Introduction: The purpose of study was to investigate the treatment planning capabilities and delivery accuracy for single isocenter multiple brain metastases treatment on an integrated 0.35T magnetic resonance imaging guided Linear Accelerator (MR-Linac) platform. Methods: The MR-Linac comprises a double-donut superconducting wide bore magnet with 0.345 T field strength and a ring-gantry mounted Linac system. The Linac has a 6X flattening filter free (FFF) beam with a dose rate up to 600 MU/min. The multileaf collimator (MLC) has a doublestack design to achieve a 4 mm spatial resolution which is half of the leaf width. Five patient plans with a total of eleven lesions were used to evaluate the dosimetric accuracy for intracranial treatment. Single-isocenter IMRT treatment plans using 10-15 coplanar beams were generated to treat the multiple metastases. The isocenter was placed at the geometric center among the lesions within each plan. All plans were calculated with 1 mm dose grid resolution using a fast Monte Carlo algorithm to a prescription dose of 16 or 18 Gy for each lesion. Plan quality was evaluated using the conformity index (CI), homogeneity index (HI) and gradient index (GI). Results: The average planning target volume was 4.8 cm3 (range 0.06-10.95 cm3). The conformity index, homogeneity index and gradient index of the plans were 1.26 ≤ 0.22, 1.22 ≤ 0.10, and 5.38 ≤ 1.44, respectively. The average absolute dose difference between measured and calculated dose to a MR compatible chamber volume was 1.64 ≤ 1.90% (range: -3.5% - 0.79%), and the percentage of points passing the 3%/1mm gamma criteria was 96.87 ≤ 1.53% (range: 93.50% - 99.00%). Conclusions: Our experience demonstrates that excellent plan quality and delivery accuracy was achievable on the MR-Linac for treating multiple brain metastases with a single isocenter

Implementation of a novel algorithm for generating synthetic CT images from magnetic resonance imaging data sets for prostate cancer radiation therapy

PURPOSE: To describe and evaluate a method for generating synthetic computed tomography (synCT) images from magnetic resonance simulation (MR-SIM) data for accurate digitally reconstructed radiograph (DRR) generation and dose calculations in prostate cancer radiation therapy. METHODS AND MATERIALS: A retrospective evaluation was performed in 9 prostate cancer patients who had undergone MR-SIM in addition to CT simulation (CT-SIM). MR-SIM data were used to generate synCT images by using a novel, voxel-based weighted summation approach. A subset of patients was used for weight optimization, and the number of patients to use during optimization was determined. Hounsfield unit (HU) differences between CT-SIM and synCT images were analyzed via mean absolute error (MAE). Original, CT-based treatment plans were mapped onto synCTs. DRRs were generated, and agreement between CT and synCT-generated DRRs was evaluated via Dice similarity coefficient (DSC). Dose was recalculated, and dose-volume metrics and gamma analysis were used to evaluate resulting treatment plans. RESULTS: Full field-of-view synCT MAE across all patients was 74.3 ± 10.9 HU with differences from CTs of 2.0 ± 8.1 HU and 11.9 ± 46.7 HU for soft tissue structures (prostate, bladder, and rectum) and femoral bones, respectively. Calculated DSCs for anterior-posterior and lateral DRRs were 0.90 ± 0.04 and 0.92 ± 0.05, respectively. Differences in D99%, mean dose, and maximum dose to the clinical target volume from CT-SIM dose calculations were 0.75% ± 0.35%, 0.63% ± 0.34%, and 0.54% ± 0.33%, respectively, for synCT-generated plans. Gamma analysis (2%/2 mm dose difference/distance to agreement) revealed pass rates of 99.9% ± 0.1% (range, 99.7%-100%). CONCLUSION: Generated synCTs enabled accurate DRR generation and dose computation for prostate MR-only simulation. Dose recalculated on synCTs agreed well with original planning distributions. Further validation using a larger patient cohort is warranted

Geometric and radiation characterization of a new double stack multileaf collimator in a low field MR-Linac.

Purpose: With real-time imaging capability and double stacked, doublefocused MLCs, the new low-field MR-linac is well-suited for SBRT treatments. However, the MLC performance must be well-characterized to facilitate high precision radiation therapy. This work evaluates the MR-Linac MLCs geometric and radiation characteristics. Methods: MLC field sizes (FS) range from 0.2×0.4 cm2 to 27.4×24.1 cm2. Each leaf is 8 mm wide with one stack offset by 4 mm, producing 4 mm aperture resolution without the tongue/groove that is observed on typical single-stack MLCs. Transmission was measured using an MR-compatible A12 ion chamber (IC) and Gafchromic film. Picket fence (PF) centering accuracy and reproducibility was performed using Gafchromic film at cardinal gantry angles. FS accuracy and penumbra measurements for FS less than 1.66 cm2 were performed in a water tank with a diode detector, and for FS from 4.15 cm2 to the maximum FS with an IC array. Penumbra results were compared to the Monte Carlo-based treatment planning system. Longitudinal MLC positional accuracy was assessed via PF tests and FS measurements run over 8 months of clinical operation. Results: The IC measured leakage was 0.12%, and a maximum point leakage of 2.62% was measured on film. Centering error was consistent with repeated testing (-0.07 ± 0.17 mm (maximum 0.25 mm) and 0.03 ± 0.21 mm (maximum = 0.38 mm)) at gantry = 0. The difference between nominal and measured FS was 0.20 ± 0.40 mm (maximum = 0.26 mm) and 0.05 ± 0.30 mm (maximum = 0.37 mm) in crossplane and inplane directions, respectively. Field penumbra differences were 0.10 ± 0.40 mm (maximum = 0.84 mm) and 0.30 ± 0.24 mm (maximum = 0.89 mm) for crossplane and inplane, respectively. PF and FS measurements during routine QA showed continued MLC accuracy within TG- 142 guidelines. Conclusion: MLC measurements met AAPM TG-50, TG- 53, and TG-142 criteria and agreed within 1%-2% of TPS calculations. Future applications include stereotactic radiosurgery, where precise and accurate MLCs are of paramount importance

Evaluation of a novel MR-compatible 3D scanner

Purpose: In MR-guided radiotherapy, there exists an unmet need for robust, 3D scanning equipment that can be used for data acquisition during acceptance/ commissioning and periodic QA testing. Avendor-provided MR-conditional prototype 3D scanning system was recently developed and tested on a clinical MR-guided Linac. Methods: Tests were performed using a 0.35 T MR-Linac delivering a 6 MV flattening filter-free beam with a bore diameter of 70 cm. The certified MR-conditional scanning system comprises webbased software, a water reservoir, an electronics control box, a hand pendant, and a 3D rectangular water tank. Scanning was performed over three sessions using 85 and 80 cm SSDs, 5 and 10 cm depths, and field sizes (FS) ranging from 0.4 × 0.2 cm2 to 27.2 × 24.1 cm2. MR-compatible A26 (field) and A1SL (reference) ionization chambers were used for FS \u3e 2.5 × 2.5 cm2 and an Edge diode detector for smaller FS. For each session, inline and crossline profiles as well as percent depth dose (PDD) measurements were acquired. Profiles and PDDs were compared across sessions and to Monte Carlo simulations and commissioning measurements acquired using either a 1D tank with manual changes in detector position applied between measurements or 2D ion chamber array. Results: For PDDs at 10 × 10 cm2 FS, dmax was constant and PDD10x was within 1% (mean: 1.4 cm and 61.6%, respectively) across all sessions. At 80 cm SSD, dmax was within 0.5 mm and PDD10x within 1% of baseline commissioning measurements. Both crossline and inline flatness and symmetry at 10 × 10 cm2 and 27.2 × 24.1 cm2 FS were within 1% of baseline measurements acquired during commissioning and field widths for all FS were within 1 mm of nominal values. Conclusion: Performance of a novel, MR-compatible 3D scanning water tank was assessed in a clinical MR-Linac system. Profile and PDD measurements were acquired using ion chambers and diodes. Data was consistent between scanning sessions and with previously acquired commissioning data