Resonance Image Guided Radiation Therapy Research Articles

Novel Virtual Reality App for Training Patients on MRI-guided Radiation Therapy

PurposePatients receiving respiratory gated magnetic resonance imaging-guided radiation therapy (MRIgRT) for abdominal targets must hold their breath for ≥25 seconds at a time. Virtual reality (VR) has shown promise for improving patient education and experience for diagnostic MRI scan acquisition. We aimed to develop and pilot-test the first VR app to educate, train, and reduce anxiety and discomfort in patients preparing to receive MRIgRT. Methods and MaterialsA multidisciplinary team iteratively developed a new VR app with patient input. The app begins with minigames to help orient patients to using the VR device and to train patients on breath-holding. Next, app users are introduced to the MRI linear accelerator vault and practice breath-holding during MRIgRT. In this quality improvement project, clinic personnel and MRIgRT-eligible patients with pancreatic cancer tested the VR app for feasibility, acceptability, and potential efficacy for training patients on using breath-holding during MRIgRT. ResultsThe new VR app experience was tested by 19 patients and 67 clinic personnel. The experience was completed on average in 18.6 minutes (SD = 5.4) by patients and in 14.9 (SD = 3.5) minutes by clinic personnel. Patients reported the app was “extremely helpful” (58%) or “very helpful” (32%) for learning breath-holding used in MRIgRT and “extremely helpful” (28%) or “very helpful (50%) for reducing anxiety. Patients and clinic personnel also provided qualitative feedback on improving future versions of the VR app. ConclusionThe VR app was feasible and acceptable for training patients on breath-holding for MRIgRT. Patients eligible for MRIgRT for pancreatic cancer and clinic personnel reported on future improvements to the app to enhance its usability and efficacy.

Ablative radiotherapy for colorectal liver metastases and intrahepatic cholangiocarcinoma

The role of radiation therapy in the management of liver cancers, both primary and metastatic, has changed drastically over the past several decades. Although conventional radiation was limited by technology, the advent of advanced image-guided radiotherapy and the rise in evidence for and popularity of stereotactic body radiotherapy have expanded the indications for radiation in these two distinct disease types. Magnetic resonance imaging–guided radiation therapy, daily online adaptive radiotherapy, and proton radiotherapy are some of many modern radiotherapy techniques that allow for increasingly efficacious treatment of intrahepatic disease while simultaneously allowing for increased normal tissue sparing, including sparing of the normal liver and the radiosensitive luminal gastrointestinal tract. Modern radiation therapy should be considered along with approaches such as surgical resection and radiofrequency ablation for the management of liver cancers of diverse histologies. Herein we describe the use of modern radiotherapy in two example settings, colorectal liver metastases and intrahepatic cholangiocarcinoma, and how external beam radiotherapy provides options within multidisciplinary discussions to elect optimal patient-specific treatments.

First treatments for Lattice stereotactic body radiation therapy using magnetic resonance image guided radiation therapy

Two abdominal patients were treated with Lattice stereotactic body radiation therapy (SBRT) using magnetic resonance guided radiation therapy (MRgRT). This is one of the first reported treatments of Lattice SBRT with the use of MRgRT. A description of the treatment approach and planning considerations were incorporated into this report. MRgRT Lattice SBRT delivered similar planning quality metrics to established dosimetric parameters for Lattice SBRT. Increased signal intensity were seen in the MRI treatments for one of the patients during the course of treatment.

Monte Carlo optimization and experimental validation of a prototype ionization chamber for accurate magnetic resonance image guided radiation therapy (MRgRT) daily output constancy measurements in solid phantoms.

To optimize the design, develop and test a prototype ionization chamber for accurate daily output constancy measurements in solid phantoms in clinical magnetic resonance-guided radiation therapy (MRgRT) radiotherapy beams. Up to 4% variations in response using commercial ionization chambers have been previously reported; the prototype ionization chamber developed here aims to minimize thesevariations. Monte Carlo simulations with the EGSnrc code system are used to optimize an ionization chamber design by increasing the thickness of a brass (high-density, nonferromagnetic, easy-to-machine) wall until results consistent with no air gap are produced for simulations with a 1.5 T and 0.35T magnetic field, with a 0.2mm air gap and varying the placement of the chamber model within the air gap. Based on the results of these simulations, prototype ionization chambers are manufactured and tested in conventional linac beams and in a 7MV Elekta Unity MR-linac. The chambers are rotated about their axes, both parallel and perpendicular to the 1.5T magnetic field, through 360º in a plastic phantom with measurements made at each cardinal angle. This reveals any variation in chamber response by varying the thickness of the air gap between the chamber and thephantom. Monte Carlo simulations demonstrate that the optimal thickness of the chamber wall to mitigate the effect of an asymmetric air gap between the chamber and the plastic phantom is 1.1mm of brass. With this thickness, the differences between simulations with and without an air gap and with asymmetric placement of the chamber within the air gap are less than 0.2%. A prototype chamber constructed with a 1.1mm brass wall thickness exhibits less than 0.3% variation in response when rotated about its axis in the plastic phantom in a beam from an MR-linac, independent of whether its axis is parallel or perpendicular to the magneticfield. The optimized ionization chamber design and validated prototype for accurate MR-linac daily output constancy measurements allows utilization of conventional phantoms and procedures in MRgRT systems. This can minimize disruption to clinical workflow for MR-linac quality assurancemeasurements.

Application of Continuous Positive Airway Pressure for Thoracic Respiratory Motion Management: An Assessment in a Magnetic Resonance Imaging–Guided Radiation Therapy Environment

PurposePatient tolerability of magnetic resonance (MR)–guided radiation treatment delivery is limited by the need for repeated deep inspiratory breath holds (DIBHs). This volunteer study assessed the feasibility of continuous positive airway pressure (CPAP) with and without DIBH for respiratory motion management during radiation treatment with an MR-linear accelerator (MR-linac). Methods and MaterialsMR imaging safety was first addressed by placing the CPAP device in an MR-safe closet and configuring a tube circuit via waveguide to the magnet bore. Reproducibility and linearity of the final configuration were assessed. Six healthy volunteers underwent thoracic imaging in a 0.35T MR-linac, with one free breathing (FB) and 2 DIBH acquisitions being obtained at 5 pressures from 0 to 15 cm-H2O. Lung and heart volumes and positions were recorded; repeatability was assessed by comparing 2 consecutive DIBH scans. Blinded reviewers graded images for motion artifact using a 3-point grading scale. Participants completed comfort and perception surveys before and after imaging sessions. ResultsCompared with FB alone, FB-10, FB-12, and FB-15 cm H2O significantly increased lung volumes (+23%, +34%, +44%; all P <.05) and inferiorly displaced the heart (0.86 cm, 0.96 cm, 1.18 cm; all P < . 05). Lung volumes were significantly greater with DIBH-0 cm H2O compared with FB-15 cm H2O (+105% vs +44%, P = .01), and DIBH-15 cm H2O yielded additional volume increase (+131% vs +105%, P = .01). Adding CPAP to DIBH decreased lung volume differences between consecutive breath holds (correlation coefficient 0.97 at 15 cm H2O vs 0.00 at 0 cm H2O). The addition of 15 cm H2O CPAP reduced artifact scores (P = .03) compared with FB; all DIBH images (0-15 cm H2O) had less artifact (P < .01). ConclusionsThis work demonstrates the feasibility of integrating CPAP in an MR-linac environment in healthy volunteers. Extending this work to a larger patient cohort is warranted to further establish the role of CPAP as an alternative and concurrent approach to DIBH in MR-guided radiation therapy.

Advances in magnetic resonance imaging guided radiation therapy

Image-guided radiation therapy using magnetic resonance imaging (MRI) is a new technology that has been widely studied and developed in recent years. The technology combines the advantages of MRI imaging, and can offer online real-time tracking of tumor and adjacent organs at risk, as well as real-time optimization of radiotherapy plan. In order to provide a comprehensive understanding of this technology, and to grasp the international development and trends in this field, this paper reviews and summarizes related researches, so as to make the researchers and clinical personnel in this field to understand recent status of this technology, and carry out corresponding researches. This paper summarizes the advantages of MRI and the research progress of MRI linear accelerator (MR-Linac), online guidance, adaptive optimization, and dosimetry-related research. Possible development direction of these technologies in the future is also discussed. It is expected that this review can provide a certain reference value for clinician and related researchers to understand the research progress in the field.

Respiratory-Gated Magnetic Resonance Image Guided Radiation Therapy for Hepatocellular Carcinoma: A Pilot Study

Hypofractionated RT has shown encouraging results in hepatocellular carcinoma (HCC) patients. However, HCC adjacent to the gastrointestinal (GI) tract should be carefully treated using the high-precision irradiation technique due to risk of radiation damage. We evaluated the feasibility of a respiratory-gated magnetic resonance image guided radiation therapy (RgMRg-RT) for hepatocellular carcinoma (HCC). Thirty-three patients with HCC underwent RgMRg-RT in our hospital from 2015 to February 2019, including patients with Child-Pugh A/B7 cirrhosis and unresectable tumors near the gastrointestinal tract. The median radiation dose was 50 Gy (range, 25–60) and median fraction number was 5 (range, 4–15). Gating was performed based on real-time magnetic resonance image without an external surrogate. The median follow-up period was 11.7 months (range, 3.6–37.9 months). The rates of local control of the target tumor at 6 months and 1 year were 90.2 and 86.9%, respectively. The overall survival rates at 6 months, 1, and 2 years were 84.3, 76.3, and 61%, respectively. The median distances from gross tumor to the esophagus, stomach, duodenum and colon were 6.1 (range, 1.9-14.3), 6.4 (0-13.4), 3.8 (0-13.5) and 4 cm (0.3-13), respectively. A total of 15 tumors (45.5%) were located within 2 cm of the gastrointestinal tract and 9 tumors (27.3%) within 1 cm. Grades 3 treatment-related bleeding was observed in one patient and one patient had radiation-induced liver disease. Hypofractionated RgMRg-RT was a safe and potentially ablative therapy for HCC. RgMRg-RT is a good alternative treatment for patients with HCCs that are unsuitable for surgical resection or local ablative therapy.

Using prediction models to evaluate magnetic resonance image guided radiation therapy plans.

Comprehensive analysis of daily, online adaptive plan quality and safety in magnetic resonance imaging (MRI) guided radiation therapy is critical to its widespread use. Artificial neural network models developed with offline plans created after simulation were used to analyze and compare online plans that were adapted and reoptimized in real time prior to treatment. Roughly one third of 60Co adapted plans were of inferior quality relative to fully optimized, offline plans, but MRI-linac adapted plans were essentially equivalent to offline plans. The models also enabled clear justification that MRI-linac plans are superior to 60Co in an overwhelming majority of cases.

Development and evaluation of machine learning models for voxel dose predictions in online adaptive magnetic resonance guided radiation therapy.

PurposeDaily online adaptive plan quality in magnetic resonance imaging guided radiation therapy (MRgRT) is difficult to assess in relation to the fully optimized, high quality plans traditionally established offline. Machine learning prediction models developed in this work are capable of predicting 3D dose distributions, enabling the evaluation of online adaptive plan quality to better inform adaptive decision‐making in MRgRT.MethodsArtificial neural networks predicted 3D dose distributions from input variables related to patient anatomy, geometry, and target/organ‐at‐risk relationships in over 300 treatment plans from 53 patients receiving adaptive, linac‐based MRgRT for abdominal cancers. The models do not include any beam related variables such as beam angles or fluence and were optimized to balance errors related to raw dose and specific plan quality metrics used to guide daily online adaptive decisions.ResultsAveraged over all plans, the dose prediction error and the absolute error were 0.1 ± 3.4 Gy (0.1 ± 6.2%) and 3.5 ± 2.4 Gy (6.4 ± 4.3%) respectively. Plan metric prediction errors were −0.1 ± 1.5%, −0.5 ± 2.1%, −0.9 ± 2.2 Gy, and 0.1 ± 2.7 Gy for V95, V100, D95, and Dmean respectively. Plan metric prediction absolute errors were 1.1 ± 1.1%, 1.5 ± 1.5%, 1.9 ± 1.4 Gy, and 2.2 ± 1.6 Gy. Approximately 10% (25) of the plans studied were clearly identified by the prediction models as inferior quality plans needing further optimization and refinement.ConclusionMachine learning prediction models for treatment plan 3D dose distributions in online adaptive MRgRT were developed and tested. Clinical integration of the models requires minimal effort, producing 3D dose predictions for a new patient’s plan using only target and OAR structures as inputs. These models can enable improved workflows for MRgRT through more informed plan optimization and plan quality assessment in real time.

Abstract P4-12-21: Implementing a real-time magnetic resonance imaging guided accelerated partial breast irradiation program

Abstract Motivation: Clinical evidence has shown that accelerated partial breast irradiation (APBI) for early stage breast cancer patients result in local recurrence control rates and cosmetic outcomes comparable to whole breast irradiation. Approaches to APBI include brachytherapy, intraoperative radiation therapy and external beam radiation therapy (EBRT). EBRT is the least invasive approach, most widely available, and does not require additional training. For EBRT APBI, based on NSABP B39/RTOG 0413 protocol, the clinical target volume (CTV) is generated by applying a 1.5 cm margin to the lumpectomy bed and the planning target volume (PTV) is generated by applying an additional 1 cm margin from the CTV to account for set up uncertainties.Real-time magnetic resonance imaging guided radiation therapy (MRgRT) allow for reduced treatment margins by decreasing treatment setup uncertainties. In this work, we share our experience in implementing a MRI guided accelerated partial breast irradiation (MRgAPBI) program into a new clinic with a linear accelerator capable of real-time MRI imaging. Methods: We have surveyed institutions in which rely on MRI guidance for APBI treatments through a comprehensive literature review and onsite visits. These visits included our radiation therapists, physicists, and radiation oncologists. There are two main components in implementing a new radiotherapy program: 1) patient simulation and immobilization techniques, and 2) treatment planning and delivery techniques. These techniques were studied and modified for our clinical workflow. Results: Literature review revealed a single institution report addressing their MRgAPBI experience. For immobilization, we adopted two techniques observed during two site visits. However, these techniques were later modified to allow for patient customization. For small breasted patients, a foam tube placed along the sternum is used and for large breasted patients, we employed custom made acrylic brackets placed superior to the shoulders which are then built into the patient's immobilization cradle for individualized set up. For patient comfort, the contralateral arm is placed down along the patient’s side, whereas the ipsilateral arm is moved up away from the radiation field. This also allows for lateral clearance for beam selection. For treatment planning, consensus on optimal margin for expansion from the lumpectomy bed is of utmost importance. A single institution MRgAPBI report described a 1 cm expansion from the lumpectomy bed to PTV while NSABP B39 indicated a 2.5 cm expansion. This reduction in margin was justified by the utilization of real-time MRgRT. We have come to consensus to use a 1.5 cm expansion around the lumpectomy cavity, which accounts for the CTV expansion per NSABP B39 and to forgo the PTV expansion given the minimal setup uncertainty with online MRgRT. Our initial experience showed significant changes in the lumpectomy cavity shape and volume when the gap between patient simulation and the start of treatment exceeded a week. We have thus reduced the time between simulation and treatment to 1 week to address this issue. Conclusion: Implementing a new MRgAPBI program requires a comprehensive evaluation of available resources and published reports, as well as, team visits to expert MRgAPBI centers. Both immobilization techniques and lumpectomy margin expansions for treatment planning should be well thought of prior to starting a similar program. In our experience, time between simulation and treatment of 1 week showed minimal changes in the shape and size of the planning volume. Additional verification will need to be conducted within a prospective setting and larger cohort. In order to setup a successful program, teamwork is essential between all groups including radiation therapists, dosimetrists, physicists, and radiation oncologists. Citation Format: Eenas A. Omari, Jennifer Gambla, Hahn P. Mai, Muaiad Kittaneh, Tarita O. Thomas, Raymond B. Wynn, Anil Sethi, William Small Jr, Tamer Refaat. Implementing a real-time magnetic resonance imaging guided accelerated partial breast irradiation program [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-12-21.

Respiratory-gated magnetic resonance image guided radiation therapy for hepatocellular carcinoma: A pilot study.

532 Background: Hypofractionated RT has shown encouraging results in hepatocellular carcinoma (HCC) patients. However, HCC adjacent to the gastrointestnal (GI) tract should be carefully treated using the high-precision irradiation technique due to risk of radiation damage. We evaluated the feasibility of a respiratory-gated magnetic resonance image guided radiation therapy (RgMRg-RT) for hepatocellular carcinoma (HCC). Methods: Thirty-three patients with HCC underwent RgMRg-RT in our hospital from 2015 to February 2019, including patients with Child-Pugh A/B7 cirrhosis and unresectable tumors near the gastrointestinal tract. The median radiation dose was 50 Gy (range, 25–60) and median fraction number was 5 (range, 4–15). Gating was performed based on real-time magnetic resonance image without an external surrogate. Results: The median follow-up period was 11.7 months (range, 3.6–37.9 months). The rates of local control of the target tumor at 6 months and 1 year were 90.2 and 86.9%, respectively. The overall survival rates at 6 months, 1, and 2 years were 84.3, 76.3, and 61%, respectively. The median distances from gross tumor to the esophagus, stomach, duodenum and colon were 6.1 (range, 1.9-14.3), 6.4 (0-13.4), 3.8 (0-13.5) and 4cm (0.3-13), respectively. A total of 15 tumors (45.5%) were located within 2 cm of the gastrointestinal tract and 9 tumors (27.3%) within 1cm. Grades 3 treatment‐related bleeding was observed in one patient and one patient had radiation‐induced liver disease. Conclusions: Hypofractionated RgMRg-RT was a safe and potentially ablative therapy for HCC. RgMRg-RT is a good alternative treatment for patients with HCCs that are unsuitable for surgical resection or local ablative therapy.

Bayesian Model Selection Approach to Multiple Change-Points Detection with Non-Local Prior Distributions

We propose a Bayesian model selection (BMS) boundary detection procedure using non-local prior distributions for a sequence of data with multiple systematic mean changes. By using the non-local priors in the BMS framework, the BMS method can effectively suppress the non-boundary spike points with large instantaneous changes. Further, we speedup the algorithm by reducing the multiple change points to a series of single change point detection problems. We establish the consistency of the estimated number and locations of the change points under various prior distributions. From both theoretical and numerical perspectives, we show that the non-local inverse moment prior leads to the fastest convergence rate in identifying the true change points on the boundaries. Extensive simulation studies are conducted to compare the BMS with existing methods, and our method is illustrated with application to the magnetic resonance imaging guided radiation therapy data.

Model-Interpolated Gating for Magnetic Resonance Image–Guided Radiation Therapy

To develop and validate a technique for radiation therapy gating using slow (≤1 frame per second) magnetic resonance imaging (MRI) and a motion model. Proposed uses of the technique include radiation therapy gating using T2-weighted images and conducting additional imaging studies during gated treatments. The technique uses a physiologically guided breathing motion model to interpolate deformed target position between 2-dimensional (2D) MRI images acquired every 1 to 3 seconds. The model is parameterized by a 1-dimensional respiratory bellows surrogate and is continuously updated with the most recently acquired 2D images. A phantom and 8 volunteers were imaged with a 0.35T MRI-guided radiation therapy system. A balanced steady-state free precession sequence with a 2D frame rate of 3 frames per second was used to evaluate the technique. The accuracy and beam-on positive predictive value (PPV) of the model-based gating decisions were evaluated using the gating decisions derived from imaging as a ground truth. A T2-weighted gating offline proof-of-concept study using a half-Fourier, single-shot, turbo-spin echo sequence is reported. Model-interpolated gating accuracy, beam-on PPV, and median absolute distances between model and image-tracked target centroids were, on average, 98.3%, 98.4%, and 0.33 mm, respectively, in the balanced steady-state free precession phantom studies and 93.7%, 92.1%, and 0.86 mm, respectively, in the volunteer studies. T2 model-interpolated gating in 6 volunteers yielded an average accuracy and PPV of 94.3% and 92.5%, respectively, and the mean absolute median distance between modeled and imaged target centroids was 0.86 mm. This work demonstrates the concept of model-interpolated gating for MRI-guided radiation therapy. The technique was found to be potentially sufficiently accurate for clinical use. Further development is needed to accommodate out-of-plane motion and the use of an internal MR-based respiratory surrogate.

(OA06) Interfraction Variations in Stomach Volume Increase Dose to Stomach and Duodenum in Pancreas Stereotactic Body Radiotherapy and Is Mitigated by Daily Adaptive Magnetic Resonance Image Guided Radiation Therapy

Magnetic Resonance Image Guided Radiation Therapy (MR-IGRT) permits clear daily visualization of the target and organs at risk (OARs) for daily dose adaptation. Using MR-IGRT with Cobalt-60 for locally advanced pancreatic cancer (LAPC) stereotactic body radiation therapy (SBRT), we noticed varied stomach volumes despite precautions not to eat prior to treatment. We hypothesized that larger stomach volumes would be associated with higher doses to OARs and that online adaptive re-planning would reduce OAR doses while maintaining prescription dose coverage. Ten LAPC patients received five fraction SBRT totaling 33-40Gy as limited by OAR constraints. Patients were instructed to avoid food and drink only sips of water for at least 3 hours prior to treatment. Treatment plans provided 90% coverage of the planning target volume (PTV) at prescription isodose (PID). While patients could have received adaptive or non-adaptive radiotherapy with MR-IGRT, all fifty fractions underwent simulated adaptation. First the original (as planned) doses were compared to the dose delivered only after 3D couch shifts (non-adaptive). An adaptive plan was created for each fraction mimicking the process for daily adaptive radiotherapy. Specifically, contours were adjusted to the anatomy visualized on the setup MRI for each fraction, and a “warm start” optimization run was performed to find a new optimizer solution with the day’s updated contours starting with the original optimizer objectives and control points. Adaptive plans were normalized to 90% PTV coverage at PID. We used linear regression and Bayesian approach to evaluate the association between changes in stomach volume and changes in radiation dose to visceral OARs for both non-adaptive and adaptive planning techniques. Only 8 non-adaptive MR-IGRT fractions maintained at least 90% PTV coverage compared to all 50 adaptive MR-IGRT fractions, and 23 non-adaptive fractions met all OAR objectives compared to 29 adaptive fractions. The original stomach volume (OSV) mean was 285.98cc, with a range from 125.6cc-855.8cc. The mean interfraction change from the OSV was +146.8cc, with a range from -403.76cc to +1271.13cc and a standard deviation of 245.57cc. The mean percentage change from the OSV was 53.2%, with a range from -47.18% to 334.32% and a standard deviation of 68.82%. Eight interfraction changes in stomach volume were greater than 500cc. For non-adaptive plans, Bayesian approach revealed a significant association between percentage increases in stomach volume and increased stomach V33 (p=.0041), stomach maximum dose (p=.0095) and duodenum V33 (p=.0208). Adaptive MR-IGRT showed no association between percentage increases in stomach volume and increased stomach V33 (p=.2763), stomach maximum dose (p=.3174) or duodenum V33 (p=.7736). This study demonstrates that for SBRT of LAPC, interfraction variations in stomach volume occur despite standard precautions and are associated with increased dose to the stomach and duodenum in non-adaptive plans. Adaptive planning limits dose to the stomach and duodenum and may permit safe SBRT dose escalation.

Optimization of treatment planning workflow and tumor coverage during daily adaptive magnetic resonance image guided radiation therapy (MR-IGRT) of pancreatic cancer

BackgroundTo simplify the adaptive treatment planning workflow while achieving the optimal tumor-dose coverage in pancreatic cancer patients undergoing daily adaptive magnetic resonance image guided radiation therapy (MR-IGRT).MethodsIn daily adaptive MR-IGRT, the plan objective function constructed during simulation is used for plan re-optimization throughout the course of treatment. In this study, we have constructed the initial objective functions using two methods for 16 pancreatic cancer patients treated with the ViewRay™ MR-IGRT system: 1) the conventional method that handles the stomach, duodenum, small bowel, and large bowel as separate organs at risk (OARs) and 2) the OAR grouping method. Using OAR grouping, a combined OAR structure that encompasses the portions of these four primary OARs within 3 cm of the planning target volume (PTV) is created. OAR grouping simulation plans were optimized such that the target coverage was comparable to the clinical simulation plan constructed in the conventional manner. In both cases, the initial objective function was then applied to each successive treatment fraction and the plan was re-optimized based on the patient’s daily anatomy. OAR grouping plans were compared to conventional plans at each fraction in terms of coverage of the PTV and the optimized PTV (PTV OPT), which is the result of the subtraction of overlapping OAR volumes with an additional margin from the PTV.ResultsPlan performance was enhanced across a majority of fractions using OAR grouping. The percentage of the volume of the PTV covered by 95% of the prescribed dose (D95) was improved by an average of 3.87 ± 4.29% while D95 coverage of the PTV OPT increased by 3.98 ± 4.97%. Finally, D100 coverage of the PTV demonstrated an average increase of 6.47 ± 7.16% and a maximum improvement of 20.19%.ConclusionsIn this study, our proposed OAR grouping plans generally outperformed conventional plans, especially when the conventional simulation plan favored or disregarded an OAR through the assignment of distinct weighting parameters relative to the other critical structures. OAR grouping simplifies the MR-IGRT adaptive treatment planning workflow at simulation while demonstrating improved coverage compared to delivered pancreatic cancer treatment plans in daily adaptive radiation therapy.

Adaptive versus non-adaptive MRI-guided radiation treatments for pancreatic cancer: A dosimetric study.

336 Background: Combination MRI and radiation therapy systems enable magnetic resonance image guided radiation therapy (MR-IGRT). MR-IGRT allows clear visualization of the target and organs at risk (OARs) allowing for dose adaptation. Using adaptive MR-IGRT with Cobalt-60 for stereotactic body radiation therapy (SBRT) in locally advanced pancreatic cancer (LAPC), we hypothesized that MR-IGRT would improve dose to pancreatic tumor without increasing doses to OARs. Methods: Ten LAPC patients received five fraction SBRT with a total dose of 33-40 Gy. For each fraction, the original plan dose was compared to the dose that would be delivered if the original radiotherapy plan was applied to the anatomy that day (non-adaptive). An adaptive plan was then created for each fraction. The plan was re-optimized based on the anatomy as seen on the daily MRI and re-normalized so the volume of the PTV receiving 100% of the prescription (PTV100) would be 90%. Both the non-adaptive and adaptive doses to the target volume and the OARs were recorded to evaluate the value of adaptation. We used a paired t-test to compare PTV100 between the adaptive and non-adaptive techniques and Chi2 tests to compare the probability of dose constraint failures for OARs. Results: Adaptive MR-IGRT improved target coverage. Mean PTV100 for adaptive and non-adaptive techniques was 89.9% [88.4-90.4] and 78.4% [27.3-96.6] respectively, p = 0.0017. There were no statistically significant differences for violations of dose constraints of OARs using adaptive vs. non-adaptive techniques. Point maxima violations above 35 Gy to duodenum occurred in 6 adaptive fractions (renormalized to 90%) vs. 12 non-adaptive fractions (p = 0.118); to stomach in 8 adaptive fractions vs. 9 non-adaptive fractions (p = 0.790), and to bowel in 9 adaptive fractions vs. 6 non-adaptive fractions (p = 0.401). When adapting, attention must be paid to other OARs in the area: Spinal cord point maxima were violated in 4 adaptive fractions. Conclusions: This study demonstrates that adaptive techniques significantly increase SBRT dose delivered to LAPC without significantly increasing dose constraint violations to OARs. Adaptive MR-IGRT may allow for further SBRT dose escalations.

Feasibility of Stereotactic Body Radiation Therapy for Non–small Cell Lung Cancer Using a Linear Accelerator-Based Magnetic Resonance Image Guided Radiation Therapy System

To describe a linear accelerator (linac) based magnetic resonance image guided radiation therapy system (MR-IGRT) and demonstrate the feasibility of delivering stereotactic body radiation therapy (SBRT) for non-small cell lung cancer (NSCLC) treatment. The linac-based MR-IGRT (MRL) system currently being installed at our institution integrates a 0.35T MRI with a flattening-filter free (FFF) linac delivering 6MV at 600 cGy/min. A novel double-focused and double-stack multileaf collimator (MLC) with an effective leaf width of 4 mm provides modulation for step-and-shoot IMRT delivery. The ability of a similar Cobalt-based system for real time, real anatomy gating has already been demonstrated, and a number of patients with NSCLC were treated. Here, we investigate the feasibility of planning and delivery with the linac-based system for SBRT, primarily focusing on plan quality as compared to conventional linacs (CL) and delivery times. Four MRL plans were generated for NSCLC patients who were previously planned and treated on a CL using 6MV FFF beams with a total dose of 54 Gy in 3 fractions. MRL plans were compared to clinical plans (CP) by evaluating conformity numbers with 100% (CN100), 95% (CN95) of prescription dose (Rx), heterogeneity indices (HI), maximum dose and dose-volume histograms of organs-at-risks (OARs), and delivery times. All MRL plans achieved PTV coverage and OAR sparing within clinical constraints and were comparable to clinically-treated plans as indicated in Table 1. Three plans showed an average of 8% and 12% improvement on PTV conformity CN100 and CN95, respectively. There is no clear trend for treatment time comparison but in general they only differ by 3 mins for a 10-min long treatment. The MRL is capable of generating clinically acceptable SBRT plans targeting NSCLC. With real-time MR gating, novel double-stack MLC design and fast FFF delivery, the system is able to achieve a more conformal and precise dose to the target therefore having great potential for further lung tumor dose escalation and reduction of treated volumes. We expect that a higher dose rate version will be available soon to further reduce the treatment time.Abstract 3536IDPlanTumor LocationVPTV Rx (%)VPTV 95%Rx (%)HICN100CN95Max Cord (Gy)Max Esophagus (Gy)Max Heart (Gy)Max Carina (Gy)Mean Lung-ITV (Gy)V20 Lung-ITV (%)Treatment time (min)1MRLLeft upper lobe97.999.61.160.920.844.4910.5521.828.843.193.110.2CP97.399.91.230.860.785.779.6818.519.223.023.07.22MRLLeft lower lobe99.11001.160.800.641.268.3310.970.452.412.45.7CP98.71001.150.840.690.456.2210.010.281.872.27.83MRLRight upper lobe99.299.91.160.740.665.138.670.730.781.210.711.8CP99.81001.170.720.581.418.160.310.421.081.09.04MRLLeft lower lobe97.299.71.260.890.8313.8714.0712.8912.676.216.26.9CP96.6991.200.780.709.5218.0115.6816.576.319.310.0 Open table in a new tab

Two-and-a-half-year clinical experience with the world's first magnetic resonance image guided radiation therapy system

PurposeMagnetic resonance image guided radiation therapy (MR-IGRT) has been used at our institution since 2014. We report on more than 2 years of clinical experience in treating patients with the world's first MR-IGRT system. Methods and materialsA clinical service was opened for MR-IGRT in January 2014 with an MR-IGRT system consisting of a split 0.35T magnetic resonance scanner that straddles a ring gantry with 3 multileaf collimator-equipped 60Co heads. The service was expanded to include online adaptive radiation therapy (ART) MR-IGRT and cine gating after 6 and 9 months, respectively. Patients selected for MR-IGRT were enrolled in a prospective registry between January 2014 and June 2016. Patients were treated with a variety of radiation therapy techniques including intensity modulated radiation therapy and stereotactic body radiation therapy (SBRT). When applicable, online ART was performed and gating on sagittal 2-dimensional cine MR was used. The charts of patients treated with MR-IGRT were reviewed to report on the clinical and treatment characteristics of the initial patients who were treated with this novel technique. ResultsA total of 316 patients have been treated with the MR-IGRT system, which has been integrated into a high-volume clinic. The cases were most commonly selected for improved soft tissue visualization, ART, and cine gating. Seventy-six patients were treated with 3-dimensional conformal radiation therapy, 146 patients with intensity modulated radiation therapy, and 94 patients with SBRT. The most commonly treated disease sites were the abdomen (28%), breast (26%), pelvis (22%), thorax (19%), and head and neck (5%). Sixty-seven patients were treated with online ART over a total of 244 adapted fractions. Cine treatment gating was used for a total of 81 patients. ConclusionsMR-IGRT has been successfully implemented in a high-volume radiation clinic and provides unique advantages in the treatment of a variety of malignancies. Additional clinical trials are in development to formally evaluate MR-IGRT in the treatment of multiple disease sites with techniques such as SBRT and ART.

(P097) Two-and-a-Half Year Clinical Experience With Magnetic Resonance Image Guided Radiation Therapy

(P097) Two-and-a-Half Year Clinical Experience With Magnetic Resonance Image Guided Radiation Therapy

Dosimetric Comparison of Real-Time MRI-Guided Tri-Cobalt-60 Versus Linear Accelerator-Based Stereotactic Body Radiation Therapy Lung Cancer Plans.

Purpose:Magnetic resonance imaging–guided radiation therapy has entered clinical practice at several major treatment centers. Treatment of early-stage non-small cell lung cancer with stereotactic body radiation therapy is one potential application of this modality, as some form of respiratory motion management is important to address. We hypothesize that magnetic resonance imaging–guided tri-cobalt-60 radiation therapy can be used to generate clinically acceptable stereotactic body radiation therapy treatment plans. Here, we report on a dosimetric comparison between magnetic resonance imaging–guided radiation therapy plans and internal target volume–based plans utilizing volumetric-modulated arc therapy.Materials and Methods:Ten patients with early-stage non-small cell lung cancer who underwent radiation therapy planning and treatment were studied. Following 4-dimensional computed tomography, patient images were used to generate clinically deliverable plans. For volumetric-modulated arc therapy plans, the planning tumor volume was defined as an internal target volume + 0.5 cm. For magnetic resonance imaging–guided plans, a single mid-inspiratory cycle was used to define a gross tumor volume, then expanded 0.3 cm to the planning tumor volume. Treatment plan parameters were compared.Results:Planning tumor volumes trended larger for volumetric-modulated arc therapy–based plans, with a mean planning tumor volume of 47.4 mL versus 24.8 mL for magnetic resonance imaging–guided plans (P = .08). Clinically acceptable plans were achievable via both methods, with bilateral lung V20, 3.9% versus 4.8% (P = .62). The volume of chest wall receiving greater than 30 Gy was also similar, 22.1 versus 19.8 mL (P = .78), as were all other parameters commonly used for lung stereotactic body radiation therapy. The ratio of the 50% isodose volume to planning tumor volume was lower in volumetric-modulated arc therapy plans, 4.19 versus 10.0 (P < .001). Heterogeneity index was comparable between plans, 1.25 versus 1.25 (P = .98).Conclusion:Magnetic resonance imaging–guided tri-cobalt-60 radiation therapy is capable of delivering lung high-quality stereotactic body radiation therapy plans that are clinically acceptable as compared to volumetric-modulated arc therapy–based plans. Real-time magnetic resonance imaging provides the unique capacity to directly observe tumor motion during treatment for purposes of motion management.