This study shows that the TG- 51 calibration is feasible in the presence of 0.35T MF and the measurement agreement is within the range of results obtainable for conventional treatment machines. Drs. Green, Goddu, and Mutic served as scientific consultants for ViewRay, Inc. Dr. Mutic is on the clinical focus group for ViewRay, Inc., and his spouse holds shares in ViewRay, Inc.
Purpose: To describe the commissioning process for the first installed commercial MRI‐guided IMRT system (ViewRay, Village of Oakwood, OH) and outline quality assurance methods for this novel treatment modality. Methods: The ViewRay™ System (510(k) pending) consists of a 0.35‐T double‐doughnut MRI coupled with a gantry that houses three Co‐60 sources,each with an activity up to 15,000 Ci (120° apart). IMRT delivery is enabled by doubly‐focused MLCs that serve as the only beam‐shaping collimators for each head, allowing a maximum field size of 27.3 cm2 at the 105‐cm isocenter. MRI imaging is used prior to and during delivery for setup evaluation, adaptive radiotherapy, and gating. The challenges in commissioning as well as periodic and patient‐specific QA arise due to the presence of the magnetic field, unique geometry of this device which is not compatible with many of conventional RT QA devices and techniques, and the penumbra of a 2‐cm wide Co‐60 source. The following devices were used for commissioning and quality assurance tests: radiochromic and radiographic film, ionization chambers (Exradin A12 & A16), Sun Nuclear's IC Profiler (Melbourne, FL), and a water tank. Tests were conducted to evaluate the beam profiles, penumbra, and PDDs. Also, tests to check MLC accuracy and reproducibility were evaluated. Results: Tests were developed to validate geometric performance of the device including a comprehensive set of MLC tests. Due to the low strength of the magnetic field, the mean free path of electrons in the ionization chamber volume is too long to have a noticeable curvature; therefore the magnetic field is not expected to have a noticeable effect on dose measurement. Conclusions: While the presence of the magnetic field limited the choice of QA devices, it was found that satisfactory methods for MRI‐IMRT machine QA exist and can be successfully employed. Drs. Green, Goddu, and Mutic served as scientific consultants for ViewRay, Inc. Dr. Mutic is on the clinical focus group for ViewRay, Inc., and his spouse holds shares in ViewRay, Inc.
Purpose: To develop a statistical and anatomical population‐based model that can be used to validate the accuracy and integrity of head and neck normal tissue structures of individual patients for use in preplanning and/or online adaptive radiation therapy. Methods: Normal tissue contours from 29 patients treated for head and neck cancers were used in development of the model. For each patient, DICOM plan and structure files were exported from the treatment planning system to an in‐house developed software program which calculated anatomic metrics for volume, shape, and intra‐structure distances for all structures. A statistical analysis of these metrics produced population specific rules that were used within the software program to evaluate the accuracy of head and neck contours for subsequent patients. The contour assessment program included only metrics for which the standard deviation was less than a heuristically determined limit of 15% of the mean for that metric. To verify the softwares utility, 42 common contouring errors were intentionally introduced within 9 specific structures for 9 different patients. These errors included incorrect laterality, position, size and shape, inclusion of small isolated pixels, deleted segments, and empty structures. The evaluation of all 9 head and neck structure sets was blinded to the nature and number of the generated errors. Results: The contour accuracy and integrity program correctly identified 40 of 42 generated errors. Small modifications to the structures shape and volume were the most difficult to correctly identify; however the program correctly identified all positional and laterality errors, deleted/isolated segments, small pixels, and deleted contours. Conclusion: Rules developed from a statistical analysis of anatomic population‐based metrics can provide much of the necessary information to correctly and efficiently evaluate the accuracy and integrity of a unique patient contour structure set for IMRT preplanning or for an online adaptive radiation therapy protocol.
Purpose: Bowel toxicity can be difficult to manage in the treatment of abdominal cancers. The bowel experiences large motion during treatment, causing it to enter high‐dose regions. Real‐time MR imaging during radiotherapy allows for potential visualization and dosimetric avoidance of the bowel during treatment. To investigate the feasibility of real‐time ‘bowel‐gated’ treatment using onboard MRI, we assessed two bowel‐tracking algorithms on 32 cine imagesets acquired with the ViewRay hybrid MR‐ radiotherapy unit. Methods: The Viewray(TM) System, which is an integrated 0.35T MR‐Co‐60 system that is pending 510k approval, was used to acquire 32 cine image sets in 5 patients under an IRB‐approved trial. Each imageset was acquired in 2D (coronal or sagittal orientation) for an average duration of 0.5‐2.5 minutes at a frame rate of 4 frames/s. The initial position of the bowel was manually contoured on the first image frame. Two algorithms were evaluated in tracking bowel from its initial position throughout its motion for the duration of the cine set ‐ a normalized cross‐ correlation (NCC) algorithm, and a weighted NCC (WNCC) algorithm. To assess the tracking feasibility and accuracy of these two methods, the initial contour was virtually shifted with the tracked motion and displayed on tracked cine images. The agreement between the shifted contour and the border of the bowel was manually inspected and noted for each frame. Results: Both algorithms successfully tracked 31/32 cases in 100% of frames. The WNCC algorithm outperformed the NCC algorithm in speed, with a mean processing speed of .007s versus .013s, respectively, and captured a greater range of motion in all cases. Conclusions: The demonstrated feasibility of bowel tracking on cine MR imagesets indicates its potential successful use in real‐time bowel tracking and gated radiotherapy. While both algorithms performed well, the WNCC algorithm was superior in processing speed and sensitivity to bowel motion.
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