Adaptive MR-Guided Stereotactic Body Radiation Therapy (AMR-SBRT) for Oligometastatic or Unresectable Primary Abdominal Malignancies: Results of a Prospective Phase I Trial

Price

et al. 2020

To present a tumor motion control system during free breathing using direct tumor visual feedback to patients in 0.35 T magnetic resonance‐guided radiotherapy (MRgRT). We present direct tumor visualization to patients by projecting real‐time cine MR images on an MR‐compatible display system inside a 0.35 T MRgRT bore. The direct tumor visualization included anatomical images with a target contour and an auto‐segmented gating contour. In addition, a beam‐status sign was added for patient guidance. The feasibility was investigated with a six‐patient clinical evaluation of the system in terms of tumor motion range and beam‐on time. Seven patients without visual guidance were used for comparison. Positions of the tumor and the auto‐segmented gating contour from the cine MR images were used in probability analysis to evaluate tumor motion control. In addition, beam‐on time was recorded to assess the efficacy of the visual feedback system. The direct tumor visualization system was developed and implemented in our clinic. The target contour extended 3 mm outside of the gating contour for 33.6 ± 24.9% of the time without visual guidance, and 37.2 ± 26.4% of the time with visual guidance. The average maximum motion outside of the gating contour was 14.4 ± 11.1 mm without and 13.0 ± 7.9 mm with visual guidance. Beam‐on time as a percentage was 43.9 ± 15.3% without visual guidance, and 48.0 ± 21.2% with visual guidance, but was not significantly different ( P = 0.34). We demonstrated the clinical feasibility and potential benefits of presenting direct tumor visual feedback to patients in MRgRT. The visual feedback allows patients to visualize and attempt to minimize tumor motion in free breathing. The proposed system and associated clinical workflow can be easily adapted for any type of MRgRT.

Section: Methodsmentioning

confidence: 99%

Section: C | Clinical Workflow Of Direct Tumor Visualizationmentioning

confidence: 99%

Direct tumor visual feedback during free breathing in 0.35T MRgRT

Kim

Price

et al. 2020

“…Magnetic resonance guided radiotherapy (MRgRT) has become an important treatment modality for improving treatment outcomes by increasing local control rates and reducing normal tissue toxicity. 1 , 2 , 3 Systems that combine a magnetic resonance imaging (MRI) scanner and a linear accelerator (Linac) provide beam gating, real‐time target tracking, and the ability to perform online adaptive radiotherapy. 3 , 4 , 5 Additionally, soft tissue contrast and structure identification with on‐board MRI systems are superior to that provided by on‐board X‐ray systems, both for 2D planar images and volumetric images such as cone beam CT (CBCT), without the addition of any excess ionizing radiation.…”

Section: Introductionmentioning

confidence: 99%

First clinical experience of correcting phantom‐based image distortion related to gantry position on a 0.35T MR‐Linac

Shin

Quinn³

et al. 2021

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

“…Currently, MRgRT with real-time two-dimensional (2D) tumor tracking is clinically available 5,6 and MRgRT treatments have been demonstrated to produce excellent local control rates with little toxicity. 5,7,8 Tumors benefiting most from real-time 2D tumor tracking are those near the diaphragm where organs experience the maximal respiratory motion, including the lower lobe of the lungs and the liver dome.…”

Section: Introductionmentioning

confidence: 99%

“…Magnetic Resonance‐guided radiotherapy (MRgRT) is a highly desirable image‐guided radiotherapy (IGRT) modality for moving targets because it provides continuous imaging of patient anatomy with the superior soft tissue contrast provided by MRI, without the ionizing radiation imparted by x‐ray imaging, during radiation beam delivery. Currently, MRgRT with real‐time two‐dimensional (2D) tumor tracking is clinically available 5,6 and MRgRT treatments have been demonstrated to produce excellent local control rates with little toxicity 5,7,8 . Tumors benefiting most from real‐time 2D tumor tracking are those near the diaphragm where organs experience the maximal respiratory motion, including the lower lobe of the lungs and the liver dome.…”

Section: Introductionmentioning

confidence: 99%

Clinical experience of MRI^4D QUASAR motion phantom for latency measurements in 0.35T MR‐LINAC

Kim

Lotey

et al. 2020

PurposeIn MRgRT, accuracy of treatment depends on the gating latency, when real‐time targeting and gating is enabled. Gating latency is dependent on image acquisition, processing time, accuracy, efficacy of target tracking algorithms, and radiation beam delivery latency. In this report, clinical experience of the MRI4D QUASAR motion phantom for latency measurements on a 0.35‐T magnetic resonance‐linear accelerator (MR‐LINAC) with two imaging speeds and four tracking algorithms was studied.Materials/MethodsBeam‐control latency was measured on a 0.35‐T MR‐LINAC system with four target tracking algorithms and two real‐time cine imaging sequences [four and eight frames per second (FPS)]. Using an MR‐compatible motion phantom, the delays between phantom beam triggering signal and linac radiation beam control signal were evaluated for three motion periods with a rigid target. The gating point was set to be 8 mm above the full exhalation position. The beam‐off latency was measured for a total of 24 combinations of tracking algorithm, imaging FPS, and motion periods. The corresponding gating target margins were determined using the target motion speed multiplied by the beam‐off latency.ResultsThe largest measured beam‐off latency was 302 ± 20 ms with the Large Deforming Targets (LDT) algorithm and 4 s motion period imaged with 8‐FPS cine MRI. The corresponding gating uncertainty based on target motion speed was 3.0 mm. The range of the average beam‐off latency was 128–243 ms in 4‐FPS imaging and 47–302 ms in 8‐FPS imaging.ConclusionsThe gating latency was measured using an MRI4D QUASAR motion phantom in a 0.35‐T MR‐LINAC. The latency measurements include time delay related to MR imaging method, target tracking algorithm and system delay. The gating uncertainty was estimated based on the beam‐off latency measurements and the target motion.