Dosimetric validation of a magnetic resonance image gated radiotherapy system using a motion phantom and radiochromic film

Abstract

PurposeMagnetic resonance image (MRI) guided radiotherapy enables gating directly on the target position. We present an evaluation of an MRI‐guided radiotherapy system's gating performance using an MRI‐compatible respiratory motion phantom and radiochromic film. Our evaluation is geared toward validation of our institution's clinical gating protocol which involves planning to a target volume formed by expanding 5 mm about the gross tumor volume (GTV) and gating based on a 3 mm window about the GTV.MethodsThe motion phantom consisted of a target rod containing high‐contrast target inserts which moved in the superior‐inferior direction inside a body structure containing background contrast material. The target rod was equipped with a radiochromic film insert. Treatment plans were generated for a 3 cm diameter spherical planning target volume, and delivered to the phantom at rest and in motion with and without gating. Both sinusoidal trajectories and tumor trajectories measured during MRI‐guided treatments were used. Similarity of the gated dose distribution to the planned, motion‐frozen, distribution was quantified using the gamma technique.ResultsWithout gating, gamma pass rates using 4%/3 mm criteria were 22–59% depending on motion trajectory. Using our clinical standard of repeated breath holds and a gating window of 3 mm with 10% target allowed outside the gating boundary, the gamma pass rate was 97.8% with 3%/3 mm gamma criteria. Using a 3 mm window and 10% allowed excursion, all of the patient tumor motion trajectories at actual speed resulting in at least 95% gamma pass rate at 4%/3 mm.ConclusionsOur results suggest that the device can be used to compensate respiratory motion using a 3 mm gating margin and 10% allowed excursion results in conjunction with repeated breath holds. Full clinical validation requires a comprehensive evaluation of tracking performance in actual patient images, outside the scope of this study.

Characterization and longitudinal assessment of daily quality assurance for an MR‐guided radiotherapy (MRgRT) linac

“…for an MRgRT program . Quality assurance of the gating capabilities of the MRIdian can be monitored with an end‐to‐end procedure with a motion phantom as described by Lamb et al …”

Multislice motion modeling forMRI‐guided radiotherapy gating

“…6 Magnetic resonance image (MRI)-guided radiotherapy is an emerging technology that enables respiratory motion compensation based on images that have excellent soft-tissue contrast. 7,8 The ViewRay MRIdian (ViewRay, Inc. Oakwood Village, OH) is a commercially available device that performs radiotherapy gating based on target position measured directly in real-time MR images. MRIdian gating images can be acquired in a single sagittal slice at four frames per second (FPS), or across three sagittal slices at 2 FPS.…”

An image regression motion prediction technique for MRI‐guided radiotherapy evaluated in single‐plane cine imaging

“…Targets were tracked using a deformable image registration algorithm similar to that used by the ViewRay system. 3,24 Image registrations were performed using the Elastix package. [25][26][27] A multi-resolution b-spline deformable registration was performed using mutual information as a similarity measure.…”

“…Magnetic resonance image (MRI)-guided radiotherapy enables the continuous acquisition of images during treatment to monitor tumor motion. [1][2][3] For those images to be useful for linear accelerator gating, they must be acquired, reconstructed, and the position of the tumor (or gating structure) with respect to the gating window must be determined. Since these steps require time, the gating software will determine whether to turn the beam on or off after the tumor has entered or exited the gating window, respectively.…”

“…MRI-guided radiation therapy system latencies have been reported to be in the range of 200-500 ms. [1][2][3]5,6 Motion prediction offers one method to mitigate the effects of system latency. If the gating system could predict when the tumor exited the gating window, the radiation beam could be preemptively turned off.…”