Effect of respiration-induced motion on a three-dimensional magnetic resonance imaging-based adaptive radiotherapy workflow in a 1.5T magnetic resonance linear accelerator.

Background: The long acquisition time of three-dimensional (3D) magnetic resonance imaging (MRI) makes it vulnerable to motion-induced artifacts such as blurring and ghosting, which may compromise target delineation and dose accuracy. Although motion management strategies such as four-dimensional MRI and cine MRI have been proposed, the specific influence of respiratory parameters-particularly amplitude and frequency-on the geometric and dosimetric precision of magnetic resonance-guided adaptive radiotherapy (RT) remains inadequately quantified. This study thus aimed to systematically evaluate how respiratory-induced linear translational motion affects delineation accuracy and dose distribution in MRI-based adaptive RT.

Methods: An MR-compatible motion phantom was employed to replicate patient-specific respiratory-induced translational motion, with amplitude and frequency variations extracted from real patient waveforms being incorporated. Eight distinct respiratory patterns were generated, and MR images were acquired with standard spin-spin relaxation time-weighted (T2W) three-dimensional (3D) Cartesian sequences for each pattern. The internal target volume (ITV) delineated by clinicians based on 3D MR images was compared with the reference ITV (ITV_ref) generated with the digital phantom. Key delineation metrics [e.g., Dice similarity coefficient (DSC), Hausdorff distance (HD), and mean surface distance (MSD)] and dosimetric parameters (e.g., dose received by 95% of the volume) were evaluated, and statistical analyses were performed to assess the correlations between respiratory motion characteristics and the observed variations.

Results: Respiratory amplitude significantly affected delineation accuracy and dosimetric consistency. The DSC decreased linearly with increasing amplitude, from 0.96 at 2.50 mm to 0.83 at 12.50 mm, while the HD and MSD increased proportionally (2.62 to 6.32 mm and 0.08 to 0.64 mm, respectively). Dosimetric analysis showed a notable reduction in ITV_ref dose coverage at higher amplitudes, with the dose received by 95% of the volume decreasing by 481.55 cGy at 12.50 mm relative to the prescribed total dose of 4,500 cGy. In contrast, respiratory frequency had minimal impact, with changes remaining within clinically acceptable ranges.

Conclusions: This study investigated the impact of respiratory-induced linear translational motion on ITV delineation and dosimetric accuracy in MR-guided RT. It was found that large respiratory amplitudes significantly compromised geometric and dosimetric precision, whereas frequency had minimal influence. The results emphasize the limitations of 3D Cartesian MRI due to motion-averaged artifacts and support the development of advanced imaging techniques for improving ITV delineation accuracy.