ESTRO 2025 - Abstract Book
S3348
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2025
demonstrates how the new T2w cine-MRI sequence enables MRI-guided MLC-tracking for accurate and time efficient motion management. Material/Methods: All experiments were conducted on a 1.5T Unity MR-linac (Elekta AB, Sweden) using a clinical interleaved coronal and sagittal T2w cine-MRI sequence (resolution=2.2x2.2x5.0mm 3 , T acq =2.0s per slice). 3D motion vectors were continuously streamed to in-house developed MLC tracking software [2]. The QUASAR MRI 4D motion phantom (IBA QUASAR, London-ON, Canada) featuring a movable insert with Ø3cm spherical clinical-target-volume (CTV), was positioned on a 20° inclined ramp (Fig1). This inclination tilted the phantom's motion axis, effectively decomposing the motion into CC and AP directions. The phantom was programmed with patient-derived prostate motion (CC avg =0.3mm/min; AP avg =0.1mm/min), linear drift motion (CC=0.9mm/min; AP=0.3mm/min), or was used statically as reference. An ultra-hypofractionated prostate IMRT plan (11-beam, 2x12Gy) was created with 2mm CTV-to-PTV margins. A rectum, urethra, and bladder were delineated as mock OARs. A pre-beam adapt-to-shape plan adaptation corrected for interfractional variability. MLC-tracking was dosimetrically validated using a prototype hybrid dosimetry array combining EBTXD-film with eight time-resolved plastic scintillation dosimeters (PSDs)[3]. The film was placed sagittally through the CTV’s centre, while the PSDs were positioned at the CTV centre and along its periphery, aligned with the phantom's translational axis.
Results: Motion-induced dose blurring mainly occurred at the CTV edges (Fig2). Linear and patient-derived motion caused dose reductions compared to static of 40-57% and 10-16% around the anterior-cranial periphery and increases of 15-62% and 5-10% around the posterior-caudal periphery. MLC-tracking reduced these differences to 0-8% for both motion patterns, effectively restoring the static dose. MLC-tracking improved local 1%/1mm gamma pass-rates w.r.t. the static reference, increasing from 8.3% to 86.5% for linear motion and from 65.3% to 97.9% for patient-derived motion, while maintaining a constant delivery time of 10,5 minutes.
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