Search

PurposeThe total time of radiation treatment delivery for pancreatic cancer patients with daily online adaptive radiation therapy (ART) on an MR-Linac can range from 50 to 90 min. During this period, the target and normal tissues undergo changes due to respiration and physiologic organ motion. We evaluated the dosimetric impact of the intrafraction physiological organ changes.MethodsTen locally advanced pancreatic cancer patients were treated with 50 Gy in five fractions with intensity-modulated respiratory-gated radiation therapy on a 0.35-T MR-Linac. Patients received both pre- and post-treatment volumetric MRIs for each fraction. Gastrointestinal organs at risk (GI-OARs) were delineated on the pre-treatment MRI during the online ART process and retrospectively on the post-treatment MRI. The treated dose distribution for each adaptive plan was assessed on the post-treatment anatomy. Prescribed dose volume histogram metrics for the scheduled plan on the pre-treatment anatomy, the adapted plan on the pre-treatment anatomy, and the adapted plan on post-treatment anatomy were compared to the OAR-defined criteria for adaptation: the volume of the GI-OAR receiving greater than 33 Gy (V33Gy) should be ≤1 cubic centimeter.ResultsAcross the 50 adapted plans for the 10 patients studied, 70% were adapted to meet the duodenum constraint, 74% for the stomach, 12% for the colon, and 48% for the small bowel. Owing to intrafraction organ motion, at the time of post-treatment imaging, the adaptive criteria were exceeded for the duodenum in 62% of fractions, the stomach in 36%, the colon in 10%, and the small bowel in 48%. Compared to the scheduled plan, the post-treatment plans showed a decrease in the V33Gy, demonstrating the benefit of plan adaptation for 66% of the fractions for the duodenum, 95% for the stomach, 100% for the colon, and 79% for the small bowel.ConclusionPost-treatment images demonstrated that over the course of the adaptive plan generation and delivery, the GI-OARs moved from their isotoxic low-dose region and nearer to the dose-escalated high-dose region, exceeding dose-volume constraints. Intrafraction motion can have a significant dosimetric impact; therefore, measures to mitigate this motion are needed. Despite consistent intrafraction motion, plan adaptation still provides a dosimetric benefit.

Purpose: The superior soft-tissue contrast of MRI-guided radiotherapy offers enhanced localization accuracy of the spinal cord in spine Stereotactic Body Radiotherapy (SBRT). This work includes a planning study for spine-SBRT on an MR-Linac. Additionally, a patient with spine metastasis was treated using an adaptive radiation therapy workflow. We report our initial experience of targeting accuracy, image-guided localization, on-line adaptive planning, and treatment with real-time intrafraction imaging with automatic beam gating.Methods: Six spine-SBRT patients were retrospectively re-planned to 18 Gy in 1-fraction on a commercial, Monte Carlo-based MR-Linac treatment planning system. Plans were generated using 9–13 step-and-shoot intensity-modulated radiation therapy 6 MV-flattening filter free beams and optimized to achieve plan quality criteria recommended by RTOG-0631. One thoracic vertebral body clinical case was treated to 27 Gy in 3-fractions utilizing ART, where daily anatomical changes were accounted for via re-planning and treatment in an on-line manner to account for limited ability to correct rotational setup uncertainties.Results: Plans met all critical-tissue constraints outlined in RTOG-0631 and AAPM Task Group-101, while covering 90% of the target with the prescription dose. Clinically, visibility of the spinal cord allowed for patient setup focusing on spinal cord-alignment. Utilization of the online ART workflow, while re-contouring the target and spinal cord, enabled an increase in prescription dose coverage from 89 to 95% in two of three fractions while maintaining acceptable doses to organs-at-risk. Real-time MR-cine imaging demonstrated sufficient quality for the automatic beam gating algorithm to provide intrafraction motion management of the spinal canal utilizing a 3.0 mm gating boundary and 1–2% region of excursion allowance, in the sagittal plane. A decrease in coverage, below the 95% threshold was noted in post-treatment volumetric imaging due to lateral movement not observed during real-time gating.Conclusion: Achieved plan quality and deliverability was within accepted standards. MR-guidance with an on-line ART workflow offered increased accuracy in the localization of the spinal cord at the time of treatment to enhance both tissue sparing and target volume coverage. Increased spatial resolution of cine-images, and tracking in three-dimensions would be beneficial for future spine-SBRT treatments on the MR-Linac.

Purpose: Patient tolerability of magnetic resonance (MR)–guided radiation treatment delivery is limited by the need for repeated deep inspiratory breath holds (DIBHs). This volunteer study assessed the feasibility of continuous positive airway pressure (CPAP) with and without DIBH for respiratory motion management during radiation treatment with an MR-linear accelerator (MR-linac). Methods and Materials: MR imaging safety was first addressed by placing the CPAP device in an MR-safe closet and configuring a tube circuit via waveguide to the magnet bore. Reproducibility and linearity of the final configuration were assessed. Six healthy volunteers underwent thoracic imaging in a 0.35T MR-linac, with one free breathing (FB) and 2 DIBH acquisitions being obtained at 5 pressures from 0 to 15 cm-H2O. Lung and heart volumes and positions were recorded; repeatability was assessed by comparing 2 consecutive DIBH scans. Blinded reviewers graded images for motion artifact using a 3-point grading scale. Participants completed comfort and perception surveys before and after imaging sessions. Results: Compared with FB alone, FB-10, FB-12, and FB-15 cm H2O significantly increased lung volumes (+23%, +34%, +44%; all P