Intrafraction Motion Management With MR-Guided Radiation Therapy

Facilitating 1.5T MR-Linac adoption: Workflow strategies and practical tips

BACKGROUND

MR-guided radiotherapy (MRgRT) offers new opportunities but also introduces workflow complexities requiring dedicated optimization. Implementing magnetic resonance linear accelerator (MR-Linac) technology comes with challenges such as prolonged treatment times and workflow integration issues.

PURPOSE

We present here our experience and share practical tips and tricks to streamline MR-Linac implementation, optimize workflow efficiency, and improve coordination.

METHODS

The first 150 patients treated with a 1.5T MR-Linac Unity® at our institution were analyzed. Treatments were assessed based on session recordings, difficulties encountered were identified, and solutions documented.

RESULTS

A total of 726 fractions were delivered, with a mean treatment time of 48 minutes. Key optimizations included standardized operating procedures (SOPs) and structured briefing sheets, leading to reduced delays and improved treatment consistency.

CONCLUSION

Strategic workflow standardization and optimized communication tools significantly improved the ability to deliver high-quality, patient-centered care by streamlining processes and enhancing coordination among team members. These insights provide practical guidance for centers integrating MR-Linac technology.

Characterization of an MR-compatible motion platform for quality assurance of motion-compensated treatments on the 1.5 T MR-linac

BACKGROUND

Novel motion-compensated treatment techniques on the MR-linac can address adverse intra-fraction motion effects. These techniques involve beam gating or intra-fraction adaptations of the treatment plan based on real-time magnetic resonance imaging (MRI) performed during treatment. For quality assurance (QA) of these workflows, a multi-purpose motion platform is desirable. This platform should accommodate various phantoms, enabling multiple QA workflows.

PURPOSE

This study aims to evaluate the new IBA QUASAR Motion MR Platform for use in the 1.5 T MR-linac.

METHODS

The motion platform was assessed for several magnetic resonance (MR) characteristics, including spurious noise generation and B0&B1 homogeneity. In addition, the motion platform's motion accuracy and beam attenuation were assessed. An application was shown with a ScandiDos Delta4 Phantom+ MR demonstrating patient-specific plan QA of gated treatments using time-resolved dosimetry that includes motion based on a patient's respiratory motion trace.

RESULTS

All MR characterization measurements were within the set tolerances for MRI QA. The motion platform motion accuracy showed excellent agreement with the reference, with a standard deviation of the amplitude of 0.01  mm (20 kg load) for the motor's self-estimated positions and 0.22 mm (no load) for the images acquired with the electronic portal imager. Beam attenuation was found to be 11.8%. The combination of the motion platform and Delta4 demonstrated motion-included dosimetry at high temporal and spatial resolutions. Motion influenced the measured dose in non-gated treatments by up to -20.1%, while gated deliveries showed differences of up to -1.7% for selected diodes.

CONCLUSION

The motion platform was found to be usable in a 1.5 T magnetic field, and for all MR characterization experiments, no influence from the motion platform was observed. This motion platform enables to perform motion-included QA, with a measurement device of choice.

Intrafractional Motion in Online-Adaptive Magnetic Resonance-Guided Radiotherapy of Adrenal Metastases Leads to Reduced Target Volume Coverage and Elevated Organ-at-Risk Doses

BACKGROUND/OBJECTIVES

Stereotactic body radiotherapy is frequently used in patients with adrenal metastases. Motion of adherent radiosensitive organs at risk (OARs) and tumors influence OAR toxicity and tumor control. Online-adaptive Magnetic Resonance-guided radiotherapy (MRgRT) can address and mitigate interfractional changes. However, the impact of intrafractional variations in adrenal MRgRT is unknown.

METHODS

A total of 23 patients with 24 adrenal metastases were treated with MRgRT. After daily plan adaptation and before beam application, an additional (preRT) 3d MRI was acquired. PreRT target volumes and OARs were retrospectively recontoured in 200 fractions. The delivered, online-adapted treatment plans, as well as non-adapted baseline plans, were calculated on these re-contoured structures to quantify the dosimetric impact of intrafractional variations on target volume coverage and OAR doses with and without online adaptation. Normal tissue complication probabilities (NTCPs) were calculated.

RESULTS

The median time between the two MRIs was 56.4 min. GTV and PTV coverage (dose to 95% of the PTV, D95%, and volume covered by 100% of the prescription dose, V100%) were significantly inferior in the preRT plans. GTV Dmean was significantly impaired in left-sided metastases, but not in right-sided metastases. Compared to non-adapted preRT plans, adapted preRT plans were still significantly superior for all GTV and PTV metrics. Intrafractional violations of OAR constraints were frequent. D0.5cc and the volume exposed to the near-maximum dose constraint were significantly higher in the preRT plans. The volume exposed to the D0.5cc constraints in single fractions escalated up to 1.5 cc for the esophagus, 3.2 cc for the stomach, 5.3 cc for the duodenum and 7.3 cc for the bowel. This led to significantly elevated NTCPs for the stomach, bowel and duodenum. Neither PTV D95%, nor gastrointestinal OAR maximum doses were significantly impaired by longer fraction duration.

CONCLUSIONS

Intrafractional motion in adrenal MRgRT caused significant impairment of target volume coverage (D95% and V100%), potentially undermining local control. Frequent violation of gastrointestinal OAR constraints led to elevated NTCP. Compared to non-adaptive treatment, online adaptation still highly improved GTV and PTV coverage.

Feasibility of celiac axis delineation and treatment on combined magnetic resonance imaging and linear accelerator systems

Trials have been performed on irradiating celiac plexus for pancreatic cancer pain management. Images from a combined magnetic resonance imaging and linear accelerator system (MR-linac) for ten patients were assessed for delineation of celiac ganglia, aiming for smaller target volumes and reducing treatment risks versus standard linac-based treatments. MRI-linacs showed superior soft tissue contrast, enabling increased dose to ganglia while irradiating smaller target volumes versus regular linacs (median: 0.8 cm3 vs. 32.2 cm3, p < 0.05 for ten pairs of plans). While further studies are needed, MR-linac treatments could improve targeting precision, minimize dose to organs-at-risk and enhance effectiveness in palliative care.

Real-time 3D MR guided radiation therapy through orthogonal MR imaging and manifold learning

BACKGROUND

In magnetic resonance image (MRI)-guided radiotherapy (MRgRT), 2D rapid imaging is commonly used to track moving targets with high temporal frequency to minimize gating latency. However, anatomical motion is not constrained to 2D, and a portion of the target may be missed during treatment if 3D motion is not evaluated. While some MRgRT systems attempt to capture 3D motion by sequentially tracking motion in 2D orthogonal imaging planes, this approach assesses 3D motion via independent 2D measurements at alternating instances, lacking a simultaneous 3D motion assessment in both imaging planes.

PURPOSE

We hypothesized that a motion model could be derived from prior 2D orthogonal imaging to estimate 3D motion in both planes simultaneously. We present a manifold learning technique to estimate 3D motion from 2D orthogonal imaging.

METHODS

Five healthy volunteers were scanned under an IRB-approved protocol using a 3.0 T Siemens Skyra simulator. Images of the liver dome were acquired during free breathing (FB) with a 2.6 mm × 2.6 mm in-plane resolution for approximately 10 min in alternating sagittal and coronal planes at ∼5 frames per second. The motion model was derived using a combined manifold learning and alignment approach based on locally linear embedding (LLE). The model utilized the spatially overlapping MRI signal shared by both imaging planes to group together images that had similar signals, enabling motion estimation in both planes simultaneously. The model's motion estimates were compared to the ground truth motion derived in each newly acquired image using deformable registration. A simulated target was defined on the dome of the liver and used to evaluate model performance. The Dice similarity coefficient and distance between the model-tracked and image-tracked contour centroids were evaluated. Motion modeling error was estimated in the orthogonal plane by back-propagating the motion to the currently imaged plane and by interpolating the motion between image acquisitions where ground truth motion was available.

RESULTS

The motion observed in the healthy volunteer studies ranged from 12.6 to 38.7 mm. On average, the model demonstrated sub-millimeter precision and > 0.95 Dice coefficient compared to the ground truth motion observed in the currently imaged plane. The average Dice coefficient and centroid distance between the model-tracked and ground truth target contours were 0.96 ± 0.03 and 0.26 mm ± 0.27 mm respectively across all volunteer studies. The out-of-plane centroid motion error was estimated to be 0.85 mm ± 1.07 mm and 1.26 mm ± 1.38 mm using the back-propagation (BP) and interpolation error estimation methods.

CONCLUSIONS

The healthy volunteer studies indicate promising results using the proposed motion modeling technique. Out-of-plane modeling error was estimated to be higher but still demonstrated sub-voxel motion accuracy.

Surrogate gating strategies for the Elekta Unity MR-Linac gating system

PURPOSE

MRI-guided adaptive radiotherapy can directly monitor the anatomical positioning of the intended target during treatment with no additional imaging dose. Elekta has recently released its comprehensive motion management (CMM) solution that enables automatic radiation beam-gating on the Unity MR-Linac. Easily visualized targets that are distinct from the surrounding anatomy can be used to drive automatic gating decisions from the MRI cine imaging. However, poorly visualized targets can compromise the tracking and gating capabilities and may require surrogate tracking structures. This work presents strategies to generate robust tracking surrogates for a variety of treatment sites, enabling a wider application of CMM.

METHODS

Surrogate tracking strategies were developed from a cohort of patients treated using the CMM system on the Unity MR-Linac for treatment sites of the lung, pancreas, liver, and prostate. These sites posed challenging visualization or tracking of the primary target thereby compromising the tracking accuracy. Surrogate structures were developed using site-specific strategies to improve the imaging textured detail within the tracking volume while avoiding the dynamic overwhelming hypo- or hyper-intense anatomical structures. These surrogate volumes were applied within the anatomical positioning monitoring system as a proxy that drove the CMM gating decisions on the treatment unit.

RESULTS

Robust site-specific surrogate structures were developed. Surrogate tracking structures for centrally located thoracic targets were created by expanding the target peripherally away from the heart and great vessels and into the lung. Pancreas surrogates required a vertically expanded column intersecting with the inferior liver edge. For the liver and prostate, surrogate structures consisted of a uniform expansion of the target, with liver surrogates intersecting the proximal liver edge or diaphragm while avoiding nearby ribs.

CONCLUSION

These surrogate strategies have enabled the gating of complex moving targets among different treatment sites at our institution.

First dosimetric evaluation of clinical raster-scanned proton, helium and carbon ion treatment plan delivery during simultaneous real-time magnetic resonance imaging

This work presents an experimental dosimetric evaluation of raster-scanning particle beam delivery during simultaneous in-beam magnetic resonance (MR) imaging. Using an open MR scanner at an experimental treatment room, radiochromic film comparisons for protons, helium and carbon ions, each with and without simultaneous in-beam cine MR imaging, yielded 2D gamma pass rates ≥ 98.8 % for a 3 % / 1.5 mm criterion, and ≥ 99.9 % for 5 % / 1.5 mm. These results constitute a first experimental confirmation that time varying magnetic fields of MR gradients do not result in clinically relevant additional dose perturbations.

Dose-volume parameter evaluation of a sub-fractionation workflow for adaptive radiotherapy of prostate cancer patients on a 1.5 T magnetic resonance imaging radiotherapy system

Background and purpose

This study focuses on evaluating a sub-fractionation workflow for intrafraction motion mitigation of prostate cancer patients on a 1.5 T magnetic resonance imaging radiotherapy system.

Materials and methods

The investigated workflow consisted of two sub-fractions where intrafraction drift correction steps were applied based on a daily reference plan. However, the daily contours were only rigidly shifted to match the intrafraction anatomies and therefore the clinical dosimetric constraints might be violated. In this work, daily contours were deformed to match the intrafraction anatomies and the online plans were re-calculated for a total of 15 patients. The deformed prostate contours were inspected by radiation oncologists and corrections were performed when necessary. Finally, a dose-volume parameter evaluation was performed on a sub-fraction level using the clinical plan parameters.

Results

Clinically acceptable coverage was reported for the target structures resulting in mean V95% of 99.7 % and 97.8 % for the clinical target volume (CTV) and planning target volume (PTV) respectively. Sub-fractions with insufficient CTV dose can be explained by the presence of intrafraction rotations and deformations that were not taken into account during intrafraction corrections. Additionally, for no sub-fraction the dose to the organs-at-risk exceeded the clinical constraints.

Conclusion

Given our results on the CTV coverage we can conclude that the sub-fractionation workflow met the dosimetric constraints for the hypofractionated treatment of the analyzed group of prostate cancer patients. A future dose accumulation study can provide further insights into the suitability of the clinical margins.

Effects of precision health management combined with dual-energy bone densitometer treatment on bone biomarkers in senile osteoporosis patients

OBJECTIVE

This study investigates the effects of precision health management combined with dual-energy X-ray absorptiometry (DXA) therapy on bone biomarkers in elderly osteoporotic patients.

METHODS

236 elderly patients diagnosed with osteoporosis between May 2020 and November 2021 were enrolled from our hospital. Patients were randomly allocated to either the observation group (n = 118), receiving precision health management alongside DXA therapy, or the control group (n = 118), receiving standard treatment. Clinical data were compared between the two groups. Protein levels of bone formation markers (BSAP, OC) and bone resorption markers (CTX, DPD, TRAP) were analyzed using Western blotting. Bone mineral density (BMD) was measured using DXA at baseline, 12 months, and 24 months. Pain levels were assessed using the Visual Analog Scale (VAS) at the same intervals. Osteoporosis knowledge and self-management confidence were evaluated using respective scales before and after intervention.

RESULTS

Baseline characteristics did not significantly differ between groups (P > 0.05). The observation group exhibited decreased BSAP and increased OC and OC protein expressions compared to the control group (P < 0.05). CTX, DPD, and TRAP protein levels were significantly lower in the observation group (P < 0.05). Prior to the intervention, there were no significant variations observed in BMD, as well as VAS, knowledge, and self-efficacy scores between the two groups (P > 0.05). However, over the course of 12 and 24 months, the observation group exhibited significant increases in BMD (P < 0.05). VAS scores were notably lower in the observation group during both follow-up assessments (P < 0.05). Furthermore, knowledge scores were higher in the observation group at 12 and 24 months (P < 0.05), while self-efficacy scores showed significant improvement in the observation group at both follow-up intervals (P < 0.05).

CONCLUSION

Precision health management combined with DXA therapy positively impacts elderly osteoporotic patients by enhancing bone biomarkers, promoting bone growth, and preventing bone loss. This approach leads to increased BMD, reduced fracture risk, improved pain management, and enhanced knowledge and self-management abilities related to osteoporosis.

Comparison of online adaptive and non-adaptive magnetic resonance image-guided radiation therapy in prostate cancer using dose accumulation

Background and purpose

Conventional image-guided radiotherapy (conv-IGRT) is standard in prostate cancer (PC) but does not account for inter-fraction anatomical changes. Online-adaptive magnetic resonance-guided RT (OA-MRgRT) may improve organ-at-risk (OARs) sparing and clinical target volume (CTV) coverage. The aim of this study was to analyze accumulated OAR and target doses in PC after OA-MRgRT and conv-IGRT in comparison to pre-treatment reference planning (refPlan).

Material and methods

Ten patients with PC, previously treated with OA-MRgRT at the 1.5 T MR-Linac (20x3Gy), were included. Accumulated OA-MRgRT doses were determined by deformably registering all fraction's MR-images. Conv-IGRT was simulated through rigid registration of the planning computed tomography with each fraction's MR-image for dose mapping/accumulation. Dose-volume parameters (DVPs), including CTV D50% and D98%, rectum, bladder, urethra, Dmax and V56Gy for OA-MRgRT, conv-IGRT and refPlan were compared using the Wilcoxon signed-rank test. Clinical relevance of accumulated dose differences was analyzed using a normal-tissue complication-probability model.

Results

CTV-DVPs were comparable, whereas OA-MRgRT yielded decreased median OAR-DVPs compared to conv-IGRT, except for bladder V56Gy. OA-MRgRT demonstrated significantly lower median rectum Dmax over conv-IGRT (59.1/59.9 Gy, p = 0.006) and refPlan (60.1 Gy, p = 0.012). Similarly, OA-MRgRT yielded reduced median bladder Dmax compared to conv-IGRT (60.0/60.4 Gy, p = 0.006), and refPlan (61.2 Gy, p = 0.002). Overall, accumulated dose differences were small and did not translate into clinically relevant effects.

Conclusion

Deformably accumulated OA-MRgRT using 20x3Gy in PC showed significant but small dosimetric differences comparted to conv-IGRT. Feasibility of a dose accumulation methodology was demonstrated, which may be relevant for evaluating future hypo-fractionated OA-MRgRT approaches.

MR signature matching (MRSIGMA) implementation for true real-time free-breathing volumetric imaging with sub-200 ms latency on an MR-Linac

PURPOSE

Develop a true real-time implementation of MR signature matching (MRSIGMA) for free-breathing 3D MRI with sub-200 ms latency on the Elekta Unity 1.5T MR-Linac.

METHODS

MRSIGMA was implemented on an external computer with a network connection to the MR-Linac. Stack-of-stars with partial kz sampling was used to accelerate data acquisition and ReconSocket was employed for simultaneous data transmission. Movienet network computed the 4D MRI motion dictionary and correlation analysis was used for signature matching. A programmable 4D MRI phantom was utilized to evaluate MRSIGMA with respect to a ground-truth translational motion reference. In vivo validation was performed on patients with pancreatic cancer, where 15 patients were employed to train Movienet and 7 patients to test the real-time implementation of MRSIGMA. Dice coefficients between real-time MRSIGMA and a retrospectively computed 4D reference were used to evaluate motion tracking performance.

RESULTS

Motion dictionary was computed in under 5 s. Signature acquisition and matching presented 173 ms latency on the phantom and 193 ms on patients. MRSIGMA presented a mean error of 1.3-1.6 mm for all phantom experiments, which was below the 2 mm acquisition resolution along the motion direction. The Dice coefficient over time between MRSIGMA and reference contours was 0.88 ± 0.02 (GTV), 0.87 ± 0.02(duodenum-stomach), and 0.78 ± 0.02(small bowel), demonstrating high motion tracking performance for both tumor and organs at risk.

CONCLUSION

The real-time implementation of MRSIGMA enabled true real-time free-breathing 3D MRI with sub-200 ms imaging latency on a clinical MR-Linac system, which can be used for treatment monitoring, adaptive radiotherapy and dose accumulation mapping in tumors affected by respiratory motion.