Ability of online adaptive replanning is desirable to correct for interfraction anatomic changes. A full-scope replanning/reoptimization with the current planning techniques takes too long to be practical. A novel online replanning strategy to correct for interfraction anatomic changes in real time is presented. The scheme consists of three steps: (1) rapidly delineating targets and organs at risk on the computed tomography of the day by modifying original planning contours using robust tools in a semiautomatic manner, (2) online segment aperture morphing (SAM) (adjusting beam/ segment apertures) by applying the spatial relationship between the planning target contour and the apertures to the new target contour, and (3) performing segment weight optimization (SWO) for the new apertures if necessary. The entire scheme was tested for direct-aperture-based IMRT on representative prostate and abdomen cases. Dose volume histograms obtained with the online scheme are practically equivalent to those obtained with full-scope reoptimization. For the days of small to moderate organ deformations, only the SAM is necessary, while for the large deformation days, both SAM and SWO are required to adequately account for the deformation. Both the SAM and SWO programs can be completed within 1 min, and the overall process can be completed within 10 min. The proposed SAM-SWO scheme is practically comparable to full-scope reoptimization, but is fast enough to be implemented for on-line adaptive replanning, enabling dose-guided RT.
Background and purpose: In this report, we describe our implementation and initial clinical experience using 4D-MRI driven MR-guided online adaptive radiotherapy (MRgOART) for abdominal stereotactic body radiotherapy (SBRT) on the Elekta Unity MR-Linac. Materials and methods: Eleven patients with abdominal malignancies were treated with free-breathing SBRT in three to five fractions on a 1.5 T MR-Linac. Online adaptive plans were generated using Adapt-To-Position (ATP) or Adapt-To-Shape (ATS) workflows based on motion averaged or mid-position images derived from a pre-beam 4D-MRI. A high performance server positioned on the local MR-Linac machine network was utilized for 4D-MR image reconstruction. A parallel contour editing approach was employed in the ATS workflow. Intravoxel incoherent motion (IVIM) and T2 mapping sequences were acquired during adaptive planning in both ATP and ATS workflows for treatment response monitoring. Adaptive plans were delivered under real-time cine image motion monitoring. Results: The shortest 4D-MRI time-to-image was the motion averaged image, followed by mid position and respiratory binned images. In this cohert of patients, 50% of treatments utilized the ATS workflow; the remaining treatments utilized the ATP workflow. Mid-position images were utilized as daily planning images for two of the eleven patients. The mean daily adaptive plan secondary dose calculation and ArcCheck 3D Gamma passing rates were 97.5% (92.1-100.0%) and 99.3% (96.2-100.0%), respectively. The median overall treatment times for abdominal SBRT was 46 and 62 min for ATP and ATS workflows, respectively. Conclusion: We have successfully implemented and utilized a 4D-MRI driven MRgOART process with ATP and ATS workflows for free-breathing abdominal SBRT on a 1.5 T Elekta Unity MR-Linac.
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