Online Adaptive MR-Guided Radiotherapy (MRgRT) in UltraCentral (UC) Lung Lesions: Cumulative Delivered Dose as Assessed with Rigid Fusion (RF) Analysis Shows Significant Improvement in Clinically Relevant Parameters

Abstract

<h3>Purpose/Objective(s)</h3> MRgRT is used to treat UC lung lesions. Adaption is triggered if the original plan does not meet criteria of target coverage or organs at risk (OAR) constraints on recontoured anatomy of the day. Current MRgRT software does not provide composite dose delivered over the complete course, partly because daily anatomy changes make accurate spatial dose accumulation difficult. Previous approaches used DVH point averages to approximate doses from full course. We aimed to more accurately quantify true delivered dose via rigid fusion (RF) of fractional doses and hypothesize that treatment with adaptive MRgRT has superior cumulative doses than non-adaptive treatment. <h3>Materials/Methods</h3> We conducted a single institution analysis of patients receiving 60 Gy in 8 fractions with adaptive MRgRT to UC lesions (per HILUS) between 2/2020 – 1/2022. We created composite doses from ‘delivered' treatment plans and ‘predicted' treatment plans. For the ‘delivered' plans, dose for clinically treated plan (adapted or non-adapted based on daily plan quality) was exported for each fraction with MR contoured anatomy of the day. Using a third-party image registration software, we performed RF for each adaptive fraction focused on aligning two regions: (1) tumor and (2) proximal airway, to separately determine local cumulative dose to those sites. For fractions that were not adapted, the initial plan was fused to daily MR. For the ‘predicted' treatment plan, initial plan dose was RF to primary tumor location for each MR contoured anatomy of the day and then summed for composite dose. <h3>Results</h3> Eleven consecutive patients with UC lesions were treated with online adaptive MRgRT. Eight (73%) patients had primary lung tumors. Median distance from proximal airway: 0 mm (0-6). Median number of adapted fractions was 7 (6-8). Differences between cumulative delivered and predicted plan dose on daily anatomy GTV D100%, PTV D95%, and Proximal Airway D0.03cc (PA0.03cc) were all statistically significant (p<0.01). The delivered dose had improvement in GTV D100% versus the initial plan (p<0.01), PTV D95% and PA0.03cc were not significant. Median follow up was 7.4 months. No grade 3+ acute or late toxicity was reported. <h3>Conclusion</h3> Online adaptive MRgRT resulted in composite delivered doses from treatment on par with initial plan dosimetric quality and significantly outperforms doses delivered with non-adapted treatment. The adapted plan coverage superiority over the non-adapted plan is explained by the ‘feathering' of dose that creates a 60 Gy isodose line while respecting OAR, which is underappreciated with just simple summing of changes via adaptive plan DVH values. This is the first ever plan evaluation study of its kind for MRgRT and aids in establishing dosimetric superiority of daily online adaptation.