Development and Validation of a Magnetic Resonance Imaging-Guided Adaptive Radiotherapy Workflow for Long, Continuous Planning Target Volumes

Introduction Owing to the limitation in the field size of the magnetic resonance (MR)-Linac, currently, tumors with a length of >20 cm cannot be treated. Thus, the present study aimed to develop an expanded magnetic resonance imaging-guided adaptive radiotherapy (MRIgART) workflow for long, continuous planning target volumes (PTVs). Methods The PTVs were divided into two sub_target volumes (PTV_sub1 and PTV_sub2). We established two isocenters and defined a field overlap region. By adjusting the MR scan range, devising the online and offline adaptive procedures, synchronizing the online adaptive processes, and constructing a pretreatment dose evaluation, a new MRIgART workflow for long PTVs was established. The new workflow was validated using an in-house-made MR phantom. Additionally, the ArcherQA Monte Carlo-based method, ArcCHECK phantom, and ionization chamber measurement method were used for dose verification. Results Two clinical scenarios were established: (1) both PTV_sub1 and PTV_sub2 followed the adapt-to-position (ATP) workflow, and (2) PTV_sub1 followed the adapt-to-shape (ATS) workflow, whereas PTV_sub2 followed the ATP workflow. The feasibility of the proposed MRIgART workflow for long, continuous PTVs was demonstrated through three independent rounds of testing and validation for each scenario. When field overlaps were utilized, the PTV length that can be treated is 40 cm minus the length of field overlap region. The average gamma pass rates for the PTV_sub1 and PTV_sub2 adaptive plans were 95.74% and 98.63%, respectively (ArcherQA vs TPS). For the field overlap region, the average gamma pass rate was 95.50% (ArcCHECK vs TPS). The difference between the ionization chamber measurements and calculated results was smaller than 2%. Conclusion This study demonstrated the feasibility, safety, and accuracy of the MRIgART workflow for long PTVs. This workflow provides an effective solution for expanding the application of MRIgART to patients with long, continuous PTVs.