Improving Target Coverage In Abdominal Adaptive Radiotherapy Using Distance-Based OAR Cropping In Ethos 2.0
Abstract
Purpose
This study evaluates whether selective distance-based cropping of organs at risk (OARs) can improve target coverage and plan robustness for abdominal stereotactic adaptive radiation therapy using the Ethos 2.0 planning system. By reducing OAR volume and thus computational burden exerted by Ethos’ intrinsic automatic optimization constraints beyond user-defined optimization constraints, this approach aims to enable higher target coverage while maintaining clinically acceptable normal tissue sparing.
Methods
Eleven patients with abdominal malignancies treated on Ethos 2.0 were included. Twenty clinical adaptive fractions requiring plan normalization due to OAR overdosing, resulting in target coverage reduction, were selected for offline replanning. The small bowel, large bowel, duodenum, and stomach were cropped within a 3 cm axial and 1.2 cm longitudinal expansion from the target volume. Each patient’s original clinical planning template was reapplied, with the only modification being the application of the cropped OAR objectives rather than full organ volumes to reduce computation burden in the Ethos optimization algorithm. Target coverage metrics, normalization requirements, and monitor units were compared between clinical adaptive plans and cropped offline replans. Paired t-tests were performed to assess statistical significance.
Results
Distance-based OAR cropping significantly improved target coverage, with mean PTV Eval D98% increasing from 88.9% to 93.4%Rx (mean increase = 4.5%, p < 0.001). Normalization factors increased significantly with OARs cropping (0.998 ± 0.008 vs 0.953 ± 0.031, p<0.001) with substantial fewer replans requiring normalization. Analysis of MU numbers per plan showed no significant difference between cropped and original plans (4378 vs 4375, p = 0.97), indicating no systematic increase in MU burden.
Conclusion
Selective distance-based OAR cropping in Ethos 2.0 improves target coverage and plan robustness without increasing doses to OARs or overall modulation, supporting its use as an effective optimization strategy to enable higher quality treatment plans in abdominal adaptive radiation therapy.