2114 - A Comparison of Single-Phase and Phase-Gated Average Verification Plans for Proton Radiotherapy

Purpose/Objective(s): To address challenges and enhance the efficiency of online adaptive radiation therapy, advancements in automation are pivotal. The creation of routine weekly verification plans for phase-gated treatments, which is a standard motion management technique for proton therapy in our clinic, requires the creation of a new phase-gated average image set. This is time-consuming and incompatible with current workflows utilizing automated software. The purpose of this study was to compare target coverage between a single, full exhale phase verification plan and a phase-gated average verification plan to determine whether a single-phase verification is an acceptable surrogate.

Materials/Methods: A group of 18 patients receiving intensity-modulated proton therapy was identified, including esophagus, lung, and liver disease sites. Each patient was planned on a phase subset average image set derived from a 4DCT. The specific phases averaged into these image sets corresponded to the phases upon which the phase-gated plan was delivered. One of each patient’s routine weekly 4DCT verification image sets, which are evaluated to ensure continuing quality during the course or treatment, was selected for analysis. A second verification plan was created using only the 50% 4DCT phase image (generally full exhale) for direct comparison against the phase subset average image set. Target coverage was compared by analyzing CTV V95% metrics assessed on both rigidly and deformably transferred CTV’s as well as physician modified CTV’s.

Results: The target coverage comparison results were analyzed as an aggregate group, inside individual disease site groups, and were also parsed by whether the verification triggered the need for a replan. The mean difference in V95% between the phase subset average and single-phase verification plans across all patients was < 1%. The highest average deviation and the most prominent outliers were observed for the lung patients, with an average V95% difference of less than 2% on updated physician contours and one outlier approaching 5%. This is attributed to higher degrees of anatomical heterogeneity and smaller target volumes.

Conclusion: The results suggest that single-phase verification plans offer a clinically viable and efficient alternative to phase-gated plans in many clinical scenarios, with potentially some contraindications in lung and liver proton treatment plans which require further investigation.