Adaptive on Demand: Leveraging Cone Beam Computed Tomography-Based Adaptive Online Radiotherapy System as Simulation-Free Replan Platform

Abstract

<h3>Purpose/Objective(s)</h3> Artificial intelligence powered cone beam computed tomography (CBCT) based online adaptive radiotherapy (ART) system offers a streamlined and efficient process for daily ART as the default. However, patients with gradual anatomy change from tumor response or weight loss may not significantly benefit from the daily ART. Instead, it will be more practical to adapt when changes are observed. In this work, we present a workflow and our early experience to utilize CBCT based ART system as a quick replan platform and the adapted plan is used for image guided radiotherapy (IGRT) until next adaptation. <h3>Materials/Methods</h3> Adaptive on demand is designed for patients treated with conventional fractionated dose and planned with conventional margins. The preplan is designed with 12-18 static IMRT fields with consideration of potential geometry/density change in beam arrangement and optimization to ensure the robustness of ART plan quality. Robustness evaluation is performed as needed. A plan revision process is required in the ART treatment planning system (TPS) to switch between adaptive and non-adaptive fractions in a treatment course. The plan is converted to the adaptive treatment upon physician's request. After each ART, the adapted plan is imported to TPS and physicist checks the quality of the synthetic CT (SCT), which is a deformed planning CT to CBCT geometry, and determines SCT/CBCT to be used as reference for the upcoming IGRT fractions. <h3>Results</h3> A total of 23 HN patients and 13 lung patients were treated with ART on demand from 09/2021-01/2022. 110 on-demand ART fractions analyzed. On average, each HN/lung patient received 1.9 (range 1-6)/5.2 (range 4-6) ART and 90.6%/85.3% frequency that the adapted plan was selected for treatment. Overall, the adapted plans show statistically significant improvements in tumor coverage and dose conformity comparing to original plan and meet the OAR objectives. Detailed dosimetric benefits are presented in other abstracts. Given the SCT is generated via deformable registration, the distortion of the SCT was observed in some cases. While dosimetric impact are negligible for the majority of cases, the geometric distortions of bony anatomy can lead to setup difficulty for the IGRT fractions and two patients were rescanned due to this reason. Cutting out the mask is recommended when it becomes loose to avoid SCT picking up mask as body and results in unsatisfactory planning target volume cropping from skin. The conversion to adaptive fraction requires 15 minutes while setting up adapted plan for IGRT fraction takes 25 minutes. Physician's time at console time is 18/14 minutes for HN and lung. <h3>Conclusion</h3> Leveraging CBCT based ART system as replan platform is feasible and satisfactory in plan quality. Physicist check on the SCT quality is crucial to prevent potential target generation uncertainties and setup error. This process significantly improved the turnaround time for replan and saves a trip for rescan.