SU‐E‐J‐161: Dynamic Gating Window for Compensation of Baseline Shift in Respiratory‐Gated Radiation Therapy

Purpose: To analyze and evaluate the necessity and use of dynamic gating techniques for compensation of baseline shift during respiratory‐gated radiation therapy of lung tumors. Methods: Motion tracking data from 30 lung tumors over 592 treatment fractions were analyzed for baseline shift. The finite state model (FSM) was used to identify the end‐of‐exhale (EOE) breathing phase throughout each treatment fraction. Using duty cycle as an evaluation metric, several methods of end‐of‐exhale dynamic gating were compared: An a posteriori ideal gating window, a predictive trend‐line‐ based gating window, and a predictive weighted point‐based gating window. These methods were evaluated for each of several gating window types: Superior/inferior (SI) gating, anterior/posterior beam, lateral beam, and 3D gating. Results: In the absence of dynamic gating techniques, SI gating gave a 39.6% duty cycle. The ideal SI gating window yielded a 41.5% duty cycle. The weight‐based method of dynamic SI gating yielded a duty cycle of 36.2%. The trend‐line‐based method yielded a duty cycle of 34.0%. Conclusions: Dynamic gating was not broadly beneficial due to a breakdown of the FSM's ability to identify the EOE phase. When the EOE phase was well defined, dynamic gating showed an improvement over static‐window gating.