TH‐C‐M100F‐07: The Feasibility of a Hybrid Active Breath‐Hold/Gating Technique for Lung Radiotherapy

Purpose: Deep inspiration active breath‐hold enables a reproducible tumor position but a long breath‐hold is not practical for many patients with lung disease. Respiratory gating generally requires a surrogate (tracking blocks, strain gauges, etc.) that may not always correspond with actual tumor motion. In this study, we evaluated the feasibility of a hybrid system that used a short shallow inspiration active breath‐hold to gate a linear accelerator. Methods and Materials: Breath‐hold volume deviation and tolerance for the hybrid breath‐hold/gating technique were evaluated in four subjects. The subjects underwent breathing exams on an active breath‐hold device, breathing at normal and fast inhalation flow rates and two different breath‐hold volumes (0.6 L and 1.5 L). Latency in the active breath hold system was also measured with a simulated flow signal and a pressure switch. Gated beam stability (Elekta Synergy) was evaluated with an ion chamber and commercial diode arrays. Results: The short breath‐hold paradigm was tolerated well by all subjects. Active breath hold latency was measured to be 157.0 ± 7.8 milliseconds. The deviation between measured and intended breath‐hold volume was dependent on the inhalation flow rate of the patient (r² = 0.9363). The percentage dose difference between nongated and gated delivery was less than 0.5% for breath‐holds longer then 3 seconds. Flatness and symmetry measured on SunNuclear's Profiler was relatively unaffected by gated delivery. Only 4% and 2% of diodes failed strict evaluation parameters in Mapcheck for 6 MV and 18 MV IMRT fields respectively. Conclusions: The results of this study show that using a short active breath‐hold for gating a linear accelerator is feasible. Acknowledgement: Supported in part by NIH R01 CA116249.