A novel optical respiratory gating system with a hybrid phase‐amplitude algorithm for spot‐scanning proton therapy

BACKGROUND: Respiratory motion remains a major challenge in thoracic and abdominal proton treatments, causing interplay effects and dose distortions. In synchrotron-based systems, conventional gating significantly prolongs beam delivery and overall treatment time, which limits its clinical applicability and patient suitability. PURPOSE: The present study was aimed to investigation of the gating system with the novel hybrid phase-amplitude algorithm specified for synchrotron-based proton therapy systems with spot-scanning beam delivery in the phantom test. This system was experimentally evaluated for robustness to target motion irregularities in terms of dose distortion and irradiation time. METHODS: A dynamic phantom simulated the free-breathing motion using a radiochromic film immersed in water. The film was irradiated in a single fraction with gating for regular and irregular motion patterns. The phase-amplitude gating algorithm was implemented to provide two-signal control of the synchrotron for beam injection and extraction. The measured dose distributions were analyzed using gamma index calculation with a 5%/3 mm criterion. RESULTS: Gating mitigated the interplay effect, resulting in a gamma passing rate (GPR) of 98.5 ± 0.1% (mean ± SD) at regular motion, but the irradiation time was increased from 155 ± 2 s (mean ± SD) to 209 ± 9 s compared to no gating. The GPR fluctuated unsystematically as a function of motion irregularity (p = 0.111), resulting in a median GPR of 96.0% (range 89.2%-98.6%), and the irradiation time increased from 209 ± 9 s to 238 ± 8 s compared to regular motion (p = 0.002). Robustness to random motion irregularities up to 30% resulted in 67% acceptable dose distributions and 14% increase in irradiation time. CONCLUSIONS: The algorithm ensured full synchronization of beam injection and extraction with the respiratory cycle, which allowed a significant reduction in treatment time. The phase-amplitude gating together with fractionation and rescanning can be potentially effective in the treatment of thoracic and abdominal tumors using synchrotron-based systems with scanning beams. The introduction of a system-specific quantitative threshold for patient respiratory irregularity is mandatory for deciding to use gated treatment or not.