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.
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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.