Gantry‑Dependent Transient Output Variation In a Compact Synchrocyclotron Proton Therapy System
Abstract
Purpose
To evaluate transient output variation immediately following gantry rotation in a compact synchrocyclotron‑based proton therapy system, with focus on quantifying magnitude of output deviation between initial post‑rotation measurement and subsequent stabilized measurements and assessing how this deviation depends on gantry angle.
Methods
Nine gantry angles from 0° to 180° in 22.5° increments were evaluated over a 6‑month period on a Mevion S250i synchrocyclotron system. At each angle, four consecutive output measurements were acquired using a consistent setup. First measurement was designated as first‑shot value to represent beam output immediately after gantry rotation. The subsequent three measurements were averaged to represent stabilized output at that angle. For each angle, difference between first‑shot value and stabilized average was calculated, along with corresponding upper and lower extrema, mean, and standard deviation of transient behavior.
Results
The three stabilized measurements at each gantry angle showed reasonable repeatability, indicating stable output once gantry settled. In contrast, first‑shot measurement differed from stabilized average by up to 1.96%, observed at G90. The magnitude of this first‑shot deviation was angle‑dependent where over 6‑month evaluation, 7/9 angles exhibited first‑shot differences that exceeded 1%, while all stayed within 2%. The largest mean first‑shot biases over 6‑month period were observed at G0 and G180 with -0.8% and -0.65 respectively, and standard deviation across angles was approximately 0.5%.
Conclusion
This work quantifies an angle‑dependent transient output effect occurring immediately after gantry rotation in a compact synchrocyclotron‑based proton therapy system. While stabilized output demonstrated reasonable stability, first‑shot value consistently differed from stabilized average, with magnitude varying by gantry angle and remaining within a 2% tolerance. These findings establish a quantitative foundation for understanding transient output variation in this system and may support future investigations into clinical and engineering implications, although such interpretation was beyond scope of this study.