KEY POINTS
- This proof-of-concept phantom study evaluated an optical respiratory gating system for synchrotron-based spot-scanning proton therapy. The hybrid algorithm synchronized both proton injection and beam extraction with the respiratory cycle.
- A dynamic water phantom reproduced 12 mm peak-to-peak motion with regular breathing and random amplitude and period irregularities of 5% to 30%. A 2 Gy, single-field plan was delivered to a moving radiochromic film.
- Without gating, motion reduced the gamma passing rate at 5%/3 mm to 74.7% ± 3.6%. With the selected hybrid gating window during regular motion, the passing rate increased to 98.5% ± 0.1%, while irradiation time increased from 155 ± 2 seconds to 209 ± 9 seconds.
- Wider gating windows shortened irradiation time but increased residual motion and dose distortion. All tested distributions met the predefined gamma thresholds when average residual motion remained at or below approximately 1.3 mm.
- With irregular motion, the median 5%/3 mm gamma passing rate was 96.0% but ranged from 89.2% to 98.6%; 8 of 12 distributions met the acceptance thresholds. At 30% irregularity, irradiation time increased to 238 ± 8 seconds (p=0.002 versus regular motion).
CLINICAL TAKEAWAY
Synchronizing both beam injection and extraction with respiration may make gating more time-efficient for synchrotron-based spot-scanning proton therapy. However, this was a homogeneous, one-dimensional phantom experiment using a single field and fraction, simplified breathing patterns, and relatively permissive gamma criteria, so the findings are technically relevant but not ready for clinical implementation.