Impact of x-ray contamination in the dose monitor on beam abortion during uninterrupted continuous delivery in proton therapy

OBJECTIVE: Real-time Gated Proton Therapy (RGPT) enables precise irradiation of moving targets and is an important motion-management technique in proton therapy. Uninterrupted continuous delivery (UCD), in which fluoroscopic X-rays remain on during proton irradiation, may improve beam stability and treatment efficiency, particularly in synchrotronbased systems using continuous beams. However, scattered fluoroscopic X-rays may be detected by the dose monitor (DM) and mistakenly counted as proton monitor units, potentially causing beam aborts. This study aimed to quantify scattered fluoroscopic X-rays detected by the DM during RGPT and evaluate their impact on beam delivery during simultaneous fluoroscopy and proton irradiation. METHODS: Scattered fluoroscopic X-ray signals detected by the DM were measured using an RGPT system. Phantom experiments evaluated scattered X-ray dose rates while varying phantom thickness, fluoroscopic parameters, and range-shifter conditions. Beam-abort simulations were conducted under UCD conditions for both phantom and clinical liver cases.Abort probability was assessed as a function of proton MU per spot, scattered X-ray MU per frame, and fluoroscopy frame rate (FPS). RESULTS: DM signals from scattered fluoroscopic X-rays increased with tube voltage, tube current, field size, and phantom thickness, and were higher without a range shifter. The maximum dose rate was 0.0211 MU/frame at 125 kV, 80 mAs, 200 × 200 mm², and PW-30 cm without a range shifter, representing an 11.7-fold increase compared with in-air conditions. Beam-abort simulations showed abort proportions below 2% for monoenergetic fields and below 3% for clinical liver plans across all tested conditions. SIGNIFICANCE: With the DM positioned closer to the X-ray tube, scattered fluoroscopic X-ray contamination increased with phantom thickness, differing from prior reports. Optimization of proton dose rate, imaging parameters, and system geometry can minimize beam interruptions, supporting the clinical feasibility of UCD.