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) avoids frequent gating interruptions, which improves proton beam extraction stability and treatment efficiency in synchrotron-based systems. However, because fluoroscopic x-rays remain on during proton irradiation in UCD, scattered fluoroscopic x-rays may be detected by the dose monitor (DM) and mistakenly counted as proton monitor units (MUs), 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 using plastic water® (PW) and anthropomorphic phantoms while varying phantom thickness, fluoroscopic parameters, and range-shifter conditions. Beam-abort simulations were conducted under UCD conditions for both phantom and clinical liver treatment plans. Abort probability was assessed as a function of proton MU per spot, scattered x-ray MU per frame, and fluoroscopy frames per second.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 (RS). The maximum dose rate was 0.0211 MU/frame at 125 kV, 80 mA, 200 × 200 mm2, and PW-30 cm without a RS, representing an 11.7-fold increase compared to the in-air conditions. The results obtained from the anthropomorphic phantom experiments further validated the findings observed in the PW phantom study. 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.