Dosimetric impact and safety of uninterrupted fluoroscopic-gated proton therapy

Fluoroscopic-gated proton therapy (FGPT) enables precise dose delivery to tumors affected by respiratory motion by tracking internal fiducial markers and delivering the proton beam when the marker is within a gating window. However, scatter radiation from fluoroscopic x-rays may be detected by the dose monitor (DM) and mistakenly counted as proton monitor units (MU). To mitigate this issue, proton beam delivery is typically interrupted during the fluoroscopy pulse, a method known as interrupted continuous delivery (ICD). If the contamination from scattered fluoroscopic x-rays is sufficiently low compared to the proton beam current, uninterrupted continuous delivery (UCD), in which fluoroscopic x-rays are delivered concurrently with proton beams, may be feasible. UCD can enhance beam stability and improve treatment efficiency in particular for synchrotron-based proton therapy systems using continuous beams.

This study aimed to measure the contamination of scattered fluoroscopic x-rays in DM and to evaluate the dose distribution when fluoroscopic x-rays and proton beams are delivered simultaneously in FGPT.

x-ray contamination was measured using a proton therapy system equipped with the FGPT system. Fluoroscopic x-rays were delivered for 30 s under both in-air and solid phantom conditions, with thicknesses of 10 and 30 cm, and DM counts were recorded. The dose rate was evaluated under various fluoroscopic conditions, both with and without the range shifter and mini ridge filter. To assess the dosimetric impact, six virtual treatment plans were created using anthropomorphic phantoms for cases involving the prostate, lung (both superficial and deep), liver (both superficial and deep), and pancreas. Scatter fluoroscopic x-ray contamination was measured at the beam angles specified in the treatment plans under clinical setup conditions. Dose distributions were recalculated assuming simultaneous delivery with proton beam currents of 0.1, 1, 2, 4, 8, and 40 MU/s, and these were compared to the treatment plans.

The scattered fluoroscopic x-ray contamination in the DM increased with higher tube voltage, tube current, frame rate, and field size. The maximum dose rate was 0.247 MU/s with 125 kV tube voltage, 80 mA tube current, 30 s^-1 frame rate, 19 × 19 cm² field size, and solid phantom of 10 cm. The dose rate of scattered fluoroscopic x-rays peaked at a solid phantom thickness of approximately 5 cm. In an anthropomorphic phantom, scattered fluoroscopic x-ray contamination varies based on anatomical site and beam angle. Dose evaluations indicated that if the proton beam current was ≥2 MU/s at the target and ≥1 MU/s at organs at risk, the differences in dose metrics were within 1% compared to the treatment plans. These beam currents are achievable with clinical systems.

Scattered fluoroscopic x-rays were confirmed to contaminate the DM and be counted as proton MUs. However, under clinically realistic beam current conditions, their impact on dose distribution in UCD FGPT was negligible. These findings support the feasibility of implementing UCD FGPT.