Dosimetric Evaluation of Target Motion Effects in Spot-Scanning Proton Therapy: A Phantom Study

Intrafractional tumor motion leads to dose distortion and limits free-breathing treatment in spot-scanning proton therapy. The motion threshold for active motion mitigation depends on the beam delivery system and should be evaluated independently for each facility. To evaluate motion effects as a function of peak-to-peak motion and period during single-field, single-fraction and single-field, multi-fraction irradiation of the moving target. An in-house dynamic phantom was used to simulate peak-to-peak motion of 5 mm, 10 mm, and 20 mm with periods of 2 s, 4 s, and 8 s. The dose distribution in the moving target was measured using radiochromic films. During the perpendicular motion, the film was fixed and moved perpendicular to the beam direction without changing the water equivalent thickness (WET). During longitudinal motion, the film was fixed and moved along the beam direction, causing a change in WET. Gamma index analysis was used with criteria of 3%/3 mm and 3%/2 mm to analyze the dose distributions. For single-fraction irradiation, varying the period did not result in a significant difference in any of the metrics used (p > 0.05), except for the local dose within the PTV (p < 0.001). In contrast, varying peak-to-peak motion was significant (p < 0.001) for all metrics except for the mean PTV dose (p ≈ 0.88) and the local dose (p ≈ 0.47). Perpendicular motion caused a greater decrease in gamma passing rate (3%/3 mm) than WET variations (65 ± 5% vs. 85 ± 4%) at 20 mm peak-to-peak motion. The implementation of multi-fraction irradiation allowed to reduce hot and cold spots, but did not reduce dose blurring. The motion threshold varied from 7 mm to 11 mm and depended on the number of fractions, the type of motion, the acceptance criteria and the calculation method used.