Presenter(s)
J. Xie, D. Zhang, Y. Zhao, T. Qiu, M. Zhou, X. Wang, J. Li, S. Wang, B. Tang, Y. Huang, J. Lang, L. C. Orlandini, and X. Xin; Sichuan Cancer Hospital and Research Institute, University of Electronic Science and Technology of China, Chengdu, China
Purpose/Objective(s):
Moderate hypofractionation for prostate cancer is increasingly adopted in proton therapy. However, geometric uncertainties may challenge treatment planning accuracy. We quantified interfraction displacement to characterize the magnitude of daily setup corrections and assessed intrafraction motion to determine whether current robustness settings adequately account for observed geometric variations.Materials/Methods:
This retrospective study included twelve patients with prostate cancer treated with moderately hypofractionated PBS proton therapy delivering 60 Gy (RBE) to the prostate and 50 Gy (RBE) to the seminal vesicles in 20 fractions. Two CBCT scans were acquired per fraction: CBCT1 for 6-DoF setup correction and CBCT2 immediately prior to beam delivery to verify positional stability. Interfraction displacement was quantified by registering CBCT1 to the simulation CT, while intrafraction motion was derived from CBCT1–CBCT2. Treatment plans were optimized assuming 3.5% range uncertainty and patient-specific geometric robustness settings of 3–5 mm (LL/SI) and 5–6 mm (AP). Translational and rotational displacements were recorded per fraction to compute 3D motion vectors and drift velocity over the CBCT1–CBCT2 interval. The proportion of fractions exceeding specified displacement magnitudes was calculated. Inter-patient variability was assessed using the Kruskal–Wallis test based on patient-level data.Results: A total of 240 fractions were analyzed; median target volumes were 47.9 cm³ (range, 14.4–82.5) for the prostate and 25.9 cm³ (range, 9.5–46.3) for the seminal vesicles. The 3D interfraction displacement vector reached a maximum of 10.2 mm, exceeding 3 mm, 5 mm, and 10 mm in 61.1%, 27.6%, and 0.4% of analyzed fractions, respectively. Intrafraction motion remained limited, exceeding 2 mm, 3 mm, and 4 mm in 8.2%, 1.8%, and 0.9% of fractions, respectively. Exceedances approaching 4 mm in the 3D vector were primarily driven by motion in the AP direction (for which the minimum robustness margin was 5 mm), consistent with the applied robustness margins, while lateral and superior–inferior components remained well below the 3-mm robustness margin. Table 1 summarizes the analyzed 3D motion parameters, reported as median and 95th percentile (P95) values. No significant inter-patient differences were observed (all p > 0.05).
Conclusion: Interfraction displacement constitutes the principal geometric variation in PBS proton therapy for prostate cancer, reinforcing the importance of daily image guidance. Intrafraction motion remained within the applied robustness margins, highlighting the importance of appropriately designed geometric margin strategies to account for residual motion.
Table 1. 3D motion parameters across 240 fractions| Intrafraction | Interfraction | |||
| (Median, P95) | p | (Median, P95) | p | |
| 3D disp. (mm) | (0.73, 2.24) | 0.643 | (3.53, 8.19) | 0.758 |
| 3D rot. (°) | (0.32, 0.87) | 0.152 | (0.77, 2.57) | 0.584 |
| Drift velocity (mm/min) | (0.31, 1.06) | 0.895 | - | - |