Impact of Machine Delivery Characteristics and Patient Motion on Interplay In Free-Breathing Proton Therapy with Mevion S250i
Abstract
Purpose
Respiratory motion during proton therapy introduces uncertainties that can lead to dose inhomogeneities, primarily due to the interplay between tumor motion and spot-scanning beam delivery. Understanding and predicting this effect is essential for robust treatment planning, especially under free-breathing conditions. This study aims to investigate the interplay effect on lung and esophageal cancer patients treated in free-breathing with the Mevion S250i machine.
Methods
We retrospectively analyzed 36 patients (18 lung, 18 esophageal) treated under free-breathing conditions. Patients were selected based on tumor location and motion amplitude. Interplay was evaluated by simulating 24 breathing scenarios per patient, varying breathing periods (2–5 s) and starting phases, combined with machine delivery sequences extracted from log files. To assess fractionation effects, 10 treatment courses were simulated by randomly sampling one breathing scenario, thus one 4D dose distribution per fraction. Interplay results were compared with our institution’s 4D robust evaluation (4DRE) method, which neglects the interplay effect.
Results
Our analysis showed that most patients' 4D dose distributions converged toward the planning dose within five fractions, except for one lung and one esophageal case, indicating persistent motion sensitivity. The 4DRE confirmed these findings, identifying the same patients as non-robust to organ motion. The two patients failing the interplay and 4DRE showed the smallest target volume and large tumor amplitude (>1cm) among the patient cohort.
Conclusion
This analysis confirmed the strong role of fractionation in mitigating the interplay effect. For the Mevion system, the larger spot size mitigates the need for volumetric or energy-layer repainting, even in cases with significant tumor motion, contrasting with requirements for smaller-spot systems such as IBA machines. These results highlight the importance of institution-specific evaluations to accurately quantify motion impact and guide robust planning strategies.