Interplay effect evaluation on lung and esophageal cancer patients treated with compact proton system in free...

Pencil beam scanning proton therapy is sensitive to respiratory motion, which can cause dose discrepancies due to the interplay between tumor and beam delivery motion. This study evaluates the clinical impact of interplay for lung and esophageal cancer patients treated in free-breathing, without rescanning.

Interplay was evaluated in 36 patients (18 lung, 18 esophageal) using both predictive and a posteriori approaches. The predictive method, for pre-treatment assessment, employed 24 synthetic sinusoidal breathing patterns to model respiratory motion. The a posteriori method incorporated patient-specific breathing traces and machine log files. Fraction-accumulated dynamic 4D dose distributions were generated and compared with the institutional 4D robustness evaluation.

The predictive method agreed closely with a posteriori evaluation, with a median Relative Distribution Error < 1% across all dose-volume metrics. Interplay-induced underdosage in the first fraction (-1% to -2%) decreased after ∼5 fractions, with cumulative D_98%, D_95% and V_95% values converging to within 2% of planned doses. Two plans, one lung and one esophageal, showed persistent underdosage exceeding 5%, consistent with 4D robustness evaluation failures. For the esophageal case, a revised beam arrangement restored robustness.

Internal target volume-based robust optimization combined with machine-specific beam delivery characteristics provides motion robustness for most lung and esophageal patients treated in free-breathing. Additional motion mitigation was not required for most patients treated under conventional fractionation. However, small targets with large motion remain at risk of interplay-induced underdosage, warranting caution, as this combination may compromise plan robustness.