KEY POINTS
- Investigators evaluated interplay in 36 patients: 18 lung and 18 esophageal cancers, representing 45 proton plans, treated in free breathing on a Mevion S250i compact PBS system without rescanning.
- Plans used ITV-based 3D robust optimization with 3% range and 5-mm setup uncertainty. Lung treatments used 20–30 fractions, while esophageal regimens predominantly used 23–28 fractions.
- A predictive pre-treatment model used 24 synthetic breathing traces, while the a posteriori calculation used actual respiratory traces and machine-log timing. Agreement was strong, with median relative distribution error <1% across evaluated DVH metrics.
- During the first fraction, interplay typically reduced target metrics by approximately 1–2%. Fraction accumulation rapidly averaged this out: lung metrics generally converged to within 1% of the nominal plan after about five fractions, and esophageal metrics to within 2%.
- Two plans were clear exceptions. One lung case and one esophageal case retained >5% underdosage in at least one target metric throughout treatment; both also failed the institution’s conventional 4D robustness evaluation.
- In the problematic esophageal case, changing the beam arrangement restored robustness, highlighting that interplay susceptibility was not determined by motion amplitude alone but also by target size, geometry and beam direction.
- The conclusions are specific to conventional fractionation and this machine’s relatively large spot size and fast energy-layer switching. Anatomical changes were not modeled, and the authors explicitly caution that hypofractionated treatment requires separate evaluation.
CLINICAL TAKEAWAY
For this compact proton system, robust ITV planning plus conventional fractionation made additional rescanning unnecessary for most free-breathing thoracic cases. The practical warning is the combination of small target + large respiratory motion, where patient-specific 4D robustness and beam-angle review remain important.