Purpose: Intensity-modulated proton therapy for lung tumors with large tumor movement is challenging. Often a 5mm tumor movement cut-off is used for proton patient selection. We propose a robust and easily implementable treatment planning strategy for lung tumors with motion larger than 5mm, and a 4DCT robust evaluation strategy.
Methods: We created an internal target volume (ITV) based treatment planning strategy. The plans were robustly optimized on the 4DCT-based average-CT. Clinical plan acceptability was judged on robust evaluation, computing voxel-wise minimum/maximum (VWmin/max) doses over 28 error-scenarios on the average-CT. Plans were calculated for sixteen lung patients with 4-26mm tumor motion in RayStation (RaySearch) with Monte Carlo dose-engine for a Mevion S250i Hyperscan system (Mevion Medical Systems). We developed a 4D robust evaluation (4DRobAvg). The 28 scenario-doses were computed on all eight 4DCT-phases. For each scenario, the doses on the individual phases were deformed to the reference phase (50%expiration) and combined to an average scenario-dose. From these 28 phase-averaged scenario-doses, VWmin/max were computed. The 4DRobAvg was compared to two other 4D evaluation strategies: re-computing nominal dose on each 4DCT-phase (4DNom), and computing robust VWmin/max doses on each phase (4DRobInd).
Results: A clinically acceptable plan was feasible for all patients with adequate tumor coverage and no organs-at-risk over-dosage. The 4DNom and 4DRobInd evaluations under- or overestimated the dosimetric effect of tumor movement, respectively; 4DNom on average showed 1Gy higher target dose, and 4DRobInd showed target under-dosage for five patients, not observed in 4DRobAvg. The accuracy of dose deformation in 4DRobAvg was acceptable and dose differences were generally less than 1Gy.
Conclusion: The proposed ITV-based planning strategy on the average-CT was found clinically feasible even for large tumor movement. The proposed 4D robust evaluation, 4DRobAvg, was shown to give an easily interpretable understanding of the effect of respiratory motion on the dose distribution.
Funding Support, Disclosures, and Conflict of Interest: This work was partly supported by Mevion Medical Systems.