Proton lattice radiotherapy for large lung tumors: effects of robust optimization and tumor motion

PURPOSE: This study aims to investigate the impact of robust parameter settings and respiratory motion on intensity-modulated proton therapy (IMPT)-based LRT plans for bulky lung tumors. METHODS: Eight patients with bulky lung tumors were retrospectively analyzed. Target and organs at risk (OARs) were contoured in the RayStation treatment planning system. An in-house script was developed in RayStation to generate sphere vertices. First, a non-robustly optimized plan (NRO plan) was generated without robust optimization. Both single-field and multi-field optimized IMPT-LRT plans were created. Then, for each optimization method, seven robust parameter settings (1mm-9mm) were applied.to investigate the effect of setup uncertainty on LRT dose distribution. At last, four-dimensional dynamic dose (4DDD) was calculated by incorporating dynamic pencil beam scanning delivery with respiratory motion to evaluate the impact of motion on PVDR and dose distributions. RESULTS: PVDRs decreased as setup uncertainty parameters increased. Multi-field optimization resulted in slightly higher PVDR values. Most of the PVDR values were in the range of 2 -4.Doses to OARs increased with larger setup uncertainties. Respiratory motion resulted larger PVDR deviations for single-field plan than multi-field plan. The 4DDD maintained the volume of high dose region inside GTV in single-field plan, while decreased in multi-field plan. The volume of high dose region decreased in 4DDD for both single-field and multi-field plans. CONCLUSION: Increased setup uncertainty resulted in a reduction of the PVDR. Under respiratory motion, the single-field plan maintained high-dose coverage within the target but did not preserve the PVDR. In contrast, the multi-field plan retained the PVDR at the expense of high-dose volume within the target. With appropriate vertex contouring, neither single-nor multi-field plans introduced additional high-dose volume to the target boundary, thereby ensuring safe beam delivery in proton LRT during respiratory motion.