2258 - Investigating Scaled and Iso-Layer Repainting to Mitigate Interplay Effect for Carbon Ion Pencil Beam Scanning over the Full Breathing Cycle

S. Ding¹, H. Liu², G. Huang³, Z. Piao³, H. Song³, Y. Li², and X. Huang¹; ¹State Key Laboratory of Oncology in South China, Guangdong Provincial Clinical Research Center for Cancer, Department of Radiation Oncology, Sun Yat-sen University Cancer Center, Guangzhou, Guangdong, China, ²State Key Laboratory of Oncology in South China; Collaborative Innovation Center for Cancer Medicine; Department of Radiation Oncology, Sun Yat-sen University Cancer Center., Guangzhou, China, ³Sun-Yat sen University Cancer Center, Guangzhou, China

Purpose/Objective(s): The interplay effect between dynamic pencil carbon ion beams and motion of the lung tumor presents a challenge in treating lung cancer patients in pencil beam scanning (PBS) carbon ion therapy. The main purpose of current study was to evaluate the interplay effect carbon ion therapy for lung cancers and investigate the difference in the mitigation of interplay effects between scaled and iso-layer repainting. Materials/

Methods: The current retrospective study included ten lung cancer patients. The total dose prescription to the clinical target volume (CTV) was 60 Gy(RBE) with a fractional dose of 2 Gy(RBE). The reference plans were optimized on T20 phase without repainting by Phoenix TPS. Then, the reference plans were recalculated on all ten phases in the 4DCT data set to evaluate interplay effect. For comparison, the interplay effect was also evaluated for conventional photon radiotherapy. We applied scaled and iso-layer repainting techniques to the reference plans and evaluated the difference in the mitigation of interplay effects based on the dosimetric metrics.
Results: The mean values of target coverage, homogeneity and conformity index were reduced by 15.2%, 28.2% and 13.8% because of interplay effects. The mean values of lung V5Gy, V20Gy and V30Gy were improved by 1.2%, 4.2% and 3.8%, respectively. The interplay effect of carbon ion plans was greater dosimetric influence than photon plans. Compared to the reference plans without repainting, scaled and iso-layer repainting techniques improved the target coverage and reduce lung dose.
Conclusion: For carbon ion lung cancer radiotherapy, the significant dose changes were caused by interplay effect. The repainting techniques can mitigate interplay effect over the full breathing cycle for carbon ion pencil beam scanning radiotherapy.