Conformity and robustness of gated rescanned carbon ion pencil beam scanning of liver tumors at NIRS

Abstract Purpose Pencil beam scanning offers excellent conformity, but is sensitive to organ motion. We conducted a simulation study to validate our rescanning approach in combination with gating in the irradiation of liver tumors. Materials and methods 4DCT imaging was performed under free-breathing conditions in 30 patients with hepatocellular carcinoma. Dose distributions for a two-field approach were calculated for layered phase controlled rescannings (PCR) under organ motion conditions. A total dose of 45 Gy(RBE) was delivered to respective field-specific target volumes (FTVs) in two fractions, each composed of two orthogonal uniform fields of 11.25 Gy(RBE) at beam angles of either 0° and 90° or 0° and 270°. The number of rescannings was changed from 1 to 10. Results Good dose conformity was achieved with 4× PCR or more, and over 95% of the prescribed dose was delivered to the CTV independent of the use of gating. D95, Dmax/min and dose homogeneity were similar with or without gating, whereas V10 dose to the liver as well as maximal doses to healthy tissue (esophagus and cord) were about 40% lower with gating. However, total time increased by about 50% with gating. Conclusions Gated rescanning provides good target coverage and homogeneity with maximal sparing of healthy tissue. Our results suggest that carbon-ion pencil beam scanning may soon be available for the safe treatment of liver tumors.