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

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 degrees. The number of rescannings was changed from 1 to 10. Results: Good dose conformity was achieved with 4x PCR or more, and over 95% of the prescribed dose was delivered to the CTV independent of the use of gating. 095, 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. (C) 2014 Elsevier Ireland Ltd. All rights reserved.