A simplified non-coplanar volumetric modulated arc therapy for the whole brain radiotherapy with hippocampus avoidance

PURPOSE: To evaluate the feasibility of using a simplified non-coplanar volumetric modulated arc therapy (NC-VMAT) and investigate its dosimetric advantages compared with intensity modulated radiation therapy (IMRT) and coplanar volumetric modulated arc therapy (C-VMAT) for hippocampal-avoidance whole brain radiation therapy (HA-WBRT). METHODS: Ten patients with brain metastase (BM) were included for HA-WBRT. Three treatment plans were generated for each case using IMRT, C-VMAT, and NC-VMAT, respectively. RESULTS: The dosimetric results of the three techniques complied roughly with the RTOG 0933 criteria. After dose normalization, the V(30Gy) of whole brain planned target volume (WB-PTV) in all the plans was controlled at 95%. Homogeneity index (HI) of WB-PTV was significantly reduced in NC-VMAT (0.249 ± 0.017) over IMRT (0.265 ± 0.020, p=0.005) and C-VMAT (0.261 ± 0.014, p=0.020). In terms of conformity index (CI), NC-VMAT could provide a value of 0.821 ± 0.010, which was significantly superior to IMRT (0.788 ± 0.019, p<0.001). According to D(2%) of WB-PTV, NC-VMAT could provide a value of 35.62 ± 0.37Gy, significantly superior to IMRT (36.43 ± 0.65Gy, p<0.001). According to D(50%) of WB-PTV, NC-VMAT can achieve the lowest value of 33.18 ± 0.29Gy, significantly different from IMRT (33.47 ± 0.43, p=0.034) and C-VMAT (33.58 ± 0.37, p=0.006). Regarding D(2%), D(98%), and D(mean) of hippocampus, NC-VMAT could control them at 15.57 ± 0.18Gy, 8.37 ± 0.26Gy and 11.71 ± 0.48Gy, respectively. D(2%) and D(mean) of hippocampus for NC-VMAT was significantly lower than IMRT (D(2%): 16.07 ± 0.29Gy, p=0.001 D(mean): 12.18 ± 0.33Gy, p<0.001) and C-VMAT (D(2%): 15.92 ± 0.37Gy, p=0.009 D(mean): 12.21 ± 0.54Gy, p<0.001). For other organs-at-risk (OARs), according to D(2%) of the right optic nerves and the right lenses, NC-VMAT had the lowest values of 31.86 ± 1.11Gy and 7.15 ± 0.31Gy, respectively, which were statistically different from the other two techniques. For other organs including eyes and optic chiasm, NC-VMAT could achieve the lowest doses, different from IMRT statistically. CONCLUSION: The dosimetry of the three techniques for HA-WBRT could roughly comply with the proposals from RTOG 0933. After dose normalization (D(95%)=30Gy), NC-VMAT could significantly improve dose homogeneity and reduce the D(50%) in the brain. Besides, it can reduce the D(2%) of the hippocampus, optic nerves, and lens. With this approach, an efficient and straightforward plan was accomplished.