Dosimetric comparison of noncoplanar volumetric modulated arc therapy and helical tomotherapy in whole brain radiotherapy with hippocampus avoidance

Background: Whole-brain radiotherapy(WBRT) can alleviate symptoms in patients with brain metastases. However, WBRT may damage the hippocampus.The helical tomotherapy (HT) presents a significant dosimetric advantage in the hippocampus avoidance WBRT(HA-WBRT). How to design linear accelerator radiation therapy plan for HA-WBRT is a challenge for medical physicists. Objective: The aim of this study is to investigate the dosimetric differences between noncoplanar VMAT(NCVMAT) and HT in HA-WBRT, exploring the feasibility of NC-VMAT for HA-WBRT. Methods: Fifteen patients with HA-WBRT were chosen randomly. For each patient, the Monaco TPS was used to design four distinct NC-VMAT plans based on the quantity of non-coplanar arcs (NC-VMATA, NC-VMATB,NC-VMATC,NC-VMATD) for HA-WBRT, totaling 60 plans. The HT TPS was used to generate HT plans, totaling 15 plans.The prescribed dose was 30 Gy in 10 fractions. Treatment plans were established based on the RTOG 0933 criteria.Under the premise that the 95% isodose curve covers the target area, dose-volume histogram(DVH) was applied to evaluate the WB-PTV, CI, HI, Dmax, Dmean, Dmin and doses of OARs in NC-VMAT and HT plans. Paired t-test was performed to compare the differences between each two radiation therapy plans, and p < 0.05 was considered statistically significant. Results: The RTOG0933 criteria could not be satisfied in NC-VMATA with 1 non-coplanar beam. NC-VMATB with 2 non-coplanar beams, the condition Dmax<16 Gy was not satisfied for 8 case in the hippocampus. NC-VMATC with 3 non-coplanar beams, the condition D98% > 25 Gy was not satisfied for 5 case in the WB-PTV. NC-VMATD with 4 non-coplanar beams and HT, all plans complied with the "Per protocol" requirements in the RTOG 0933 standard.We compared NC-VMATD with HT and found that there were no differences in the D98% and D2% of WB-PTV. However, the HT was significantly superior to NC-VMATD in terms of CI, HI, Dmean in HP, Dmax in Lens, Dmean in eyes (p < 0.05). The Dmax, D100% of HP and the dosimetry of other OARs had no significant difference (p > 0.05).In contrast, the average beam on time in NC-VMATD was greater advantage(506s Vs 687s, p < 0.05). NC-VMAT and HT planning templates were devised, which facilitated physicists in rapidly designing high-quality HA-WBRT plans. Conclusion: NC-VMAT proves to be effective in managing HA-WBRT.The plan quality improved along with the increase of non-coplanar arcs. NC-VMAT(one full arc and four noncoplanar partial arcs) with FFF irradiation mode is most recommended plan design pattern. NC-VMAT provides a practical clinical treatment option for HAWBRT based on linear accelerators.