A Non-coplanar Volumetric Modulated Arc Therapy For The Whole Brain Radiotherapy With Hippocampus Avoidance

Background:BM(Brain Metastase)is the most common intracranial tumor in adults.Many primary malignant tumors will eventually develop into BM,with poor prognosis and serious threaten to patients’ life and health.WBRT(Whole Brain Radiotherapy)is considered to be an important treatment for multiple BM,which can control the progression and recurrence of intracranial tumors.However,the neurotoxicity caused by WBRT due to the large irradiation range cannot be ignored.There are increasing evidences that hippocampus damage from intracranial irradiation plays an important role in the decline of neurological function.HA-WBRT(Hippocampal-avoidance Whole Brain Radiation Therapy)is a strategy to reduce the radiation dose to the hippocampus and improve the neurocognitive function of patients with BM.At present,the most commonly used radiotherapy techniques include TOMO(Helical Tomotherapy),IMRT(Intensity Modulated Radiation Therapy)and VMAT(Volumetric-modulated Arc Therapy).In addition,more and more clinical studies have been conducted to continuously optimize the radiotherapy plan of HA-WBRT by improving the radiation field arrangement and adjusting the tilt of the patient’s head during CT simulation.Objective: To evaluate the feasibility of using a simplified NC-VMAT(Noncoplanar Volumetric-modulated Arc Therapy)and investigate its dosimetric advantages compared with IMRT and C-VMAT(Coplanar Volumetric-modulated Arc Therapy)for HA-WBRT in patients with BM.Methods: Ten patients with BM in the Department of Radiotherapy,The First Affiliated Hospital of Dalian Medical University from 2010 to 2022,were included for HA-WBRT.Three kind of treatment plans were generated for each case using IMRT,C-VMAT and NC-VMAT,respectively.Dosimetric parameters from the three techniques were compared by a one-way ANOVA(Analysis of Variance)and the LSD(Least Significant Distance)post-hoc tests.p values of <0.05 were regarded to indicate a statistically significant difference.Results: The dosimetric results of the three techniques complied with the RTOG 0933 criteria roughly.V30 Gy of WB-PTV(Whole Brain Planned Target Volume)was the highest for NC-VMAT(93.66 ± 0.74%),followed by C-VMAT(92.35 ± 1.51%)and IMRT(90.85 ± 0.77%),and NC-VMAT was statistically superior to IMRT(p<0.001)and C-VMAT(p=0.006).HI of WB-PTV was significantly reduced in NC-VMAT(0.260 ± 0.014)over IMRT(0.276 ± 0.172,p=0.048)and C-VMAT(0.283 ± 0.023,p=0.008).In terms of conformity index(CI),NC-VMAT could provide a value of 0.809 ± 0.010,and it was significantly superior to IMRT(0.772 ± 0.019,p<0.001).However,there was no significant improvement compared with C-VMAT(0.802 ± 0.012,p=0.196).According to D98% of WB-PTV,NC-VMAT could provide a value of 27.10 ± 0.40 Gy,significantly superior to C-VMAT(26.28 ± 0.57 Gy,p<0.001).According to D2% of WB-PTV,there was no significant difference between the three techniques.In terms of Dmax and D100% of the hippocampus,NC-VMAT could control them at 16.26 ± 0.25 Gy and 8.03 ± 0.22 Gy,respectively.Dmax of the hippocampus for NC-VMAT was significantly lower than IMRT(16.68 ± 0.67 Gy,p=0.024).For other organs-at-risk(OARs),in terms of Dmax of bilateral lenses,NC-VMAT had the lowest values,which were statistically different from IMRT(left: p=0.006;right: p=0.008).Besides,NCVMAT could reduce the Dmax of the right eye to 24.20 ± 2.74 Gy,which was significantly superior to IMRT(26.77 ± 2.89 Gy,p=0.004).For other organs including the optic nerves and optic chiasm,no statistical differences were detected by the three techniques.Conclusions: For HA-WBRT,the dosimetric results of the three techniques roughly met the RTOG 0933 criteria.Compared with IMRT and C-VMAT,NC-VMAT can provide the optimal target homogeneity and prescription dose coverage,while significantly reducing the maximum doses to the hippocampus and lenses.