Objective:To investigate the dosimetric differences between non-coplanar volumetric modulated arc therapy (VMAT) and non-coplanar fixed-field intensity-modulated radiotherapy (IMRT) of Monaco treatment planning system (TPS) in hippocampus-avoidance whole-brain radiation therapy with simultaneous integrated boost(HA-WBRT+SIB) for brain metastases,so as to provide a practical and feasible treatment plan for clinical work. Methods:A total of 22 patients with brain metastases were retrospectively selected. Two radiotherapy plans,which included non-coplanar VMAT and non-coplanar fixed field IMRT of HA-WBRT+SIB,were designed for each patient. The dose distribution,the machine units (MU) of plan,and verification of pre-treatment plan in target area and organs at risk (OARs) between two plans were compared under meeting the prescribed dose requirements of target volume. Results:There were no significant differences in covered volumes (V50 and V55) of 50 and 55 Gy dose,maximum dose (Dmax),heterogeneity index(HI) and conformity index(CI) of planning gross tumor volume(PGTV) between the two plans (P>0.05). For whole-brain planning target volume (PTV-brain-SIB),there was no significant difference in the dose that was covered by 98％ volume (D98％) between IMRT and VMAT (P<0.05). VMAT plan could significantly improve the covered volume of 30 Gy dose (V30) of PTV-brain-SIB,and decrease HI value and increase CI value,and the differences of them between VMAT and IMRT were significant (t=-6.366,3.289,-8.343,P<0.05),respectively. The Dmax values of left and right eyes of VMAT plan were significantly lower than those of IMRT plan,and the differences were significant (t=5.784,5.351,P<0.05),respectively. The Dmax value of right inner ear of VMAT plan was higher than that of IMRT plan,and the difference was significant (t=-2.848,P<0.05). VMAT could significantly increase D100％ value of left and right hippocampus,which was higher than that of IMRT plan,and the differences of them between two plans were significant (t=-2.564,-6.578,P<0.05),respectively. The MU of IMRT plan was significantly lower than that of VMAT plan,and the difference was significant (t=-6.249,P<0.05). The treatment time of VMAT plan was (294.1±16.4) s,which was significantly less than that of IMRT plan,and the difference of that between two plans was significant (t=-7.400,P<0.001). Under the standards of 3％/2 mm and 2％/2 mm of γ analysis,the γ pass rates of IMRT plan were respectively (99.1±0.6)％ and (97.6％±1.0)％,which were significantly higher than those of VMAT plan,and the differences of them were significant (t=-2.733,t=-5.780,P<0.001),respectively. Conclusion:The non-coplanar VMAT and non-coplanar fixed field IMRT based on the Monaco TPS can produce clinically acceptable results for HA-WBRT+SIB in patients with brain metastases. Compared with IMRT,VMAT has better advantage in dose distribution of target volume,and higher treatment efficiency,but IMRT can better protect the hippocampus and reduce the number of MU.
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