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Purpose:The purpose of this study was to test feasibility of clinical usage of flattening filter free(FFF)beam for treatment of lung stereotactic ablative radiotherapy(SABR).Methods:Ten patients were treated with SABR and 6 MV FFF beam for this study.All plans using volumetric modulated arc therapy(VMAT)were optimized in the Eclipse treatment planning system(TPS)using Acuros XB(AXB)and were delivered in Varian True Beam linear accelerator equipped with a high-definition(HD)multi leaf collimator.The prescription dose used was 48 Gy in 4 fractions of 12 Gy.In order to compare the plan using a 6-MV FF beam,the SABR plan was recalculated under the condition of the same beam settings used in the plan employing the FFF beam.The cumulative dose-volume histograms(DVH)for the PTV and OARs were analyzed.Technical parameters,such as total Monitor units(MUs)and delivery time,were also recorded and accessed.Results:Differences in a DVH for the PTV and OARs between plans with the FFF and the FF beam were statistically insignificant but showed a marked reduction in treatment delivery time.SABR plan with the FFF beam required an increased amount of MU.The average MUs were 2.91%larger for plans with the FFF beam compared to those of the FF beam.Conclusion:This study confirmed that the use of the FFF beam for lung SABR provided treatment efficiency relative to 6 MV for the FF beam,and the strategy is particularly beneficial for high dose conformity to lung and decreased intra-fraction movements because of the shorter treatment delivery time.Future studies are necessary to assess clinical outcome and toxicity.