Abstract: Objective The aim of this study was investigate the dosimetric difference between VMAT and IMRT by VARIAN IX linear accelerator using 6 MV X-ray in flatting-filter-free mode.Methods A retrospective study was conducted on 53 patients with rectal cancer treated by preoperative radiotherapy in Liaoning Cancer Hispital from September 2016 to September 2018.Each patient in flatting-filter-free mode was used the same optimization conditions to perform VMAT and IMRT plans.The dosimetric differences were compared between target areas and organs in two modes of plans.Results In terms of target dose,there were statistically significant of differences in D2%,D98%,D_mean,CI and HI of the two plans.FFF-IMRT compared to FFF-VMAT,D2% increased by 3.1%(P=0.001),D98% decreased by 4.5%(P=0.001),D_mean decreased by 2.4%(P=0.026),CI decreased(P＜0.001),and HI increased(P=0.005).In terms of organ at risk,there were statistically significant of differences in D2%,D_mean of bladder,D2%,D_mean of double femoral heads and D_mean of pelvis between the two plans.FFF-IMRT compared to FFF-VMAT,D2% of bladder increased by 2.7%(P＜0.001)and D_mean decreased by 20.4%(P＜0.001),D2% of double femoral head decreased by 4.2%(P＜0.001),D_mean decreased by 11.2%(P＜0.001),and D_mean of pelvis decreased by 8.9%(P＜0.001).However,there was no significant of differences between D2% and D_mean of intestines(P＞0.05).The planned machine hops of FFF-IMRT were(978±127)MU,which was 1.86 times of the planned machine hops of FFF-VMAT(527±63)MU(P＜0.001).The planned execution time of FFF-IMRT was(510±252.2)s,which was 2.21 times that of FFF-VMAT(232±52.5)s(P＜0.001).Conclusion In the flatting-filter-free mode,the conformity and uniformity of VMAT in target area are better than those of IMRT,the treatment time of VMAT was shorter and the execution efficiency of the plan was higher.However,IMRT could better protect the organs of patients.

Key words: Flatting-filter-free mode, Rectal cancer, Volume arc intensity modulation, Intensity modulated radiotherapy