Abstract: ObjectiveTo evaluate the interfractional three-dimensional (3D) setup error and propose optimum margin of planning target volume (PTV) in radiotherapy for intracranial tumor at risk of using intensity modulated radiotherapy (IMRT) guided with cone-beam computed tomography (CBCT). Methods Thirty-six patients received radiotherapy of IMRT imaging-guided with megavolt MVision CBCT scans equipped in ONCOR Linear Accelerator. The initial five fractional CBCT volume-imaging and data obtained by CBCT scans after initial setup and after re-positioning were dimensionally matched with planned CT images. ResultsThe interfractional setup errors on X, Y and Z axes were (2.22±1.37) mm, (2.62±1.50) mm and (2.77±1.34) mm, respectively, for 36 intracranial tumors. Comparing with the pre-correction position, the post-correction setup errors decreased significantly on all three axes (P<0.05), whereas there were no statistically significant difference in five fractional setup errors (P>0.05). While without correction of CBCT setup errors，PTV margin in three dimension were Lat(cm)-X：2.94 mm, Long(cm)-Y：3.31mm，Vert(cm)-Z：3.52 mm.While with correction of CBCT setup errors, PTV margin in three dimension were，Lat (cm)-X: 1.00 mm, Long (cm)-Y: 1.19 mm，Vert (cm)-Z: 1.68 mm. ConclusionThe set-up margins were <4 mm in all directions. Measurement and correction of interfractional setup errors and PTV margins before fractional radiotherapy using megavolt MVision CBCT could help to improve the precision of IMRT radiotherapy for intracranial tumors.