【Objective】 To evaluate the inter-fraction and intra-fraction setup error during the treatment with cone beam computer tomography (CBCT) and provide theoretical basis for clinical target volume-planning target volume (CTV-PTV) margins for nasopharyngeal carcinoma patients treated with volumetric-modulated arc therapy (VMAT). 【Methods】 Seventeen consecutive NPC patients treated with VMAT were prospectively enrolled for the study between October 2010 and January 2012. For each patient

three CBCT scans were obtained after conventional positioning

online correction with 2 mm tolerance daily and VMAT delivery weekly

and the scans were registered to the planning CT to determine inter-fraction and intra-fraction errors. The MPTV were calculated with the recipe

VMargin = 2.5Σ + 0.7σ (Σ

systematic error; σ

random error). 【Results】 The average absolute errors of the pre-correction

post-correction

and intra-fraction error were 1.1-1.2 mm

0.6-0.7 mm and 0.4-0.6 mm in the three dimensions. The corresponding Σ were 0.9-1.3 mm

0.4-0.5 mm and 0.2-0.4 mm

σ were 0.7-0.9 mm

0.7 mm and 0.5-0.7 mm. Gradual increases in both inter-fraction and intra-fraction three-dimensional (3D) displacement were observed with time and treatment (P = 0.004

P = 0.009). The total MPTV accounting for pre-correction and intra-fraction error were 3.4-4.0 mm and margins accounting for post-correction and intra-fraction error were 1.7-2.3 mm.【Conclusions】 VMAT irradiation technology personalized MPTV should be adopted for the design of VMAT plan. Displacement increased as a function of time.