Objective To explore the inter-fraction setup errors and affecting factors from data of daily fan-beam megavoltage computed tomography(MVCT). Methods A total of 37consecutive NPC patients treated with tomotherapy were hospitalized during the period of February 2015 to September 2015. For each patient,one MVCT scan was obtained after conventional positioning ,online correction and tomotherapy delivery daily ,and the scans were put into the planning computed tomography to determine inter-fraction setup errors. The MPTV was calculated with the equation:MPTV=2.5∑+0.7σ(∑:systematic error;σ:random error). Results The average absolute errors of the inter-fraction were(2.102 ± 0.0406)mm,(1.490 ± 0.0348)mm,(1.306 ± 0.335)mm and(1.392 ± 0.0384)° at three dimensions. The total MPTV accounting for inter-error was 3.4675 mm,2.9795 mm,and 2.8885 mm. Gradual increases in both inter-fraction three-dimensional displacement were observed with time and treatment(P < 0.05). Univariate analysis revealed that weight loss and retraction of neck lymph nodes were affecting factors of set-up errors. Conclusions 3 mm margins uniformly expended from clinical target volume to planning target volume may not be suitable. The personalized margin should be adopted for the design of IMRT planning. Displacement increases as a treatment course is prolonged.

Objective To evaluate the inter-fraction setup error during the treatment with megavoltage computed tomography (MVCT) and provide theoretical basis for clinical target volume-planning target volume (CTV-PTV) margins for nasopharyngeal carcinoma (NPC) patients treated with tomotherapy.Methods Thirty-seven consecutive NPC patients treated with tomotherapy were prospectively enrolled for the study between February 2015 and September 2015.For each patient,one MVCT scan was obtained after conventional positioning,online correction and tomotherapy delivery daily,and the scan was registered to the planning CT to determine inter-fraction setup error.The expanding margin for PTV (MPTV) was calculated with the recipe:MPTV =2.5∑ + 0.76 (∑:systematic error;6:random error).Results The average absolute errors of the inter-fraction were (2.102 ± 0.040 6) mm,(1.490 ± 0.034 8) mm,(1.306 ± 0.335) mm and (1.392 ± 0.038 4) ° in the three dimensions.Gradual increases in both inter-fraction three-dimensional displacement were observed with time and treatment (P ＜ 0.05).The total MPTV ac counting for inter-error were 3.467 5 mm,2.979 5 mm and 2.888 5 mm.Conclusions Tomotherapy irradiation technology personalized MPTV should be adopted for the design of tomotherapy plan.Displacement increased as a function of time.