RESUMO
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.