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OBJECTIVE The aim of the present study was to determine the electronic portal imaging (EPI) evaluation differences between the therapists in the reference of cone beam computed tomography (CBCT). METHODS In the present study, 62 EPI images belonging to 13 head and neck patients were evaluated separately by four therapists as offline, and the amount of shift in the center of fields was determined. CBCT obtained at the same time with the EPI images was accepted as reference, and the amount of shift in the center of fields was compared separately for each therapist with the results of EPI. RESULTS According to our results, the amount of shift in the center of fields had changed between therapists with 0-9.4 mm in the reference of CBCT. The probability of shifting center of fields to be >3 mm was 60% for the first therapist, 35% for the second therapist, 63% for the third therapist, and 50% for the fourth therapist. The probability of shifting center of fields to be >5 mm was 24%, 8%, 27%, and 14.5%, respectively. Analysis of variance for repeated measures test was applied to center shift values, and there were a significant difference in the groups (sig. <0.05) and a significant difference between the groups (sig. <0.05). CONCLUSION The usage of CBCT for the verification of treatment fields eliminates the differences in interpersonal evaluation. CBCT improves the set-up accuracy such that planning tumor volume expansion margin can be safely dropped. Therefore, CBCT should be the preferred imaging modality in intensity-modulated radiation therapy planning. [ABSTRACT FROM AUTHOR]