Frequency of cone beam computed tomography guidance in radiotherapy for nasopharyngeal carcinoma

Abstract

Objective To analyze setup errors in cone beam computed tomography (CBCT)-guided radiotherapy for nasopharyngeal cancer (NPC) and evaluate the use frequency of CBCT. Methods Twenty-six patients newly diagnosed with NPC who received CBCT scan no less than 3 times per week during the whole course (7 weeks) of intensity-modulated radiotherapy (IMRT) were enrolled as subjects. Two setup errors were recorded each week: the setup error in the first CBCT analysis (Ef) and the setup error in the subsequent CBCT analysis (Ec). Comparisons of Ef between two different weeks, Ec between two different weeks, and between Ef and Ec in the same week were made by analysis of variance. Results During the 7 weeks, there were no significant differences in Ef in the medial-lateral (ML), superior-inferior (SI), or anterior-posterior (AP) direction between any two weeks (all P>0.05); there were also no significant differences in Ec in the ML or SI direction between any two weeks (all P>0.05); there was a significant difference in Ec in the AP direction between two different weeks (P 0.05); there were also no significant differences between Ef and Ec in the AP direction in any week (P>0.05) except for the first week (P<0.05). Conclusions In IMRT for NPC, the setup errors in the ML and SI directions are stable in the whole course of radiotherapy, while the setup errors in the AP direction are different between the first week and subsequent weeks. Daily CBCT scan in the first week and weekly CBCT scan in the subsequent weeks are highly recommended for most patients to reduce the use frequency of CBCT. Key words: Tomography, X-ray computed; Use frequency; Nasopharyngeal neoplasms/radiotherapy