Abstract
Objective The purpose of the study was to use cone beam CT (CBCT) scans to evaluate setup errors in image-guided radiotherapy with thoracic esophageal carcinoma, in order to refer for adaptive radiotherapy. Methods CBCT scans were performed before and at the end of image-guided radiotherapy with 23 thoracic esophageal carcinoma patients. The registration method and setup error in different lesions and treatment weeks were measured. The difference of treatment time was single factor analysis of variance. Results Compared with bone registration and gray value registration, the setup errors in x and z axis have significant differences. Without CBCT, the adequate coverage margins required for the CTV in x, y and z axis with upper, middle and lower esophageal carcinoma were 6 mm, 17 mm, 6 mm; 4mm, 17 mm, 6 mm and 11 mm, 11 mm 4 mm, respectively. The adequate coverage margins of the plan target volume (PTV) were 2 mm, 2 mm and 4 mm in x, y and z axis if the CBCT was applied in irradiation during the image guided radiotherapy. When compared with setup errors at different weeks, no significant difference was found among them in y axis (P=0.001, 0.000) , but the setup errors of the fifth week in x and z axis were larger significantly than those of other weeks (P=0.858) . Conclusions Gray value registration is recommended in image-guided radiotherapy with thoracic esophageal carcinoma. The adequate coverage margins are different in x, y and z axis with different lesions. The coverage margin of the PTV could be reduced significantly as CBCT are applied during the irradiation. The setup errors measured during earlier treatment period can be considered to guide subsequent irradiation. The setup errors in x and z axis become larger during the later irradiation course. Re-simulation should be recommended at the the fifth treatment week. Key words: Esophageal neoplasms/ image guided radiotherapy; Setup error; Tomography, X-ray computed, cone beam
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