| Objective: This work was to analyze the set-up errors in image guided IMRT for tumors using the cone-beam computed tomography (CBCT).Methods 53 patients were treated with the Varian 21EX IGRT system,including 28 head and neck tumors and 25 thoracic tumors. All the patients had received kilovoltage cone-beam computed tomography (KV-CBCT) before radiotherapy was delivered at the first, the sixth, the eleventh and the sixteenth fraction, separately. The acquired CBCT images were co-registered with the corresponding simulation CT images and set-up errors of iso-center positions on the axes of X, Y and Z were obtained for statistical analyses. Corresponding CTV to PTV margins were also calculated .Results All the 53 patients received a total of 212 CBCT scans. The acquired CBCT images had a high quality clearly showing soft tissue organs , especially the tumor size, location and the surrounding structures.In the 134 scans , the setup errors on the three axes were less than 3.00 mm , including 103 scans of the head and neck cases and 31 of the chest cases. The mean±standard deviation in the X,Y and Z directions were (1.78±1.18)mm(,1.00±0.73)mm, (1.15±0.77)mm in head and neck tumors and were (2.77±1.30)mm, (2.47±1.29)mm,(2.60±1.27)mm in the thoracic patients,respectively.The CTV to PTV margins in the head and neck patients were 5.30 mm, 3.00 mm, 3.40mm,and in the thoracic cases were 7.80 mm,7.10 mm,7.40 mm, respectively.Conclusion Setup errors in the three directions of all the CBCT scans were detected. In the head and neck cases, the setup errors were minor, with errors in 90 percent scans in the range of 3.00 mm and no evident differences on the three axes. In the chest cases, the setup errors were more evident and in most conditions the locations should be corrected online. Differences in the three directions were not evident, either. CTV to PTV margins on the three axes were 5.30 mm, 3.00 mm, 3.40 mm in the head and neck cases and 7.80 mm, 7.10 mm, 7.40 mm in the thoracic cases. |
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