[Interfractional and intrafractional setup errors in radiotherapy for tumors analyzed by cone-beam computed tomography].

Background & Objective: Both interfractional and intrafractional setup errors may affect the precision of radiotherapy. This study was to analyze the interfractional and intrafractional setup errors in radiotherapy for tumors using cone-beam computed tomography(CBCT).

Methods: Of the 51 patients received radiotherapy, 19 had head and neck tumors, 25 had thoracic tumors, and seven had abdominal-pelvic tumors. Patients received CBCT scans after initial setup, after re-positioning and after radiation delivery. The CBCT images were registered to the planning CT images, and setup errors on X,Y, Z axes were analyzed.

Results: A total of 1,934 CBCT scans were performed on 51 patients, of which 955 were performed after initial setup, 525 after re-positioning and 454 after radiation delivery. The interfractional setup errors on X, Y and Z axes were (1.2+/-0.9) mm,(1.2+/-1.1) mm and (1.0+/-0.8) mm, respectively, for head and neck tumors; (2.3+/-1.9) mm, (4.2+/-3.7) mm and (2.4+/-2.1) mm, respectively, for thoracic tumors; (1.7+/-1.5) mm,(4.7+/-3.6) mm and (2.1+/-1.6) mm, respectively, for abdominal-pelvic tumors. Comparing with the post-correction position, the post-treatment setup errors in head and neck tumors increased significantly on all three axes (P < 0.05), whereas the difference was not significant in trunk tumors (P > 0.05).

Conclusions: Measurement and correction of interfractional setup errors before each fraction using CBCT could help to improve the precision of radiotherapy. The intrafractional setup error variations are obvious in head and neck tumors and should be taken into account during treatment planning. The intrafractional setup errors in trunk tumors need further study.