ABSTRACT
Objective To assess set-up errors measured with kilovoltage cone-beam CT (KV-CBCT), and the impact of online corrections on margins required to account for set-up variability during IMRT for patients with prostate cancer. Methods Seven patients with prostate cancer undergoing IMRT were enrolled onto the study. The KV-CBCT scans were acquired at least twice weekly. After initial set-up using the skin marks, a CBCT scan was acquired and registered with the planning CT to determine the setup errors using an auto grey-scale registration software. Corrections would be made by moving the table if the setup errors were considered clinically significant ( i. e. , ＞ 2 mm). A second CBCT scan was acquired immediately after the corrections to evaluate the residual error. PTV margins were derived to account for the measured set-up errors and residual errors determined for this group of patients. Results 197 KV-CBCT images in total were acquired. The random and systematic positioning errors and calculated PTV margins without correction in mm were:a) Lateral 3. 1,2. 1,9. 3;b) Longitudinal 1.5, 1.8, 5. 1 ;c) Vertical 4. 2,3.7, 13.0. The random and systematic positioning errors and calculated PTV margin with correction in mm were:a) Lateral 1.1,0. 9, 3.4;b) Longitudinal 0. 7, 1.1, 2. 5;c) Vertical 1.1, 1.3, 3.7. Conclusions With the guidance of online KV-CBCT, set-up errors could be reduced significantly for patients with prostate cancer receiving IMRT. The margin required after online CBCT correction for the patients enrolled in the study would be appoximatively 3-4 mm.