Objective: To determine inter-fraction errors from setup and target motion of prostate cancer treated with intensity modulated
radiation therapy [IMRT] by using on-board cone beam computed tomography [CBCT].

Materials and Methods: We analyzed retrospective data of on-board CBCT images of 14 prostate cancers over 395 sessions. We performed image matching of CBCT and planning CT, using two methods, prostate soft tissue matching to determine total error (tumor motion and setup error), and bone matching for setup error only. Target motion values were estimated by subtracting setup error from total error. Systematic error of target motion and setup error were calculated from individual mean values.

Results: Inter-fraction errors for target motion were 0.9+0.6 mm (0 to 8 mm), 0.7+0.5 mm (0 to 6 mm), and 0.5+0.3 mm (0 to 6 mm) for vertical [Vrt], longitudinal [Lng], and lateral [Lat] directions, respectively. Inter-fraction of setup errors were 2.7+1.2 mm (0 to 13 mm) for Vrt direction, 2.2+1.0 mm (0 to 8 mm) for Lng, and 2.8+1.6 mm (0 to 14 mm) for Lat.

Conclusion: With bone matching, planning target volume [PTV] margins should be at least 6 mm in all directions. Systematic setup errors are within 3 mm in all directions.

Keywords: Target motion, Setup error, Inter-fraction, Cone-beam CT, Prostate cancer