Residual set-up errors and margins in on-line image-guided prostate localization in radiotherapy

Background and purposeImage-guided on-line correction of the target position allows radiotherapy of prostate cancer with narrow set-up margins. The present study investigated the residual set-up error after on-line prostate localization and its impact on margins.Materials and methodsProstate localization based on two orthogonal X-ray images of gold markers implanted in the prostate was performed with an on-board imager at four treatment sessions for 90 patients. The set-up error in the sagittal plane residual after couch adjustment was evaluated on lateral verification portal images.ResultsThe set-up error was less than 3.0 mm in 92% of the cases in the anterior–posterior (AP) direction and in 95% of the cases in the cranio-caudal (CC) direction. The set-up error was dominated by internal prostate motion taking place during the set-up procedure. Set-up margins were calculated using two formalisms: margins designed to ensure a minimum CTV dose of 95% for 90% of the patient population were 3.6 mm (AP) and 3.5 mm (CC). Patient-independent normal distributed set-up errors would result in margins of 4.3 mm (AP) and 4.0 mm (CC) to ensure complete CTV inclusion in the PTV with 90% probability.ConclusionInternal prostate motion during the set-up procedure was the main contributor to residual set-up errors.