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


METHODS
Prostate 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.


RESULTS
The set-up error was less than 3.0mm 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 3.6mm (AP) and 3.5mm (CC). Patient-independent normal distributed set-up errors would result in margins of 4.3mm (AP) and 4.0mm (CC) to ensure complete CTV inclusion in the PTV with 90% probability.


CONCLUSIONS
Internal prostate motion during the set-up procedure was the main contributor to residual set-up errors.
Poulsen
Per Rugaard
PR
Department of Medical Physics, Aarhus University Hospital, Denmark. prp@as.aaa.dk
Muren
Ludvig Paul
LP
Høyer
Morten
M
D006801:Humans; D008297:Male; D009038:Motion; D009862:Online Systems; D011467:Prostate / Q000000981:diagnostic imaging* / Q000502:physiology; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D014057:Tomography, X-Ray Computed