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Residual set-up errors and margins in on-line image-guided prostate localization in radiotherapy
Per Rugaard Poulsen1, Ludvig Paul Muren, Morten Høyer
1Department of Medical Physics, Aarhus University Hospital, Denmark. prp@as.aaa.dk
Background And Purpose:
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.
Materials And 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.
Conclusion:
Internal prostate motion during the set-up procedure was the main contributor to residual set-up errors.
