Purpose: The use of marker-based on-line image guided radiotherapy for prostate cancer has considerably reduced the treatment margins to sub-cm. In this study we have quantified the residual set-up errors remaining after isocenter correction, studied their development during beam delivery and estimated their impact on margins.
Methods and materials: After initial on-line patient set-up based on orthogonal kV x-ray images of implanted fiducial markers, continuous electronic portal imaging was performed during treatment delivery in 10 of 39 treatment sessions for 20 prostate cancer patients. The cranio-caudal (CC) position of the centre-of-mass of the three markers was found using a threshold technique on every single image frame for all patients, typically 12-14 images for 5 treatment beams in every fraction. The CC prostate position was determined relative to its initial position at treatment onset and relative to its planned position within the field aperture. These results allowed determination of the CC intrafraction prostate motion and the intrafraction progression of the geometrical CC error, respectively.
Results: At treatment onset the standard deviation (SD) of the set-up error was 1.0mm in the lateral direction and 1.5mm in the cranio-caudal (CC) direction. It did not depend significantly on the duration of the set-up procedure (mean: 3.0 min, span 1.2-14.6 min). The distribution of CC prostate positions relative to the position at treatment onset broadened from 0 to 1.4mm during the treatment session, while the corresponding CC setup error distribution broadened from 1.5 to 1.9 mm. This broadening means that the necessary CC setup margin increased by around 1mm during the treatment fraction.
Conclusions: Large differences in the intrafraction CC prostate motion patterns were found, however, intrafraction motion only results in a modest additional CC set-up margin of around 1mm relative to the margins needed for the residual set-up error at treatment start.