Feasibility of gold fiducial markers as a surrogate for GTV position in image-guided radiotherapy of rectal cancer.

Large interfraction errors of the COG and the fiducials relative to bony anatomy were found. Therefore, despite the observed fiducial displacement relative to the COG, the use of fiducials as a surrogate for GTV position reduces the required margins in the AP and CC direction for a GTV boost using image-guided radiotherapy of rectal cancer. This reduction in margin may be larger in patients with tumors located in the mid- and upper rectum compared to the lower rectum.