Isocenter accuracy in frameless stereotactic radiotherapy using implanted fiducials

The stereotactic radiotherapy (SRT) system verifies isocenter accuracy in patient space. In this study, we evaluate isocenter accuracy in frameless SRT using implanted cranial gold markers. We performed frameless SRT on 43 intracranial tumor patients between August 1997 and December 2000. The treatment technique was determined by the tumor shape and volume, and by the location of critical organs. The coordinates of anterior-posterior and lateral port film were inputted to ISOLOC software, which calculated ( 1) the couch moves translation distance required to bring the target point to the isocenter, and ( 2) the intermarker distance comparisons between the CT study and the treatment machine films. We evaluated the isocenter deviation based on the error between orthogonal film target coordinates and isocenter coordinates. The mean treatment isocenter deviations ( x, y, z) were −0.03, 0.14, and −0.04 mm, respectively. The systematic component isocenter standard deviations were 0.28, 0.31, and 0.35 mm (1 SD), respectively, and the random component isocenter standard deviations were 0.53, 0.52, and 0.50 mm (1 SD), respectively. The isocenter accuracy in the frameless SRT–implanted fiducial system is highly reliable and is comparable to that of other stereotactic radiosurgery systems.