Purpose: ExacTrac is clinically used for inter- and intra- fraction shift corrections for hypofractionated prostate patients by registering to implanted fiducial markers. ExacTrac corrections for thirty-three prostate patients were collated and analyzed. Methods: ExacTrac shifts were exported for prostate patients (140 total fractions) who were treated with two 180° partial arcs with beam-on time less than 60seconds/arc. Prior to each treatment fraction, a cone beam computed tomography (CBCT) image was acquired to verify bladder and rectal filling against simulation. Following CBCT, ExacTrac was used for initial prostate localization and shifts were applied using a 6 degree-of-freedom (6D) couch. Following initial localization and in-between arcs, ExacTrac intra-fraction images were acquired and shifts were applied if prostate deviations were outside of 1.4mm/2° tolerance. Magnitude and direction of initial patient setup localization, intra-fraction corrections, frequency of re-starting patient positioning, and number of instances when patients were unable to be treated, were extracted from exported ExacTrac data. Results: The mean initial localization corrections in lateral, longitudinal, and vertical directions were -2.38mm (interquartile range: -5.16mm to -1.36mm), 1.96mm (interquartile range: -0.78 to 4.57mm), and -0.83mm (interquartile range: -4.50mm to 2.73mm) respectively. Mean initial rotations were less than 0.2° in all directions. However, 30% of lateral, 11% of longitudinal, and 8% of vertical initial rotations were greater than 2°. Following initial localization, 134 additional intra-fraction corrections were made. Patient positioning was restarted 22 times and daily treatment postponed 8 times due to inability to setup patients within tolerance or technical issues. Loosening shift tolerances to 2mm/2° would have decreased the number of intrafraction shifts from 134 to 69. Conclusion: The ExacTrac 6D image guide system is a useful tool for daily setup for hypo-fractionated prostate patients. Further analysis is warranted to determine the dosimetric effect of current and potential loosened shift tolerances.
Sandhu R, Knill C, Loughery B, Lin L, Seymour Z. Clinical experience of ExacTrac to guide six degree of freedom fiducial marker based patient positioning for hypo-prostate fractionation. Med Phys. 2022June;49(6):E720.