Document Type

Conference Proceeding

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Publication Title

Medical Physics


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.





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