Program Information
Prospective Study of the Setup Accuracy and Margins in Linear Accelerator Based Cranial Stereotactic Radiosurgery
H Sha1*, L Nedialkova2 , J McKenna3 , J DeWyngaert4 , G Jozsef5 , (1) Cornell Univ Medical Ctr/New York Hosp, New York, NY, (2) New York Presbyterian Hospital, New York, NY, (3) New York Presbyterian Hospital, New York, NY, (4) New York Presbyterian Hospital/Weill Cornell Medical College, New York, NY, (5) New York Weill Cornell Medical Ctr, New York, NY
Presentations
SU-I-GPD-J-47 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: In this study we analyze the linac-based stereotactic radiosurgery (SRS) intra-fractional setup variations in our institution. The corresponding setup margin is estimated and compared with the current CTV to PTV expansion.
Methods: In our institution, single fraction cranial SRS procedures are performed on Varian TrueBeam STx linear accelerator equipped with HD120 MLC. Patients are immobilized framelessly using Brainlab thermoplastic masks and BrainLab couch. Their isocenters are localized and verified using Brainlab ExacTrac dual stereoscopic x-ray system.A total of 351 ExacTrac calculated isocenter locations (shifts) were obtained during the treatment of 30 consecutive patients with 60 isocenters and 74 tumors. Our action limit required couch position adjustment and re-measurement if a shift exceeded 0.7 mm and 0.5° translation and rotation. The overall average and standard deviations of the setup variations were calculated, and the systematic and random errors were obtained according to the calculation method proposed by van Herk. His margin formula was then used to estimate the intrafractional setup margin.
Results: The mean intra-fractional shifts ± SD in the medial-lateral (ML), superior-inferior (SI), and anterior-posterior (AP) directions were 0.03 ± 0.20, 0.00 ± 0.20, and 0.01 ± 0.16 mm, respectively. The systematic setup errors were 0.14, 0.18, and 0.11 mm, and the random errors were 0.21, 0.22, and 0.18 mm along three orthogonal directions respectively. The margin formula resulted in a margin of 0.50, 0.61, and 0.41 mm in the principal directions and 0.39 ± 0.19 mm for 3D distance, somewhat less than the institutional action limit.
Conclusion: The margins derived from ExacTrac x-ray data verified that our institutional CTV-to-PTV margin of 2 mm is sufficient to deliver SRS single fraction treatment accurately, even if we include the ~0.3 mm uncertainty of the calibration of the ExacTrac.
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