title: Setup uncertainties in linear accelerator based stereotactic radiosurgery and a derivation of the corresponding setup margin for treatment planning.
abstract: OBJECTIVE
  In the present study, clinical stereotactic radiosurgery (SRS) setup uncertainties from image-guidance data are analyzed, and the corresponding setup margin is estimated for treatment planning purposes.
  
  
  METHODS
  Patients undergoing single-fraction SRS at our institution were localized using invasive head ring or non-invasive thermoplastic masks. Setup discrepancies were obtained from an in-room x-ray patient position monitoring system. Post treatment re-planning using the measured setup errors was performed in order to estimate the individual target margins sufficient to compensate for the actual setup errors. The formula of setup margin for a general SRS patient population was derived by proposing a correlation between the three-dimensional setup error and the required minimal margin.
  
  
  RESULTS
  Setup errors of 104 brain lesions were analyzed, in which 81 lesions were treated using an invasive head ring, and 23 were treated using non-invasive masks. In the mask cases with image guidance, the translational setup uncertainties achieved the same level as those in the head ring cases. Re-planning results showed that the margins for individual patients could be smaller than the clinical three-dimensional setup errors. The derivation of setup margin adequate to address the patient setup errors was demonstrated by using the arbitrary planning goal of treating 95% of the lesions with sufficient doses.
  
  
  CONCLUSIONS
  With image guidance, the patient setup accuracy of mask cases can be comparable to that of invasive head rings. The SRS setup margin can be derived for a patient population with the proposed margin formula to compensate for the institution-specific setup errors.
authors.affiliation: Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, USA. Electronic address: mutianzhang@unmc.edu.
authors.forename: Mutian
authors.initials: M
authors.lastname: Zhang
authors.affiliation: Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, USA; Department of Radiation Medicine, Northwell Health System, New York, USA. Electronic address: Qinghui.zhang@gmail.com.
authors.forename: Qinghui
authors.initials: Q
authors.lastname: Zhang
authors.affiliation: Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, USA. Electronic address: huagan@gmail.com.
authors.forename: Hua
authors.initials: H
authors.lastname: Gan
authors.affiliation: Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, USA. Electronic address: sl1@unmc.edu.
authors.forename: Sicong
authors.initials: S
authors.lastname: Li
authors.affiliation: Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, USA. Electronic address: szhou@unmc.edu.
authors.forename: Su-min
authors.initials: SM
authors.lastname: Zhou
mesh_terms: D006801:Humans; D010315:Particle Accelerators; D016634:Radiosurgery / Q000295:instrumentation*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061106:Radiotherapy Setup Errors; D061089:Radiotherapy, Image-Guided; D035501:Uncertainty
keywords: Linear accelerator; Margin; Radiosurgery; Setup error