title: Margin of error for a frameless image guided radiosurgery system: Direct confirmation based on posttreatment MRI scans.
abstract: OBJECTIVE
  To report on radiosurgery delivery positioning accuracy in the treatment of tremor patients with frameless image guided radiosurgery using the linear accelerator (LINAC) based ExacTrac system and to describe quality assurance (QA) procedures used.
  
  
  METHODS
  Between 2010 and 2015, 20 patients underwent radiosurgical thalamotomy targeting the ventral intermediate nucleus for the treatment of severe tremor. The median prescription dose was 140 Gy (range, 120-145 Gy) in a single fraction. The median maximum dose was 156 Gy (range, 136-162 Gy). All treatment planning was performed with the iPlan system using a 4-mm circular cone with multiple arcs. Before each treatment, QA procedures were performed, including the imaging system. As a result of the extremely high dose delivered in a single fraction, a well-defined circular mark developed on the posttreatment magnetic resonance imaging (MRI). Eight of these 20 patients were selected to evaluate treatment localization errors because their circular marks were available in posttreatment MRI. In this study, the localization error is defined as the distance between the center of the intended target and the center of the posttreatment mark.
  
  
  RESULTS
  The mean error of distance was found to be 1.1 mm (range, 0.4-1.5 mm). The mean errors for the left-right, anteroposterior, and superoinferior directions are 0.5 mm, 0.6 mm, and 0.7 mm, respectively.
  
  
  CONCLUSIONS
  The result reported in this study includes all tremor patients treated at our institution when their posttreatment MRI data were available for study. It represents a direct confirmation of target positioning accuracy in radiosurgery with a LINAC-based frameless system and its limitations. This level of accuracy is only achievable with an appropriate QA program in place for a LINAC-based frameless radiosurgery system.
authors.affiliation: Department of Radiation Oncology, Vanderbilt University Medical Center, Nashville, Tennessee; Department of Physics and Astronomy, Vanderbilt University, College of Art and sciences, Nashville, Tennessee.
authors.forename: Guozhen
authors.initials: G
authors.lastname: Luo
authors.affiliation: Department of Neurological Surgery, Vanderbilt University Medical Center, Nashville, Tennessee.
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Neimat
authors.affiliation: Department of Radiation Oncology, Vanderbilt University Medical Center, Nashville, Tennessee.
authors.forename: Anthony
authors.initials: A
authors.lastname: Cmelak
authors.affiliation: Department of Radiation Oncology, Vanderbilt University Medical Center, Nashville, Tennessee.
authors.forename: Austin N
authors.initials: AN
authors.lastname: Kirschner
authors.affiliation: Department of Radiation Oncology, Vanderbilt University Medical Center, Nashville, Tennessee.
authors.forename: Albert
authors.initials: A
authors.lastname: Attia
authors.affiliation: Department of Radiation Oncology, Vanderbilt University Medical Center, Nashville, Tennessee.
authors.forename: Manuel
authors.initials: M
authors.lastname: Morales-Paliza
authors.affiliation: Department of Radiation Oncology, Vanderbilt University Medical Center, Nashville, Tennessee; Department of Physics and Astronomy, Vanderbilt University, College of Art and sciences, Nashville, Tennessee. Electronic address: george.ding@vanderbilt.edu.
authors.forename: George X
authors.initials: GX
authors.lastname: Ding
mesh_terms: D001921:Brain / Q000000981:diagnostic imaging; D020329:Essential Tremor / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D000072662:Margins of Excision; D010300:Parkinson Disease / Q000532:radiotherapy*; D011786:Quality Control; D016634:Radiosurgery / Q000295:instrumentation* / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D013788:Thalamus / Q000528:radiation effects