title: Image-guided stereotactic radiosurgery for cranial lesions: large margins compensate for reduced image guidance frequency.
abstract: BACKGROUND
  We investigated patient positioning during radiosurgery of cranial lesions, and calculated clinical target volume (CTV) to planning target volume (PTV) margins using a modified common margin recipe. We simulated CTV-to-PTV margins for reduced image guidance, and repositioning for the first table angle only.
  
  
  METHODS
  Patients were immobilized with a thermoplastic mask. Positioning was verified and corrected using the ExacTrac/Novalis Body. Each patient was repositioned before each beam. A common margin recipe was adapted for estimation of CTV-to-PTV margins. Necessary margins were estimated to correct positioning for the initial table angle only in comparison.
  
  
  RESULTS
  In total, 269 radiosurgery treatments with 967 different-angle setups (mean 3.6 different angles) were performed on 190 patients. Residual translational errors were (one standard deviation) 0.3 mm in left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions, with a mean three-dimensional vector of 0.5 mm. Margins for residual errors after correction were calculated in LR, SI, and AP directions as 0.8 mm. For simulated reduced frequency setup correction, we calculated CTV-to-PTV margins as 1.9, 1.9, and 1.6 mm, respectively.
  
  
  CONCLUSIONS
  The ExacTrac/Novalis Body system allows for accurate positioning of the patient with a residual error comparable to invasive mask fixation. If verification is only performed after initial positioning, adaption of CTV-to-PTV margins should be considered.
authors.affiliation: Department for Radiation Oncology, Charité School of Medicine and University Hospital Berlin, Augustenburger Platz 1, 13353 Berlin, Germany. Harun.Badakhshi@charite.de.
authors.forename: Harun
authors.initials: H
authors.lastname: Badakhshi
authors.forename: David
authors.initials: D
authors.lastname: Kaul
authors.forename: Peter
authors.initials: P
authors.lastname: Wust
authors.forename: Edzard
authors.initials: E
authors.lastname: Wiener
authors.forename: Volker
authors.initials: V
authors.lastname: Budach
authors.forename: Volker
authors.initials: V
authors.lastname: Buadch
authors.forename: Reinhold
authors.initials: R
authors.lastname: Graf
mesh_terms: D001932:Brain Neoplasms / Q000473:pathology / Q000601:surgery*; D015331:Cohort Studies; D006801:Humans; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted; D061089:Radiotherapy, Image-Guided; D047368:Tumor Burden
keywords: Margin; frameless stereotactic radiosurgery; image guidance; image-guided intervention