title: Dosimetric consequences of translational and rotational errors in frame-less image-guided radiosurgery.
abstract: BACKGROUND
  To investigate geometric and dosimetric accuracy of frame-less image-guided radiosurgery (IG-RS) for brain metastases.
  
  
  METHODS
  Single fraction IG-RS was practiced in 72 patients with 98 brain metastases. Patient positioning and immobilization used either double- (n = 71) or single-layer (n = 27) thermoplastic masks. Pre-treatment set-up errors (n = 98) were evaluated with cone-beam CT (CBCT) based image-guidance (IG) and were corrected in six degrees of freedom without an action level. CBCT imaging after treatment measured intra-fractional errors (n = 64). Pre- and post-treatment errors were simulated in the treatment planning system and target coverage and dose conformity were evaluated. Three scenarios of 0 mm, 1 mm and 2 mm GTV-to-PTV (gross tumor volume, planning target volume) safety margins (SM) were simulated.
  
  
  RESULTS
  Errors prior to IG were 3.9 mm ± 1.7 mm (3D vector) and the maximum rotational error was 1.7° ± 0.8° on average. The post-treatment 3D error was 0.9 mm ± 0.6 mm. No differences between double- and single-layer masks were observed. Intra-fractional errors were significantly correlated with the total treatment time with 0.7 mm ± 0.5 mm and 1.2 mm ± 0.7 mm for treatment times ≤23 minutes and >23 minutes (p<0.01), respectively. Simulation of RS without image-guidance reduced target coverage and conformity to 75% ± 19% and 60% ± 25% of planned values. Each 3D set-up error of 1 mm decreased target coverage and dose conformity by 6% and 10% on average, respectively, with a large inter-patient variability. Pre-treatment correction of translations only but not rotations did not affect target coverage and conformity. Post-treatment errors reduced target coverage by >5% in 14% of the patients. A 1 mm safety margin fully compensated intra-fractional patient motion.
  
  
  CONCLUSIONS
  IG-RS with online correction of translational errors achieves high geometric and dosimetric accuracy. Intra-fractional errors decrease target coverage and conformity unless compensated with appropriate safety margins.
authors.affiliation: Department of Radiation Oncology, University of Würzburg, Würzburg, Germany. Guckenberger_M@klinik.uni-wuerzburg.de
authors.forename: Matthias
authors.initials: M
authors.lastname: Guckenberger
authors.forename: Johannes
authors.initials: J
authors.lastname: Roesch
authors.forename: Kurt
authors.initials: K
authors.lastname: Baier
authors.forename: Reinhart A
authors.initials: RA
authors.lastname: Sweeney
authors.forename: Michael
authors.initials: M
authors.lastname: Flentje
mesh_terms: D001932:Brain Neoplasms / Q000532:radiotherapy / Q000556:secondary / Q000601:surgery*; D002277:Carcinoma / Q000532:radiotherapy / Q000556:secondary / Q000601:surgery*; D054893:Cone-Beam Computed Tomography; D006801:Humans; D009068:Movement / Q000502:physiology; D009493:Neurosurgery / Q000379:methods; D056888:Patient Positioning / Q000009:adverse effects / Q000379:methods; D061214:Patient Safety; D011829:Radiation Dosage; D011874:Radiometry; D016634:Radiosurgery / Q000009:adverse effects* / Q000379:methods* / Q000592:standards; D011880:Radiotherapy Planning, Computer-Assisted / Q000009:adverse effects; D061106:Radiotherapy Setup Errors; D061089:Radiotherapy, Image-Guided / Q000009:adverse effects / Q000379:methods; D012189:Retrospective Studies; D012399:Rotation / Q000009:adverse effects; D025321:Surgery, Computer-Assisted / Q000009:adverse effects / Q000379:methods