title: Optimization of isocenter location for intensity modulated stereotactic treatment of small intracranial targets.
abstract: OBJECTIVE
  To quantify the impact of isocenter location on treatment plan quality for intensity-modulated stereotactic treatment of small intracranial lesions.
  
  
  METHODS
  For 18 patients previously treated by stereotactic-intensity modulated radiosurgery (IMRS) or intensity-modulated radiation therapy (IMRT), a retrospective virtual planning study was conducted wherein the impact of isocenter location on plan quality was measured. Treatment indications studied included six arteriovenous malformations, six acoustic neuromas, and six intracranial metastases, ranging in volume from 0.71 to 3.21 cm(3) (mean = 2.26 cm(3)), 1.08 to 2.84 cm(3) (mean = 1.73 cm(3)), and 0.19 to 2.30 cm(3) (mean = 0.79 cm(3)), respectively. Variation of isocenter location causes the geometric grid of pencil beams into which the target is segmented for intensity-modulated treatment to be altered. The impact of this pencil-beam-grid redefinition on achievable conformity index was quantified for three collimators (Varian Millennium 120; BrainLab MM3; Nomos binary Mimic) and three treatment planning systems (TPS; Varian Eclipse v6.5; BrainLab BrainScan v5.31; Best-Nomos Corvus v6.2), resulting in the evaluation of 3,446 treatment plans.
  
  
  RESULTS
  For all patients, collimator, and TPS combinations studied, a significant variation in plan quality was observed as a function of isocenter and pencil-beam-grid relocation. Optimization of isocenter location resulted in treatment plan conformity variations as large as 109% (min = 15%, mean = 51%, max = 109%).
  
  
  CONCLUSIONS
  Optimization of isocenter location for IMRT/IMRS treatment of small intracranial lesions in which pencil-beam dimensions are comparable to target dimensions, can result in significant improvements in treatment plan quality.
authors.affiliation: Department of Radiation Oncology, University of Utah-Huntsman Cancer Institute, Salt Lake City, USA.
authors.forename: Bill J
authors.initials: BJ
authors.lastname: Salter
authors.forename: Martin
authors.initials: M
authors.lastname: Fuss
authors.forename: Vikren
authors.initials: V
authors.lastname: Sarkar
authors.forename: Brian
authors.initials: B
authors.lastname: Wang
authors.forename: Prema
authors.initials: P
authors.lastname: Rassiah-Szegedi
authors.forename: Niko
authors.initials: N
authors.lastname: Papanikolaou
authors.forename: Scott
authors.initials: S
authors.lastname: Hollingshaus
authors.forename: Dennis C
authors.initials: DC
authors.lastname: Shrieve
mesh_terms: D001932:Brain Neoplasms / Q000532:radiotherapy* / Q000601:surgery; D016371:Cranial Irradiation / Q000379:methods; D006801:Humans; D002538:Intracranial Arteriovenous Malformations / Q000532:radiotherapy* / Q000601:surgery; D009464:Neuroma, Acoustic / Q000532:radiotherapy* / Q000601:surgery; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods / Q000592:standards; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods / Q000592:standards*; D012016:Reference Values; D012189:Retrospective Studies