title: Dosimetric impact of reduced nozzle-to-isocenter distance in intensity-modulated proton therapy of intracranial tumors in combined proton-carbon fixed-nozzle treatment facilities.
abstract: BACKGROUND
  In combined proton-carbon fixed-nozzle treatment facilities with raster scanning delivery, the scattering of proton pencil beams caused by nozzle elements and the relatively large nozzle-to-isocenter distance cause a beam broadening. This may pose limitations to the achievable dose conformity. One way to counteract this effect is by delivering the treatment in a position closer to the nozzle than the room isocenter. Purpose of this study was to assess the potential dosimetric benefit of such solution, in terms of dose conformity and normal tissue sparing, in intensity-modulated proton therapy (IMPT) of intracranial tumors.
  
  
  METHODS
  For 12 patients with intracranial lesions, IMPT-plans were created at two treatment positions: nozzle-to-treatment-isocenter distance: 100 cm (room isocenter) and nozzle-to-treatment-isocenter distance: 60 cm. The resulting plans were compared in terms of dose distributions, dose-volume histograms and selected dosimetric indexes.
  
  
  RESULTS
  With comparable target coverage, statistically significant normal tissue sparing was achieved through the reduction of the distance between nozzle and treatment isocenter. The decrease in mean dose (Dmean) was 12.5% to the whole brain, 16.2% to the brainstem, 9.7% and 15.4% to the temporal lobes, 10.0% and 12.9% to the hippocampi, 11.8% and 12.5% to the optic nerves and 0.2% to the chiasm. The volume receiving at least 10% of the prescribed dose (V10%) was reduced by more than 10% for most organs at risk (OARs). The maximum dose (Dnear-max) values to most OARs remained without significant difference.
  
  
  CONCLUSIONS
  A reduced distance between nozzle and treatment isocenter leads to steeper lateral dose gradients and significantly reduces the volume of OARs adjacent to the target, which receives low to intermediate doses. Technical solutions shifting the treatment isocenter closer to the nozzle should be considered in clinical situations, where critical OARs are adjacent to the beam channel and where the integral dose should be minimized.
authors.affiliation: Department of Radiotherapy and Radiation Oncology, Philipps-University of Marburg, Marburg, Baldingerstrasse 35043, Germany. andrea.wittig@med.uni-marburg.de.
authors.forename: Urszula
authors.initials: U
authors.lastname: Jelen
authors.forename: Marta E
authors.initials: ME
authors.lastname: Bubula
authors.forename: Filippo
authors.initials: F
authors.lastname: Ammazzalorso
authors.forename: Rita
authors.initials: R
authors.lastname: Engenhart-Cabillic
authors.forename: Uli
authors.initials: U
authors.lastname: Weber
authors.forename: Andrea
authors.initials: A
authors.lastname: Wittig
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001932:Brain Neoplasms / Q000532:radiotherapy*; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D061766:Proton Therapy / Q000379:methods*; D011874:Radiometry; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D055815:Young Adult
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