title: Dosimetric comparison between proton and photon beams in the moving gap region in cranio-spinal irradiation (CSI).
abstract: OBJECTIVE
  To investigate the moving gap region dosimetry in proton beam cranio-spinal irradiation (CSI) to provide optimal dose uniformity across the treatment volume.
  
  
  METHODS
  Proton beams of ranges 11.6 cm and 16 cm are used for the spine and the brain fields, respectively. Beam profiles for a 30 cm snout are first matched at the 50% level (hot match) on the computer. Feathering is simulated by shifting the dose profiles by a known distance two successive times to simulate a 2 × feathering scheme. The process is repeated for 2 mm and 4 mm gaps. Similar procedures are used to determine the dose profiles in the moving gap for a series of gap widths, 0-10 mm, and feathering step sizes, 4-10 mm, for a Varian iX 6MV beam. The proton and photon dose profiles in the moving gap region are compared.
  
  
  RESULTS
  The dose profiles in the moving gap exhibit valleys and peaks in both proton and photon beam CSI. The dose in the moving gap for protons is around 100% or higher for 0 mm gap, for both 5 and 10 mm feathering step sizes. When the field gap is comparable or larger than the penumbra, dose minima as low as 66% is obtained. The dosimetric characteristics for 6 MV photon beams can be made similar to those of the protons by appropriately combining gap width and feathering step size.
  
  
  CONCLUSIONS
  The dose in the moving gap region is determined by the lateral penumbras, the width of the gap and the feathering step size. The dose decreases with increasing gap width or decreasing feathering step size. The dosimetric characteristics are similar for photon and proton beams. However, proton CSI has virtually no exit dose and is beneficial for pediatric patients, whereas with photon beams the whole lung and abdomen receive non-negligible exit dose.
authors.affiliation: IU Health Proton Therapy Center, Bloomington, Indiana 47408, USA. ccheng1@iuhealth.org
authors.forename: Chee-Wai
authors.initials: CW
authors.lastname: Cheng
authors.forename: Indra J
authors.initials: IJ
authors.lastname: Das
authors.forename: Shiv P
authors.initials: SP
authors.lastname: Srivastava
authors.forename: Li
authors.initials: L
authors.lastname: Zhao
authors.forename: Mark
authors.initials: M
authors.lastname: Wolanski
authors.forename: Joseph
authors.initials: J
authors.lastname: Simmons
authors.forename: Peter A S
authors.initials: PA
authors.lastname: Johnstone
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Buchsbaum
mesh_terms: D001921:Brain / Q000473:pathology / Q000502:physiology; D002648:Child; D061888:Craniospinal Irradiation / Q000009:adverse effects / Q000379:methods*; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D009038:Motion; D058958:Organs at Risk / Q000473:pathology; D017785:Photons / Q000627:therapeutic use*; D061766:Proton Therapy / Q000009:adverse effects / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D013131:Spine / Q000473:pathology / Q000502:physiology; D013909:Thorax / Q000473:pathology; D011522:Protons
chemical_list: D011522:Protons