title: Commissioning of output factors for uniform scanning proton beams.
abstract: OBJECTIVE
  Current commercial treatment planning systems are not able to accurately predict output factors and calculate monitor units for proton fields. Patient-specific field output factors are thus determined by either measurements or empirical modeling based on commissioning data. The objective of this study is to commission output factors for uniform scanning beams utilized at the ProCure proton therapy centers.
  
  
  METHODS
  Using water phantoms and a plane parallel ionization chamber, the authors first measured output factors with a fixed 10 cm diameter aperture as a function of proton range and modulation width for clinically available proton beams with ranges between 4 and 31.5 cm and modulation widths between 2 and 15 cm. The authors then measured the output factor as a function of collimated field size at various calibration depths for proton beams of various ranges and modulation widths. The authors further examined the dependence of the output factor on the scanning area (i.e., uncollimated proton field), snout position, and phantom material. An empirical model was developed to calculate the output factor for patient-specific fields and the model-predicted output factors were compared to measurements.
  
  
  RESULTS
  The output factor increased with proton range and field size, and decreased with modulation width. The scanning area and snout position have a small but non-negligible effect on the output factors. The predicted output factors based on the empirical modeling agreed within 2% of measurements for all prostate treatment fields and within 3% for 98.5% of all treatment fields.
  
  
  CONCLUSIONS
  Comprehensive measurements at a large subset of available beam conditions are needed to commission output factors for proton therapy beams. The empirical modeling agrees well with the measured output factor data. This investigation indicates that it is possible to accurately predict output factors and thus eliminate or reduce time-consuming patient-specific output measurements for proton treatments.
authors.affiliation: ProCure Proton Therapy Center, 5901 West Memorial Road, Oklahoma City, Oklahoma 73142, USA. yuanshui.zheng@okc.procure.com
authors.forename: Yuanshui
authors.initials: Y
authors.lastname: Zheng
authors.forename: Eric
authors.initials: E
authors.lastname: Ramirez
authors.forename: Anthony
authors.initials: A
authors.lastname: Mascia
authors.forename: Xiaoning
authors.initials: X
authors.lastname: Ding
authors.forename: Benny
authors.initials: B
authors.lastname: Okoth
authors.forename: Omar
authors.initials: O
authors.lastname: Zeidan
authors.forename: Wen
authors.initials: W
authors.lastname: Hsi
authors.forename: Ben
authors.initials: B
authors.lastname: Harris
authors.forename: Andries N
authors.initials: AN
authors.lastname: Schreuder
authors.forename: Sameer
authors.initials: S
authors.lastname: Keole
mesh_terms: D019047:Phantoms, Imaging; D061766:Proton Therapy; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D011522:Protons
chemical_list: D011522:Protons