title: Using field size factors to characterize the in-air fluence of a proton machine with a range shifter.
abstract: BACKGROUND
  The range shifter (RS) is used to treat shallow tumors for a proton pencil beam scanning system (PBS). Adding RS certainly complicates the commissioning of the treatment planning system (TPS) because the spot sizes are significantly enlarged with RS. In this work, we present an efficient method to configure a commercial TPS for a PBS system with a fixed RS.
  
  
  METHODS
  By combining a spiral delivery with customized control points, we were able to significantly improve measurement efficiency and obtain 250 field size factors (FSF) within three hours. The measured FSFs were used to characterize the proton fluence and fit the parameters for the double-Gaussian fluence model used in the TPS. Extensive validation was performed using FSFs measured in air and in water, absolute doses of spread-out Bragg peak (SOBP) fields, and the dose measurements carried out for patient-specific quality assurance (QA).
  
  
  RESULTS
  The measured in-air FSFs agreed with the model's prediction within 3% for all 250 FSFs, and within 2 for 94% of the FSFs. The agreement between model's prediction and measurement was within 2% for the in-air and in-water FSFs and the absolute doses for SOBP beams. The patient-specific QA of 113 fields showed an excellent gamma passing rates (96.95 ± 2.51%) for the absolute dose comparisons with gamma criteria of 2 mm and 2%.
  
  
  CONCLUSIONS
  The excellent agreement between the model's prediction and measurements proved the efficiency and accuracy of the proposed method of using FSFs to characterize the proton fluence and configure the TPS for a PBS system with fixed RS.
authors.affiliation: Department of Radiation Oncology, Mayo Clinic Arizona, Phoenix, AZ, USA. Shen.Jiajian@Mayo.edu.
authors.forename: Jiajian
authors.identifier: http://orcid.org/0000-0003-0439-2075
authors.initials: J
authors.lastname: Shen
authors.affiliation: Department of Radiation Oncology, Mayo Clinic Arizona, Phoenix, AZ, USA.
authors.forename: Jarrod M
authors.initials: JM
authors.lastname: Lentz
authors.affiliation: Department of Radiation Oncology, Mayo Clinic Arizona, Phoenix, AZ, USA.
authors.forename: Yanle
authors.initials: Y
authors.lastname: Hu
authors.affiliation: Department of Radiation Oncology, Mayo Clinic Arizona, Phoenix, AZ, USA.
authors.forename: Wei
authors.initials: W
authors.lastname: Liu
authors.affiliation: Department of Radiation Oncology, Mayo Clinic Arizona, Phoenix, AZ, USA.
authors.forename: Danairis Hernandez
authors.initials: DH
authors.lastname: Morales
authors.affiliation: Department of Radiation Oncology, Mayo Clinic Arizona, Phoenix, AZ, USA.
authors.forename: Joshua B
authors.initials: JB
authors.lastname: Stoker
authors.affiliation: Department of Radiation Oncology, Mayo Clinic Arizona, Phoenix, AZ, USA.
authors.forename: Martin
authors.initials: M
authors.lastname: Bues
mesh_terms: D000465:Algorithms; D006801:Humans; D009010:Monte Carlo Method; D016011:Normal Distribution; D061766:Proton Therapy; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*
keywords: Double Gaussians fluence; Field size factor; Proton pencil beam scanning; Range shifter; TPS commissioning