title: Theoretical Benefits of Dynamic Collimation in Pencil Beam Scanning Proton Therapy for Brain Tumors: Dosimetric and Radiobiological Metrics.
abstract: OBJECTIVE
  To quantify the dosimetric benefit of using a dynamic collimation system (DCS) for penumbra reduction during the treatment of brain tumors by pencil beam scanning proton therapy (PBS PT).
  
  
  METHODS
  Collimated and uncollimated brain treatment plans were created for 5 patients previously treated with PBS PT and retrospectively enrolled in an institutional review board-approved study. The in-house treatment planning system, RDX, was used to generate the plans because it is capable of modeling both collimated and uncollimated beamlets. The clinically delivered plans were reproduced with uncollimated plans in terms of target coverage and organ at risk (OAR) sparing to ensure a clinically relevant starting point, and collimated plans were generated to improve the OAR sparing while maintaining target coverage. Physical and biological comparison metrics, such as dose distribution conformity, mean and maximum doses, normal tissue complication probability, and risk of secondary brain cancer, were used to evaluate the plans.
  
  
  RESULTS
  The DCS systematically improved the dose distribution conformity while preserving the target coverage. The average reduction of the mean dose to the 10-mm ring surrounding the target and the healthy brain were 13.7% (95% confidence interval [CI] 11.6%-15.7%; P<.0001) and 25.1% (95% CI 16.8%-33.4%; P<.001), respectively. This yielded an average reduction of 24.8% (95% CI 0.8%-48.8%; P<.05) for the brain necrosis normal tissue complication probability using the Flickinger model, and 25.1% (95% CI 16.8%-33.4%; P<.001) for the risk of secondary brain cancer. A general improvement of the OAR sparing was also observed.
  
  
  CONCLUSIONS
  The lateral penumbra reduction afforded by the DCS increases the normal tissue sparing capabilities of PBS PT for brain cancer treatment while preserving target coverage.
authors.affiliation: Department of Radiation Oncology, University of Iowa Hospitals and Clinics, Iowa City, Iowa. Electronic address: alexandra-moignier@uiowa.edu.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Moignier
authors.affiliation: Department of Radiation Oncology, University of Iowa Hospitals and Clinics, Iowa City, Iowa.
authors.forename: Edgar
authors.initials: E
authors.lastname: Gelover
authors.affiliation: Department of Radiation Oncology, University of Iowa Hospitals and Clinics, Iowa City, Iowa.
authors.forename: Dongxu
authors.initials: D
authors.lastname: Wang
authors.affiliation: Department of Radiation Oncology, University of Iowa Hospitals and Clinics, Iowa City, Iowa.
authors.forename: Blake
authors.initials: B
authors.lastname: Smith
authors.affiliation: Department of Radiation Oncology, University of Iowa Hospitals and Clinics, Iowa City, Iowa.
authors.forename: Ryan
authors.initials: R
authors.lastname: Flynn
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Maura
authors.initials: M
authors.lastname: Kirk
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Liyong
authors.initials: L
authors.lastname: Lin
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Timothy
authors.initials: T
authors.lastname: Solberg
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Alexander
authors.initials: A
authors.lastname: Lin
authors.affiliation: Department of Radiation Oncology, University of Iowa Hospitals and Clinics, Iowa City, Iowa.
authors.forename: Daniel
authors.initials: D
authors.lastname: Hyer
mesh_terms: D000465:Algorithms; D001921:Brain / Q000473:pathology / Q000528:radiation effects; D001932:Brain Neoplasms / Q000532:radiotherapy*; D006801:Humans; D009336:Necrosis; D059351:Organ Sparing Treatments / Q000295:instrumentation / Q000379:methods; D058958:Organs at Risk / Q000528:radiation effects; D061766:Proton Therapy / Q000295:instrumentation / Q000379:methods*; D011832:Radiation Injuries / Q000517:prevention & control; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods; D012189:Retrospective Studies; D012542:Scattering, Radiation; D013997:Time Factors