title: Reduced neutron production through use of a flattening-filter-free accelerator.
abstract: OBJECTIVE
  To measure and compare neutron fluence around an accelerator operating at 18 MV, both with the flattening filter present (FF mode) and absent (flattening-filter-free [FFF] mode).
  
  
  METHODS
  The neutron fluence was measured at several locations in the patient plane using gold foil activation in neutron moderators. Differences in neutron fluence between the FF and FFF mode were assessed in three frameworks: (1) measured per monitor unit of machine-on time, (2) determined per dose on the central axis, and (3) determined for a complete course of prostate intensity-modulated radiotherapy.
  
  
  RESULTS
  Neutron fluence per monitor unit was approximately 20% lower when the accelerator was operated in the FFF mode than when it was in FF mode. The total amount of neutron fluence that would be obtained during the entire course of prostate intensity-modulate radiotherapy was 69% lower when the accelerator was operated in the FFF mode than when it was in the FF mode. This reduction in neutron fluence would correspond to a drastic reduction in the neutron dose equivalent received by the patient as a byproduct of high-energy radiotherapy. It would also correspond to a reduction in activation within the treatment vault and subsequent exposure to radiation therapists.
  
  
  CONCLUSIONS
  When feasible, operating the accelerator without a FF will benefit both patients and radiation therapists by reducing the number of unwanted neutrons and resultant exposure. This reduces the risk of negative effects from such exposure (e.g., second cancers).
authors.affiliation: Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030, USA. sfkry@mdanderson.org
authors.forename: Stephen F
authors.initials: SF
authors.lastname: Kry
authors.forename: Uwe
authors.initials: U
authors.lastname: Titt
authors.forename: Falk
authors.initials: F
authors.lastname: Pönisch
authors.forename: Oleg N
authors.initials: ON
authors.lastname: Vassiliev
authors.forename: Mohammad
authors.initials: M
authors.lastname: Salehpour
authors.forename: Michael
authors.initials: M
authors.lastname: Gillin
authors.forename: Radhe
authors.initials: R
authors.lastname: Mohan
mesh_terms: D005374:Filtration / Q000295:instrumentation; D006801:Humans; D008297:Male; D009502:Neutrons; D016273:Occupational Exposure / Q000517:prevention & control; D010315:Particle Accelerators / Q000295:instrumentation*; D055585:Physical Phenomena; D010825:Physics; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D018787:Radiation Oncology; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation*