title: Comparison of secondary neutron dose in proton therapy resulting from the use of a tungsten alloy MLC or a brass collimator system.
abstract: OBJECTIVE
  To apply the dual ionization chamber method for mixed radiation fields to an accurate comparison of the secondary neutron dose arising from the use of a tungsten alloy multileaf collimator (MLC) as opposed to a brass collimator system for defining the shape of a therapeutic proton field.
  
  
  METHODS
  Hydrogenous and nonhydrogenous ionization chambers were constructed with large volumes to enable measurements of absorbed doses below 10(-4) Gy in mixed radiation fields using the dual ionization chamber method for mixed-field dosimetry. Neutron dose measurements were made with a nominal 230 MeV proton beam incident on a closed tungsten alloy MLC and a solid brass block. The chambers were cross-calibrated against a (60)Co-calibrated Farmer chamber in water using a 6 MV x-ray beam and Monte Carlo simulations were performed to account for variations in ionization chamber response due to differences in secondary neutron energy spectra.
  
  
  RESULTS
  The neutron and combined proton plus γ-ray absorbed doses are shown to be nearly equivalent downstream from either a closed tungsten alloy MLC or a solid brass block. At 10 cm downstream from the distal edge of the collimating material the neutron dose from the closed MLC was (5.3 ± 0.4) × 10(- 5) Gy/Gy. The neutron dose with brass was (6.4 ± 0.7) × 10(- 5) Gy/Gy. Further from the secondary neutron source, at 50 cm, the neutron doses remain close for both the MLC and brass block at (6.9 ± 0.6) × 10(- 6) Gy/Gy and (6.3 ± 0.7) × 10(- 6) Gy/Gy, respectively.
  
  
  CONCLUSIONS
  The dual ionization chamber method is suitable for measuring secondary neutron doses resulting from proton irradiation. The results of measurements downstream from a closed tungsten alloy MLC and a brass block indicate that, even in an overly pessimistic worst-case scenario, secondary neutron production in a tungsten alloy MLC leads to absorbed doses that are nearly equivalent to those seen from brass collimators. Therefore, the choice of tungsten alloy in constructing the leaves of a proton MLC is appropriate, and does not lead to a substantial increase in the secondary neutron dose to the patient compared to that generated in a brass collimator.
authors: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors: Eric S
authors: ES
authors: Diffenderfer
authors: Christopher G
authors: CG
authors: Ainsley
authors: Maura L
authors: ML
authors: Kirk
authors: James E
authors: JE
authors: McDonough
authors: Richard L
authors: RL
authors: Maughan
mesh_terms: D000497:Alloys; D003300:Copper; D009502:Neutrons; D061766:Proton Therapy; D011874:Radiometry / Q000295:instrumentation*; D011878:Radiotherapy / Q000379:methods*; D011879:Radiotherapy Dosage; D014414:Tungsten; D015032:Zinc
chemical_list: D000497:Alloys; C048399:brass; D003300:Copper; D015032:Zinc; D014414:Tungsten