Comparison of secondary neutron dose in proton therapy resulting from the use of a tungsten alloy MLC or a brass collimator system
- Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania 19104 (United States)
Purpose: 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{sup -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 {sup 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 {gamma}-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) x 10{sup -5} Gy/Gy. The neutron dose with brass was (6.4 {+-} 0.7) x 10{sup -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) x 10{sup -6} Gy/Gy and (6.3 {+-} 0.7) x 10{sup -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.
- OSTI ID:
- 22098679
- Journal Information:
- Medical Physics, Vol. 38, Issue 11; Other Information: (c) 2011 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
Similar Records
Assessment of out-of-field absorbed dose and equivalent dose in proton fields
Characterization of a mini-multileaf collimator in a proton beamline
Related Subjects
62 RADIOLOGY AND NUCLEAR MEDICINE
BRASS
COBALT 60
COLLIMATORS
COMPUTERIZED SIMULATION
DOSIMETRY
ENERGY SPECTRA
IONIZATION CHAMBERS
IRRADIATION
MEV RANGE 100-1000
MONTE CARLO METHOD
NEUTRON SOURCES
NEUTRONS
PATIENTS
PROTON BEAMS
PROTONS
RADIATION DOSES
RADIOTHERAPY
TUNGSTEN ALLOYS
X RADIATION