Measurement of the distribution of neutrons produced by a 15 MV linear accelerator in a solid water phantom using CR-39 detectors
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Abstract
Background: In high energy photon therapy with >10 MV x-rays, neutrons are produced from photonuclear reactions between high energy photons and high atomic number materials in the treatment head. Although neutron dose is expected to be relatively small compared to the primary photon dose to the target, neutrons have high quality factors that are associated with the increased secondary cancer risk of the treated patient. Due to the attenuation of neutrons by the patient’s body, the distribution of neutron doses at different positions in the patient should be determined for the assessment of organ-specific secondary cancer risks.
Objectives: To determine the distribution of neutrons from a 15 MV linear accelerator at different lateral distances from the isocenter and at different depths in a solid water phantom, as a mimic for the patient body.
Materials and methods: The distribution of neutrons was measured with BARYOTRAK CR-39 detectors in term of nuclear track densities in the CR-39 detectors. A half of the detector’s surface area was covered with a boron converter and a polyethylene radiator for thermal and fast neutron measurement, while the other half had no boron converter making it only sensitive to fast neutrons. The detectors were placed in a solid water phantom at 0, 5, 10 and 15 cm lateral distances from the isocenter and at 0, 3, 7, 11, 15 and 18 cm depths from the phantom surface. The detectors were irradiated with 15 MV photon beams at 0° gantry angle for the field size of 20x20 cm2.
Results: The nuclear track density initially increased with depth, reached a maximum at 3 cm depth and decreased with depth beyond the depth of the maximum. The contributions from fast and thermal neutrons at shallow depths were competitive while at large depths most of neutrons became thermalized. The lateral distribution of the track density had a maximum at the central axis. Thermal neutrons were responsible for this behavior. In contrast, nuclear track densities generated by fast neutrons were nearly constant with the off-axis distance.
Conclusion: Nuclear track densities generated by neutrons varied with the depth and the lateral distance from the isocenter. The conversion coefficients from nuclear track density to dose should consider the energy spectrum of neutrons especially at shallow depths.
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