Neutron Spectral Measurement at the Proton Therapy Room of the National Cancer Center of Korea

Abstract

The neutrons, which are mainly induced from a beam modulator and two beam flattening scatters bombarded by high energetic protons inside the gantry of the proton therapy unit of the National Cancer Center (NCC) of Korea, were measured by using the Bonner Sphere (BS) of the Korea Atomic Energy Research Institute (KAERI). Protons were accelerated to 218 MeV using a cyclotron and guided to a gantry for modulating of the beam size and energy. The modulated proton beam irradiated the surface of a water phantom to simulate a condition of a proton therapy. Neutron spectral measurements were performed at two proton therapy rooms with a rotational and a fixed gantry respectively for different beam directions of the proton beam. The fluence average energies were 3.14 and 2.92 MeV at the two measuring positions respectively and there was a significant fraction of low energy neutrons under 0.5 eV. Dose equivalent rates were consistent to within 10% with the survey data measured by a neutron remmeter. Two kinds of neutron spectra were prepared to unfold the BS measurement data and the difference in the unfolding result for the two cases was not big for some dosimetric data.

Secondary neutron dose measurement for proton eye treatment using an eye snout with a borated neutron absorber

“…The detectable energy with CR-39 ranges from 100 KeV to 20 MeV, which fully covers most of the secondary neutrons in proton therapy [15,16]. Therefore, the dose response of the relatively fast neutrons with energies of more than 20 MeV and the scattered neutrons with energies of less than 100 KeV are relatively low compared to that of relatively low neutrons with energies ranging from the 100 KeV to 20 MeV.…”

“…We measured the secondary neutron dose equivalent per proton absorbed dose in a proton eye treatment unit in single scattering mode using a CR-39 neutron detector [10-16] at the NCC, Korea. We expected that the secondary neutrons from the beam delivery system could be reduced by using a neutron absorber before reaching the patient’s body.…”