@misc{oai:repo.qst.go.jp:00063666,
 author = {Matsufuji, Naruhiro and Yonai, Shunsuke and Matsui, Yuki and Matushita, Kaoru and Kohno, Toshiyuki and Kanai, Tatsuaki and 松藤 成弘 and 米内 俊祐 and 松井 祐樹 and 松下 郁 and 河野 俊之 and 金井 達明},
 month = {Oct},
 note = {Elevating energy loss of swift charged particles toward range end shapes unique and favorable dose distribution for treating deep-seated tumors while sparing surrounding normal tissues. The benefit is further enhanced on heavy charged particles such as carbon by their less scatting and resultant shaper dose localization associated with increasing biological effectiveness.
When colliding with target nucleus in a patients body or beam delivery devices, those incident charged particles are broken into some fragment particles. Among them, secondary neutrons are a matter of concern in connection with late side effects after therapeutic irradiation. Their wide spatial distribution together with high biological effectiveness might increase a risk of secondary cancer on normal tissues even far from the irradiated target. The aim of this study is to establish a technique to obtain spatial energy distribution of the secondary neutrons in the field of charged particle radiotherapy and to assess the ambient dose equivalent by the secondary neutrons while comparing with Monte Carlo simulation.
The experiment was performed at HIMAC (Heavy Ion Medical Accelerator in Chiba) of NIRS (National Institute of Radiological Sciences). More than 4,500 patients have been treated so far with carbon ions since 1994. Typical therapeutic irradiation was simulated by irradiating 12C 290 MeV/n beams to a water phantom as a substitution of patients body at a therapeutic irradiation cave of HIMAC. Emitted neutrons were then detected at some points in the cave with a Bonner sphere detector (BSD) containing a 6LiI(Eu) scintillator in the center. In order to discriminate neutron events from hits by charged particles in the forward angle measurements, another plastic scintillator was placed upstream of the BSD as a veto counter. The number of neutron events was normalized to the absorbed dose given to the phantom, then unfolded by SAND-II code with initial guess by Monte Carlo code PHITS. For the sake of comparison, 1H-160MeV beam was also used.
The result revealed that fast neutrons above 10 MeV were mainly produced in the water phantom while a collimator was the main source of neutron around 1MeV. Neutron ambient dose equivalent estimated by multiplying ICRP74 conversion factor to the energy spectra showed strong angular dependence in carbon fields while almost isotropic in proton fields. The extent of ambient dose equivalent by neutrons was found smaller in the field of carbon ions than that of protons., NEUDOS 11},
 title = {MEASUREMENT OF AMBIENT NEUTRON DISTRIBUTION WITH A BONNER SPHERE DETECTOR IN THE FIELD OF CHARGED PARTICLE RADIOTHERAPY},
 year = {2009}
}