DOE OSTI2014
Radiotherapy using ion beams has gained momentum in radiation oncology. While the number of proton therapy facilities that have been established or are under construction is rapidly growing, interest in heavy ion (whose atomic number Z is greater than one) radiotherapy has also been revitalized in the U.S. Helium ions, with mass and charge between protons and carbon ions, have advantages of deep penetration distances with significant peak-to-entrance dose ratios and limited dose contributions from secondary neutrons and other secondary charged fragments. In consideration of these advantages, the U.S. ion beam therapy community has expressed interests in helium ions as a potential candidate for ion beam therapy, especially pediatric cases. As such, characterizing the secondary nuclear fragment field created by primary helium ion interactions along the beam delivery devices and in human tissue is important for dose assessment and patient protection. Up to now the experimental data with respect to helium ions over the therapeutic energy range, 100 - 230 MeV/amu, is scarce compared to protons and carbon ions. Kurosawa et al. measured stopping target yields and angular spectra of neutrons, protons, deuterons and tritons produced by various target materials bombarded by 100- and 180- MeV/amu helium ions; Heilbronn et al. and Cecil et al. also measured the stopping target neutron yields by 155-, 160- and 177.5-MeV/amu helium ion bombardments. We have proposed to measure angular distributions of secondary neutrons and light charged ions produced by 100- and 230-MeV/amu helium ions bombarding stopping water and iron targets. The experiment was conducted at the Heavy Ion Medical Accelerator in Chiba (HIMAC) of the National Institute of Radiological Sciences (NIRS) in Japan this June. During the literature survey for experimental setup, we found that the ratio of the ion beam range (R) to the target depth (d) varies from 0.5 - 0.98 in the stopping target measurements either for helium ions incident on various targets or carbon ions bombarding targets as tissue surrogates. The difference in the values of ratio R/d makes comparison of experimental results complicated and require accurate corrections. Hence, how the R/d ratio and cross sectional area of the targets influence the angular distributions of secondary particles is investigated by using a Monte Carlo method in this study. The influence of the depth and the cross sectional area of stopping water and iron targets on the yield and the spectra of secondary particles induced by 100- and 230-MeV/amu He ions was studied by a Monte Carlo simulation code, PHITS. The secondary charged particles and secondary neutrons who come from breakup of projectiles are highly forward-focused, while the secondary particles produced by target breakup are nearly isotropic. As the energy of {sup 4}He ions increases from 100 MeV/amu to 230 MeV/amu, the normalized intensities of charged particles decrease more gently with increasing angles, whereas the neutron angular distribution falls quickly at angles smaller than 30 deg. and starts to exceed the normalized profile of 100 MeV/amu He from 60 deg. When the target depth increases, the impact on attenuation of secondary particles is more significant for lighter target mass and higher-energy projectiles at forward angles. Also, with deeper targets, more interactions occur between the secondary particles and the target elements, which results in more target-like fragments at large and backward angles. In respect of the cross sectional area of the stopping targets, the forward angular distributions are similar to the system with smaller cross sectional area of the targets; however, no general rule was found for how the large-angle distributions are affected by the target cross sectional area. (authors)
61 RADIATION PROTECTION AND DOSIMETRY↗