In electron radiotherapy for the periorbital region, metallic eye shields are used to protect the lens of the eye. Currently used eye shields are made of lead or tungsten. However, there has been little investigation of effective materials and appropriate thickness of the eye shield for protecting the lens and backscattered dose enhancement from the eye shield for each beam energy is not clear. The purpose of this study was to investigate the effectiveness of eye shields for attenuating the beam and to evaluate backscattered dose enhancement from the eye shield using an Electron Gamma Shower ver. 5 Monte Carlo code system. To calculate both the transmission and the backscatter dose for 4, 6, and 9 M e V, the eye shields were assumed to be made of lead and tungsten of 1 mm and 2 mm in thickness (width: 1 cm× 1 cm), which were placed on a virtual water phantom. For attenuation by the eye shield, at 9 MeV, the relative doses to the lens with lead eye shields of 1 mm and 2 mm in thickness were 51.0% and 23.5%, respectively, whereas those with tungsten eye shields of 1 mm and 2 mm in thickness were 33.5% and 7.2%, respectively. For all energies except 4 M e V, similar results were obtained, indicating that the 2 mm tungsten eye shield provided more attenuation than did the other. For 4 M e V, the 1 mm tungsten eye shield provided more attenuation than did the 2mm tungsten eye shield (21.1% vs 25.2%). The reason for this might be the difference in lateral electron scattering between 4 MeV and other energies, and the lens position in the case of the 2 mm eye shield is 1 mm deeper than that in the case of the 1 mm eye shield. For backscatter dose caused by the eye shield, dose enhancement was 1.25 to 1.5-times larger than that in the absence of eye shields, and lower beam energy caused larger dose enhancement. These results suggested that coating of that eye shield with low atomic number materials is needed to reduce the dose enhancement from backscattered electrons, especially at lower beam energy. In conclusion, in this study, tungsten was shown to be a better material for the eye shield, and optimal thickness of the eye shield might depend on the beam energy.
|出版ステータス||Published - 2014 10月 1|
|イベント||2014 21st EGS Users' Meeting in Japan - Tsukuba, Japan|
継続期間: 2014 8月 4 → 2014 8月 5
|Conference||2014 21st EGS Users' Meeting in Japan|
|Period||14/8/4 → 14/8/5|
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