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Keywords:
Dosimetric comparison, Dosimetry, SPECT, PET, Radioprotection / Radiation dose, Radiation physics
Authors:
H. Okino1, H. Hayashi1, K. Takegami1, N. Kimoto1, I. Maehata1, Y. Kanazawa1, T. Okazaki 2, T. Hashizume3, I. Kobayashi2; 1Tokushima/JP, 2Tsukuba, Ibaraki/JP, 3Tsukuba/JP
DOI:
10.1594/ecr2016/C-0024
Methods and materials
The proposed irradiation system is presented in Fig.
3. In order to achieve a small irradiation field,
the detection region was covered with phantoms having thicknesses of “t”. Here,
the size of the irradiation field (S×S) was defined as S = R + 10 mm + R,
where R was range of secondary electrons.
Figure 4 shows the properties of the Monte-Carlo simulation. In the present study,
EGS5 (electron gamma shower ver.5) code [15,16] was used. The number of photons used was 108,
the energy of photons was 100-2000 keV,
and phantom thickness was from 1-15 mm. The compositions of the phantom and detection region are represented in Fig.
4. The detection region was filled with air or PMMA (Polymethyl methacrylate) with a density of 0.001205 g/cm3,
and the phantom region filled with air or PMMA having a density of 1.00 g/cm3 or 1.19 g/cm3 were applied. We then simulated the photon and electron transportations based on the following three conditions; “Air/Air” means air in the detection region and air in the phantom region,
“PMMA/PMMA” means PMMA in the detection region and PMMA in the phantom region,
and “Air/PMMA” means air in the detection region and PMMA in the phantom region. A detailed analysis and purpose of these materials will be described later.
The range “R” of the secondary electron field was calculated as shown in Fig.
5. The graph in Fig.
5 represents energy loss of electrons as a function of electron energy [17]. When the electron with energy Ei penetrates the material with thickness of Δt,
the energy loss ΔEi is calculated by ΔEi = dE/dx(Ei)×Δt. Therefore,
R is calculated by summation of Δt until the integrated value of ΔEi agrees with the incident energy of E. In this study,
Δt is set at 0.01 cm.
Theoretically speaking,
when secondary electron equilibration is achieved,
the absorbed dose D in the detection region is equal to the air-kerma K (collision kerma Kcol). Therefore,
we evaluated the consistency between D and Kcol. A detailed description of how Kcol is obtained from energy fluencies is presented in Fig.
6. Here,
the reference value of the mass energy absorption coefficient is applied [18].
The proposed irradiation system was simply constructed,
in which the detection area was fully covered with a phantom to achieve secondary electron equilibration. Here,
the thickness of the phantom is an important parameter,
because the size of the irradiation field is decided by the range of secondary electrons. Then,
advantages and disadvantages of our simulation were evaluated by the “efficiency of Monte-Carlo simulation” and “fraction of scattered rays”. In Fig.
7,
these values are defined as mathematical expressions.