TY - JOUR
AU - Zhu, Weiguo
AB - Abstract Some therapeutic proton synchrotron accelerators will be built for cancer treatment in China. The radiation produced by proton must be carefully evaluated and shielded for occupational disease hazard assessment and environmental impact assessment. Adopting the FLUKA code, a therapeutic room model, a synchrotron hall model and a high energy transport line tunnel model are constructed to get the ambient dose equivalent rate distributions. The ambient dose equivalent rates are also calculated with an empirical formula in some regions. The results calculated by the two ways are consistent with each other. The advantages and disadvantages of the two methods have been analyzed. The ambient dose equivalent rates are no more than 2.5 μSv/h at 30 cm beyond the shielding wall and the maze door. The dose rates are <25 μSv/h at 30 cm beyond the roof of the therapeutic room. These results comply with the National Occupational Health Standard requirements. INTRODUCTION The application of therapeutic proton accelerators in the energy range of approximately 250 MeV for cancer treatment has been increased in popularity worldwide in recent years due to its physical and biological advantages(1–3). In China, some therapeutic proton accelerators will be built in Shanghai, Guangzhou and Wuwei. The therapeutic proton synchrotron accelerator has its unique advantages in radiation protection and therapeutic aspects. Compared with cyclotron, the energy of proton can be adjusted directly by synchrotron accelerator and the beam loss will be greatly reduced at the therapeutic terminal. In addition, it produces a pencil beam with a typical spot size of ~4–10 mm. Proton accelerators in this intermediate energy range could potentially produce intense secondary radiation, which must be carefully evaluated and shielded for the purpose of radiation safety in a densely populated hospital. The secondary particles, such as neutrons, photons, electrons and positrons, will be generated through nuclear interactions initiated by primary proton bombarding with surrounding materials. Among them, secondary neutrons are the dominant components and the main concern of the accelerator shielding design. Neutrons mainly result from the intranuclear cascades and subsequent evaporation reactions. Intranuclear cascades create high-energy (~100 MeV) neutrons that account for a small fraction but exhibit forward-peaked anisotropy and strong penetration of matter, whereas evaporation reactions produce copious neutrons that are mostly isotropic emission(1). The peak for evaporation neutrons is located at energies of ~2 MeV(1). In summary, the high-energy neutrons will determine the shielding requirements of a therapeutic proton synchrotron accelerator. A therapeutic proton synchrotron accelerator system includes an ion source, a linear accelerator, a low energy transport line, a synchrotron, a high energy transport line and several therapeutic rooms. The proton energy is no more than 7 MeV in ion source, linear accelerator room and low energy transport line. So the research will focus on the shielding of synchrotron hall, high energy transport line tunnel and therapeutic room, where the proton energy can reach up to 250 MeV. In this article, we explore the shielding design and corresponding dose distribution of a therapeutic proton synchrotron accelerator system, including models construction, shielding calculation methods, results analysis and comparison. Two approaches are used to conduct the radiation shielding design of the therapeutic proton synchrotron accelerator system. One is the Monte Carlo simulation and the other is the empirical formula calculation. MATERIALS AND METHODS Monte Carlo study FLUKA is a common Monte Carlo tool for simulations of particle transport and interactions with matter, covering an extended range of applications spanning from proton and electron accelerator shielding to target design, calorimetry, activation, dosimetry, detector design, cosmic rays and so on(4). FLUKA can simulate 60 different particles’ interaction and propagation in matter, including photons and electrons from 1 keV to thousands of TeV, neutrinos, muons of any energy, hadrons of energies up to 20 TeV and all the corresponding antiparticles, neutrons down to thermal energies. We have adopted the version FLUKA 2011.2c.3 to complete the simulation. Model description A synchrotron hall model, a high energy transport line tunnel model and a rotation therapeutic room model are constructed separately with FLUKA. These models are taken from the design of a hospital which will be built in Shanghai. The diagram of the whole therapeutic proton synchrotron system is shown in Figure 1. The therapeutic room will be equipped with therapeutic terminal which can allow for a 180° rotation around a patient in order to vary the proton beam direction. The shielding materials are ordinary concrete, iron and polystyrene. The three kinds of materials, of which densities and compositions are shown in Table 1, are found pre-defined in FLUKA. The element compositions of the materials are expressed by weight fractions. Table 1. Density and composition of materials. Material  Density (g/cm3)  Composition  Ordinary concrete  2.35  H (1%), C (0.1%), O (52.9%), Na (1.6%), Mg (0.2%), Al (3.3%), Si (33.7%), K (1.3%), Ca (4.4%) and Fe (1.4%)  Iron  7.87  Fe (100%)  Polystyrene  0.94  H (7.7%), C (92.3%)  Material  Density (g/cm3)  Composition  Ordinary concrete  2.35  H (1%), C (0.1%), O (52.9%), Na (1.6%), Mg (0.2%), Al (3.3%), Si (33.7%), K (1.3%), Ca (4.4%) and Fe (1.4%)  Iron  7.87  Fe (100%)  Polystyrene  0.94  H (7.7%), C (92.3%)  Figure 1. View largeDownload slide A diagram of the therapeutic proton synchrotron system. Figure 1. View largeDownload slide A diagram of the therapeutic proton synchrotron system. In the synchrotron hall and high energy transport line tunnel, the proton beam is possibly lost at iron target. When measuring the ambient dose equivalent rates around the therapeutic room, the target of proton beam is usually a water phantom(5–7). Therefore, the water phantoms and iron slab models are chosen as the targets for the simulations. Table 2 shows their densities and compositions. The element compositions of materials are also expressed by weight fractions. Table 2. Density and composition of targets. Targets  Density (g/cm3)  Composition  Water phantom  1.0  H (11.1%), O (88.9%)  Iron  7.87  Fe (100%)  Targets  Density (g/cm3)  Composition  Water phantom  1.0  H (11.1%), O (88.9%)  Iron  7.87  Fe (100%)  The water phantom is set to 40 cm (length) × 40 cm (width) × 40 cm (height) and the iron slab is set to 10 cm (length) × 10 cm (width) × 5 cm (height). The beam currents and the beam losses in different regions are shown in Table 3. Note that the beam lost at synchrotron ring in four directions each accounting for 1/4. The interactions between neutrons and therapeutic bed as well as other accelerator structures are ignored. Table 3. The beam currents and losses in different regions. Region  Beam current  Target  Beam loss  Beam loss composition  Synchrotron ring  2.9 × 1010 protons/s  Iron slab  9 × 109 protons/s  7.875 × 109 150 MeV protons/s  1.125 × 109 250 MeV protons/s  High energy transport line  2.0 × 1010 protons/s  Iron slab  2.5 × 109 protons/s  2.35 × 109 220 MeV protons/s  1.5 × 108 250 MeV protons/s  Rotation therapeutic room  1.75 × 1010 protons/s  Water phantom  1.75 × 1010 protons/s  1.645 × 1010 220 MeV protons/s  1.05 × 109 250 MeV protons/s  Region  Beam current  Target  Beam loss  Beam loss composition  Synchrotron ring  2.9 × 1010 protons/s  Iron slab  9 × 109 protons/s  7.875 × 109 150 MeV protons/s  1.125 × 109 250 MeV protons/s  High energy transport line  2.0 × 1010 protons/s  Iron slab  2.5 × 109 protons/s  2.35 × 109 220 MeV protons/s  1.5 × 108 250 MeV protons/s  Rotation therapeutic room  1.75 × 1010 protons/s  Water phantom  1.75 × 1010 protons/s  1.645 × 1010 220 MeV protons/s  1.05 × 109 250 MeV protons/s  Empirical formula calculations The Moyer formula is a common way to calculate the dose rate for neutrons produced by proton accelerators(1, 8). As for point source, the neutron ambient dose equivalent rate can be calculated by the following empirical formula (5, 6):   Ḣ=S0H0(θ)1r2exp⁡(−dρλ(θ)) (1)where Ḣ is the neutron ambient dose equivalent rate, r is the distance between the target and the detector, S0 is the beam loss, d is the thickness of shielding material, ρ is the density of barrier, H0 is the neutron ambient dose equivalent rate produced by one proton at 1 m from the target, λ(θ) is the neutron attenuation length of shielding material and θ is the scattering angle. The values of H0(θ) and λ(θ) in ordinary concrete shielding against 250 MeV protons and 220 MeV protons impinging on two targets, which can be obtained from the simulation with FLUKA, are shown in Tables 4 and 5 separately. The application of the formula with parameters with respect to different energies and shielding materials can also be found(8–21). Table 4. The values of H0(θ) and λ(θ) in ordinary concrete shielding against the 250 MeV protons impinging on two targets. θ  H0(θ) (Sv m2)  λ(θ) (g/cm2)  Iron slab  Water phantom  Iron slab  Water phantom  0°~10°  1.0E-14  1.6E-14  107  108  40°~50°  1.9E-15  1.8E-15  93  92  80°~90°  4.3E-16  3.1E-16  64  72  130°~140°  2.5E-16  3.7E-16  57  58  θ  H0(θ) (Sv m2)  λ(θ) (g/cm2)  Iron slab  Water phantom  Iron slab  Water phantom  0°~10°  1.0E-14  1.6E-14  107  108  40°~50°  1.9E-15  1.8E-15  93  92  80°~90°  4.3E-16  3.1E-16  64  72  130°~140°  2.5E-16  3.7E-16  57  58  Table 5. The values of H0(θ) and λ(θ) in ordinary concrete shielding against the 220 MeV protons impinging on two targets. θ  H0(θ) (Sv m2)  λ(θ) (g/cm2)  Iron slab  Water phantom  Iron slab  Water phantom  0°~10°  7.5E-15  4.3E-15  107  106  40°~50°  7.4E-16  1.2E-15  102  90  80°~90°  1.1E-16  1.5E-16  99  76  130°~140°  8.8E-17  3.7E-16  54  58  θ  H0(θ) (Sv m2)  λ(θ) (g/cm2)  Iron slab  Water phantom  Iron slab  Water phantom  0°~10°  7.5E-15  4.3E-15  107  106  40°~50°  7.4E-16  1.2E-15  102  90  80°~90°  1.1E-16  1.5E-16  99  76  130°~140°  8.8E-17  3.7E-16  54  58  RESULTS The ambient dose equivalent rates for rotation therapeutic room Because the proton beam can be rotated 180° around a patient in the therapeutic room, the simulations have been performed for beam impinging on target from horizontal, upper and bottom separately. Figures 2 and 3 show the ambient dose equivalent rate distributions of the therapeutic room model when the directions of the 250 MeV proton beam and the 220 MeV proton beam are horizontal separately. The highest ambient dose equivalent rates in different regions are listed in Table 6. These regions are the places where the public, doctors and nurses can reach. Table 6. The ambient dose equivalent rates outside rotation therapeutic room. Region  Shielding material and thickness  Beam direction  The highest dose rate (μSv/h)  FLUKA result  Moyer formula  30 cm beyond west wall  230 cm concrete and 50 cm iron plant  Horizontal  2.4 ± 0.2  /  30 cm beyond east wall  250 cm concrete  Down  <0.01  <0.01  30 cm beyond north wall  200 cm concrete  Down  <0.01  <0.01  30 cm beyond maze door  5 cm polystyrene  Down  1.0 ± 0.2  /  30 cm beyond roof  200 cm concrete  Up  23.3 ± 1.2  23.7 ± 2.1  Region  Shielding material and thickness  Beam direction  The highest dose rate (μSv/h)  FLUKA result  Moyer formula  30 cm beyond west wall  230 cm concrete and 50 cm iron plant  Horizontal  2.4 ± 0.2  /  30 cm beyond east wall  250 cm concrete  Down  <0.01  <0.01  30 cm beyond north wall  200 cm concrete  Down  <0.01  <0.01  30 cm beyond maze door  5 cm polystyrene  Down  1.0 ± 0.2  /  30 cm beyond roof  200 cm concrete  Up  23.3 ± 1.2  23.7 ± 2.1  Figure 2. View largeDownload slide The dose rate distributions of therapeutic room model for 1.645 × 1010 220 MeV protons/s impinging on water phantom. Figure 2. View largeDownload slide The dose rate distributions of therapeutic room model for 1.645 × 1010 220 MeV protons/s impinging on water phantom. Figure 3. View largeDownload slide The dose rate distributions of therapeutic room model for 1.05 × 109 250 MeV protons/s impinging on water phantom. Figure 3. View largeDownload slide The dose rate distributions of therapeutic room model for 1.05 × 109 250 MeV protons/s impinging on water phantom. The highest dose rates calculated by the Moyer formula in different regions are also listed in Table 6. From Table 6, we can see that the results from the two methods are consistent with each other in the same case. The maximum ambient dose equivalent rate is 2.4 μSv/h at 30 cm beyond the wall and the maze door. The maximum dose rate is 23.7 μSv/h at 30 cm beyond the roof of therapeutic room when the beam direction is up. As the neutrons energy spectra will change greatly when neutrons are transported through iron before arriving at concrete(1, 20–22), it is difficult to calculate dose rate outside west wall composed of two kinds of materials. The neutrons must be multiple scattered when they reach the maze door, so it is also difficult for the Moyer formula to calculate the ambient dose equivalent rate at 30 cm beyond the maze door. The both results are not shown in Table 6. Other areas around the therapeutic room are the synchrotron hall or the high energy transport line tunnel, so the results are not listed in Table 6. The ambient dose equivalent rates for high energy transport line tunnel The direction of proton beam is set to horizontal west to obtain the ambient dose equivalent rate distributions of high energy transport line tunnel model. Figures 4 and 5 show the dose rate distributions of high energy transport line tunnel model for injecting 250 MeV protons and 220 MeV protons separately. The ambient dose equivalent rates are also calculated by the Moyer formula. The both results are listed in Table 7. Table 7. The ambient dose equivalent rates outside high energy transport line tunnel. Region  Shielding material and thickness  Beam direction  The highest dose rate (μSv/h)  FLUKA result  Moyer formula  30 cm beyond north wall  180 cm concrete  Horizontal west  0.70 ± 0.10  0.77 ± 0.18  30 cm beyond maze door  5 cm wood  Horizontal west  0.75 ± 0.10  /  30 cm beyond roof  170 cm concrete  Horizontal west  <0.01  <0.01  Region  Shielding material and thickness  Beam direction  The highest dose rate (μSv/h)  FLUKA result  Moyer formula  30 cm beyond north wall  180 cm concrete  Horizontal west  0.70 ± 0.10  0.77 ± 0.18  30 cm beyond maze door  5 cm wood  Horizontal west  0.75 ± 0.10  /  30 cm beyond roof  170 cm concrete  Horizontal west  <0.01  <0.01  Figure 4. View largeDownload slide The dose rate distributions of high energy transport line tunnel model for 1.5 × 108 250 MeV protons/s impinging on iron target. Figure 4. View largeDownload slide The dose rate distributions of high energy transport line tunnel model for 1.5 × 108 250 MeV protons/s impinging on iron target. Figure 5. View largeDownload slide The dose rate distributions of high energy transport line tunnel model for 2.35 × 109 220 MeV protons/s impinging on iron target. Figure 5. View largeDownload slide The dose rate distributions of high energy transport line tunnel model for 2.35 × 109 220 MeV protons/s impinging on iron target. The highest ambient dose equivalent rates are <1.0 μSv/h at 30 cm beyond the shielding wall and the maze door. The dose rates are negligible at 30 cm beyond the high energy transport line tunnel roof. Other areas around the tunnel are soil or synchrotron hall. The ambient dose equivalent rates for synchrotron hall For the synchrotron hall model, there are three directions of beam loss simulated including horizontal west, horizontal east and horizontal north. Because the south of synchrotron hall is soil, the simulation is not done when the beam direction is south. Figures 6 and 7 show the ambient dose rate distributions of synchrotron hall model when the proton beam direction is horizontal north for injecting 250 MeV protons and 150 MeV protons separately. Figure 6. View largeDownload slide The dose rate distributions of synchrotron hall model for 0.28 × 109 250 MeV protons/s impinging on iron target. Figure 6. View largeDownload slide The dose rate distributions of synchrotron hall model for 0.28 × 109 250 MeV protons/s impinging on iron target. Figure 7. View largeDownload slide The dose rate distributions of synchrotron hall model for 1.97 × 109 150 MeV protons/s impinging on iron target. Figure 7. View largeDownload slide The dose rate distributions of synchrotron hall model for 1.97 × 109 150 MeV protons/s impinging on iron target. The results obtained from the simulation are listed in Table 8. From this table, what can be seen is that the ambient dose equivalent rates are <1.0 μSv/h at 30 cm beyond the shielding wall and the maze door. The dose rates are negligible at the outside of synchrotron hall roof. Other areas around the hall are soil or high energy transport line tunnel. Table 8. The ambient dose equivalent rates outside synchrotron hall. Region  Shielding material and thickness  Beam direction  The highest dose rate (μSv/h)  FLUKA result  Moyer formula  30 cm beyond west wall  250 cm concrete  Horizontal west  0.25 ± 0.05  /  30 cm beyond east wall  230 cm concrete  Horizontal east  0.30 ± 0.05  /  30 cm beyond north wall  160 cm concrete  Horizontal north  0.80 ± 0.10  /  30 cm beyond maze door  5 cm wood  Horizontal north  0.70 ± 0.10  /  30 cm beyond roof  130 cm concrete  Horizontal east  <0.01  /  Region  Shielding material and thickness  Beam direction  The highest dose rate (μSv/h)  FLUKA result  Moyer formula  30 cm beyond west wall  250 cm concrete  Horizontal west  0.25 ± 0.05  /  30 cm beyond east wall  230 cm concrete  Horizontal east  0.30 ± 0.05  /  30 cm beyond north wall  160 cm concrete  Horizontal north  0.80 ± 0.10  /  30 cm beyond maze door  5 cm wood  Horizontal north  0.70 ± 0.10  /  30 cm beyond roof  130 cm concrete  Horizontal east  <0.01  /  CONCLUSION By adopting the FLUKA code, we first constructed a synchrotron hall model, a rotation therapeutic room model and a high energy transport line tunnel model. The beams impinging on target with different angles are simulated separately to obtain the ambient dose equivalent rates. At the same time, we use the empirical Moyer formula to calculate the ambient dose equivalent rates. The results are consistent with each other in the two ways. It is a demonstration of the validity of the Moyer formula. Although the empirical formula can quickly calculate the ambient dose equivalent rate, it is difficult to calculate the results beyond the maze door and the wall composed of multiple shielding materials. No matter what kind of beam directions, the ambient dose equivalent rates are no more than 2.5 μSv/h at 30 cm beyond the shielding wall and the maze door for the three models. The dose rate is close to 25 μSv/h at 30 cm beyond the therapeutic room roof when the beam direction is up. The National Occupational Health Standard(7) stipulates that the ambient dose equivalent rates are <2.5 μSv/h at 30 cm beyond the shielding wall and the maze door, and the ambient dose equivalent rates are <100 μSv/h at 30 cm beyond roof. These calculated results conform to the standard requirements in China. The thickness for different shielding materials as listed in this paper can be taken as references when a similar therapeutic proton synchrotron accelerator is built. The calculation results can be used for occupational disease hazard assessment and environmental impact assessment. FUNDING This work was supported by the Ministry of Science and Technology of the People's Republic of China [2012YQ180118]. REFERENCES 1 Sheu, R. J., Chen, Y. F., Lin, U. T. and Jiang, S. H. Deep-penetration calculations in concrete and iron for shielding of proton therapy accelerators. Nucl. Instrum. Methods Phys. Res.  280, 10– 17 ( 2012). 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TI - STUDY ON THE SHIELDING AND DOSE RATE DISTRIBUTIONS OF THERAPEUTIC PROTON SYNCHROTRON ACCELERATOR BASED ON FLUKA
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncx068
DA - 2018-01-01
UR - https://www.deepdyve.com/lp/oxford-university-press/study-on-the-shielding-and-dose-rate-distributions-of-therapeutic-mkCRvf3bPG
SP - 1
EP - 7
VL - 178
IS - 1
DP - DeepDyve
ER -