Neutron Sources For Bnct Nuclear Reactors

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Neutron sources for BNCT currently are limited to nuclear reactors and in the present section we will only summarize information that is described in more detail in a recently published review (138). Reactor derived neutrons are classified according to their energies as thermal (En <0.5 eV), epithermal (0.5 eV <En <10 keV), or fast (En >10 keV). Thermal neutrons are the most important for BNCT because they usually initiate the 10B(n, a)7Li capture reaction. However, because thermal neutrons have a limited depth of penetration, epithermal neutrons, which lose energy and fall into the thermal range as they penetrate tissues, are now preferred for clinical therapy. A number of reactors with very good neutron beam quality have been developed and currently are being used clinically. These include: (1) Massachusetts Institute of Technology Research Reactor, shown schematically in Fig. 2 (139); (2) the FiR1 clinical reactor in Helsinki, Finland (140); (3) R2-0 High Flux Reactor (HFR) at Petten in the Netherlands (141); (4) LVR-15 reactor at the Nuclear Research Institute (NRI) in Rez, Czech Republic (142); (5). Kyoto University Research Reactor (KURR) in Kumatori, Japan (143); (6) JRR4 at the Japan Atomic Energy Research Institute (JAERI) (144); (7) the RA-6 CNEA reactor in Bariloche, Argentina (145); and (8) until June 2005, the R2-0 clinical reactor, operated by a private company Studsvik Medical AB in Sweden (146). Other reactor facilities are being designed, notably the TAPIRO reactor (147) at the Ente Nazionale Energia Atomica (ENEA) Casaccia Center near Rome, Italy, which is unique in that it will be a low-power fast-flux reactor, and a facility in Beijing, China, which will be used exclusively for BNCT (148). This reactor will have a power of 30 kW and currently is under construction adjacent to the "401" hospital in a southwestern suburb of Beijing. Two reactors that have been used in the

Fig. 2. Schematic diagram of the Massachusetts Institute of Technology Reactor (MITR). The fission converter based epithermal neutron irradiation (FCB) facility is housed in the experimental hall of the MITR and operates in parallel with other user applications. The FCB contains an array of 10 spent MITR-II fuel elements cooled by forced convection of heavy water coolant. A shielded horizontal beam line contains an aluminum and Teflon® filter-moderator to tailor the neutron energy spectrum into the desired epithermal energy range. A patient collimator defines the beam aperture and extends into the shielded medical room to provide circular apertures ranging from 16 to 8 cm in diameter. The in-air epithermal flux for the available field sizes ranges from 3.2 to 4.6 x 109 n /cm2 s at the patient position. The measured specific absorbed doses are constant for all field sizes and are well below the inherent background of 2.8 x10-12 RBE Gy cm2/« produced by epithermal neutrons in tissue. The dose distributions achieved with the FCB approach the theoretical optimum for BNCT.

Fig. 2. Schematic diagram of the Massachusetts Institute of Technology Reactor (MITR). The fission converter based epithermal neutron irradiation (FCB) facility is housed in the experimental hall of the MITR and operates in parallel with other user applications. The FCB contains an array of 10 spent MITR-II fuel elements cooled by forced convection of heavy water coolant. A shielded horizontal beam line contains an aluminum and Teflon® filter-moderator to tailor the neutron energy spectrum into the desired epithermal energy range. A patient collimator defines the beam aperture and extends into the shielded medical room to provide circular apertures ranging from 16 to 8 cm in diameter. The in-air epithermal flux for the available field sizes ranges from 3.2 to 4.6 x 109 n /cm2 s at the patient position. The measured specific absorbed doses are constant for all field sizes and are well below the inherent background of 2.8 x10-12 RBE Gy cm2/« produced by epithermal neutrons in tissue. The dose distributions achieved with the FCB approach the theoretical optimum for BNCT.

past for clinical BNCT are the Musashi Institute of Technology (MuITR) reactor in Japan (149) and the Brookhaven Medical Research Reactor (BMRR) at the Brookhaven National Laboratory (BNL) in Upton, Long Island, New York (150). The MuITR was used by Hatanaka (149) and later by Hatanaka and Nakagawa (151). The BMRR was used for the clinical trial that was conducted at the Brookhaven National Laboratory between 1994 and 1999 (28,152) and the results are described in detail later in this section. For many reasons reasons, including the cost of maintaining the BMRR, it has been deactivated and is no longer available for use.

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