Boron Neutron Capture Therapy
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Collimator opening11.5 cmø2.5cm5.5cmøFigure 4. Two-dimensional Thermal Neutron DistributionFigure 5. Depth Dose Distribution by Radiation TypeRadiation at the BNCT epithermal neutron irradiation field consists of the following components: fast neutrons, epith-ermal neutrons, and thermal neutrons deriving from the produced neutron as well as gamma radiation given off by nuclear reactions in the deceleration system. Additionally, the primary types of radiation in the water phantom include incident radiation as well as recoiled protons and recoiled oxygen generated in the phantom and gamma radiation resulting primarily from 1H (n, γ) 2H reactions. Following administration of an intravenous boron agent, the internal dose includes not only the above radiation, but also alpha particles and 7Li derived from 10B as well as gamma radia-tion resulting from reactions between the elements that make up the body (for example nitrogen and sodium chloride in bodily fluids) and neutrons. Figure 4 illustrates the distribution as epithermal neutrons are decelerated in the water phantom and thermalized. This data was observed during epithermal neutron mode irradiation at the KUR heavy-water irradiation facility. The peak depth of the thermal neutron flux is approximately 2.5 cm. This peak domain is about 5.5 cm in diameter, or approximately half the diameter of the collimator. Please note how broadening of the epithermal neutron direction of incidence and broadening due to scattering during the deceleration process in the phantom exceed similar increases seen in other radiation treatments. This charac-teristic determines the margin for BNCT planning target volume (PTV: the tumor volume for irradiation planning purposes, calculated based on three-dimensional image data of the tumor along with the extent of projected devel-opment, body movement, and irradiation field ambiguity). Figure 5 illustrates the depth dose distribution of the 10B radiation dose, nitrogen radiation dose, hydrogen radiation dose, and gamma radiation dose inside a tumor with a 10B concentration of 30 ppm as well as the total distribution (total for all types of radiation) as calculated using the radiation dose and dose distribution plan. Under these conditions, 10B radiation accounts for approximately 85% of the total exposure. It is necessary to note that this proportion does not obtain uniformly throughout the irradiation field. In order to assure the quality of BNCT, it is necessary to assess an extremely complex radiation composition as described above, to accurately evaluate the dose of each type, and to measure the benefit-versus-dose curve. Although it has not been implemented yet, real-time measurement of the 10B (n, α) 7Li and 14N (n, p) 14C reaction distributions in the body is the most important measure-ment issue that needs to be addressed.000.251.00.750.5481216Irradiation field and internal dose distributionDepth in water (cm)Dose rate (relative value)Total radiation dose10B radiation doseHydrogen radiation doseGamma radiation doseNitrogen radiation dose・Dose of each radiation concerned in BNCT is estimated as a dose-equivalent (unit; Gy-eq)・Maximum value of the total dose is normalized to 1.*1:*2:*3:The dosage released in the nuclear reaction between neutrons and 10B (which is part of a Boron compound).The dosage released from the elastic scattering between neutrons and hydrogen atoms in the body.The dosage released in the nuclear reaction between neutrons and nitrogen atoms in the body.*1 *2*3 14

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