Cell Survival Following in Vitro Irradiation at Depth in a Lucite Phantom as a Measure of Epithermal Beam RBE
The radiation field produced in tissue during BNCT consists of a mixture of components with differing linear energy transfer (LET) characteristics. In addition to the high-LET products of the 10B(n,α)7Li reaction, the interaction of the neutron beam with the nuclei of elements in tissue will deliver an unavoidable, non-specific background dose, from a mixture of high- and low-LET radiation components, to both tumor and normal tissue. Thermal neutron capture by hydrogen releases a gamma ray through the 1H(n,γ)2H reaction. The capture of thermal neutrons by nitrogen in tissue, the 14N(n,p)14C reaction, releases a high-LET proton with an energy of 590 keV. Contaminating fast neutrons (those with kinetic energies >10keV) in the epithermal neutron beam produce high-LET recoil protons through collisions with hydrogen nuclei (1H(n,n′)p reaction) in tissue. Because the energies of the nitrogen capture proton and the fast neutron recoil proton tend to be in the same range, the biological effects of these high-LET components of the dose are most conveniently measured as a combined “proton dose”. This high-LET “proton dose” must be multiplied by an experimentally determined factor for relative biological effectiveness (RBE) in order to express the total BNCT dose in photon-equivalent units.
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