Physical and tumor biological aspects and calculation model of dosage in boron neutron capture therapy (BNCT).

Fundamentally different aspects apply to dosage in boron neutron capture therapy (BNCT) compared to that in the case of normal radiotherapy with photons, electrons or heavy particles such as neutrons. The reason is that the latter only requires a knowledge of the stochastic distribution of the absorbed dose within cells, radiation quality and atomic composition of tissue in the regions of interest, whereas for the former the absolute concentration and microscopic distribution of 10B atoms in inter- and intracellular spaces of tumor and healthy cells is additionally of equal importance. The effects of radiation without 10B must always be superimposed on those of heavy particles resulting from neutron capture reactions on 10B atoms. Complex geometrical calculations are necessary with respect to ranges of the heavy particles smaller than a cell diameter. Apart from the direct effects of radiation without 10B, the dosage therefore depends on thermal neutron fluence, 10B concentration, its extreme inhomogeneous macroscopic distribution in the tumor tissue, the cellular localization of the 10B atoms in the large intercellular space, the cell membrane, within cytoplasm or the cell nucleus, the geometrical probability of hitting the cell nucleus, and that such a hit finally results in a cell killing, and a Poisson statistical enhancement factor, which describes the dose-effect relation for cell survival. The calculations necessary are demonstrated in the case of a normal and a tumor cell type, each with representative cell diameter and nucleus size. It is evident that the microscopic distribution of 10B atoms is one of the most critical parameters which is still insufficiently known.