RBE in fast neutron therapy and in boron neutron capture therapy. A useful concept or a misuse?

In high-LET radiation therapy, radiation quality and radiation quality differences have to be taken into account and specified. In fast neutron therapy, an operational approach has been adopted which is based on the concept of "clinical RBE". The paper discusses the quantities of RBE (relative biological effectiveness), reference RBE and clinical RBE and their relationship and significance in radiation therapy. In particular, the difference between the well defined RBE in radiation biology and the clinical RBE which is based on the judgement of radiotherapists is elucidated and emphasized. The clinical RBE is based on the reference RBE and implicitly includes differences in physical selectivity of the irradiation beams and clinical experience. The particular aspects of radiation quality in boron neutron capture therapy (BNCT) are due to the poor penetration of the primary beam, the inhomogeneity of the spatial distribution of 10B nuclides in the irradiated tissue and the short range of the alpha-particles emitted after neutron capture in 10B. The related problems in accounting for radiation quality in BNCT and in the applicability of the clinical RBE concept are discussed.