Contribution of Microdosimetry to the Specification of Neutron Beam Quality for the Choice of the 'Clinical RBE' in Fast Neutron Therapy

Abstract

This paper discusses the Relative Biological Effectiveness (RBE) problem in neutron therapy and the way it is handled today. The possible contribution of microdosimetry to improve the situation is suggested. 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, reference RBE and clinical RBE, 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 are elucidated and emphasised. Although it is based first on the reference RBE, the clinical RBE implicitly includes differences in physical selectivity of the irradiation beam as well as clinical experience when available. Microdosimetry can provide useful information for the selection of the clinical RBE by giving a description of the radiation quality at the point of interest. A method has been developed, using iterative and deconvolution techniques, allowing the derivation of the RBE of a given new neutron beam provided the microdosimetric spectra are available. This method has been tested by combining microdosimetric data and radiobiological data (intestinal crypt cell regeneration) obtained at nine neutron therapy beams. For a 'new' neutron beam, the RBE value computed on the basis of the microdosimetric spectra would have the same accuracy (and confidence) as RBE values measured directly. Radiobiological data, systematically obtained for late normal tissue tolerance at 2 Gy per fraction, are still missing; their correlation with the microdosimetric data would be most relevant for clinical applications.