Microdosimetric measurements (0.5 inch TEPC, 2 µm simulated diameter) were performed in similar conditions for 14 neutron beams with energies ranging from p(4)+Be to p(65)+Be used for radiobiology and therapy. They were collected in an intercomparison programme currently carried out by the EORTC at European facilities and aimed at the comparison of the radiation quality (in terms of RBE) of a given neutron beam with a reference neutron beam. Indeed, accurate physical and biological characteristics for these beams are urgently required. The large RBE differences (up to 50%) observed between different Neutron Therapy Centres must be accurately (<±5%) taken into account for the comparison of clinical results. In order to provide a practicable solution to the existing and unsolved problem of accounting for radiation quality variations in neutron therapy, RBE against lineal energy weighting functions have been established by unfolding methods using radiobiological data and microdosimetric spectra obtained under identical conditions at the neutron beams investigated. This empirical procedure enabled the determination of the best accuracy achievable in predicting RBE variations between neutron beams using their weighted mean lineal energy.
The introduction of new ICRP recommendations, especially the new Human Respiratory Tract Model (HRTM) in ICRP Publication 66 led us to focus on some specific parameters related to industrial uranium aerosols collected between 1990 and 1999 at French nuclear fuel fabrication facilities operated by COGEMA, FBFC, and the CEA. Among these parameters, the activity median aerodynamic diameter (AMAD), specific surface area (SSA), and parameters describing absorption to blood f(r), s(r) and s(s) defined in ICRP Publication 66 were identified as the most relevant influencing dose assessment. This study reviewed the data for 25 pure and impure uranium compounds. The average value of AMAD obtained was 5.7 microm (range 1.1-8.5 microm), which strongly supports the choice of 5 microm as the default value of AMAD for occupational exposures. The SSA varied between 0.4 and 18.3 m2 g(-1). For most materials, values of the absorption parameters f(r), s(r), and s(s) derived from the in vitro experiments were generally consistent with those derived from the in vivo experiments. Using average values for each pure compound allowed us to classify UO2 and U3O8 as Type S, mixed oxides, UF4, UO3 and ADU as Type M, and UO4 as Type F based on the ICRP Publication 71 criteria. Dose coefficients were also calculated for each pure compound, and average values for each type of pure compound were compared with those derived using default values. Finally, the lung retention kinetics and urinary excretion rates for inhaled U03 were compared using material-specific and default absorption parameters, in order to give a practical example of the application of this study.
The neutron beams used by various radiotherapy centres are of widely differing energies, and differences of up to 50 per cent in the relative biological effectiveness (RBE) between different beams have been found in radiobiological experiments. Moreover, at some facilities RBE variations have been observed with increasing depth in a phantom. In spite of this evidence, there is no quantitative and uniquely accepted specification of radiation quality used in practice. The urgency of an adequate solution of this problem is illustrated by the fact that in radiation therapy the usual accuracy requirement for the quantity of radiation, i.e. the absorbed dose to be delivered to the tumour, is 3.5 per cent (1 SD). In this paper a pragmatic solution for the specification of radiation quality for fast neutron therapy is proposed. It is based on empirical RBE versus lineal energy response or weighting functions. These were established by using existing radiobiological data and microdosimetric spectra measured under identical irradiation conditions at several European neutron irradiation units.
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