Can we put aside the LNT dilemma by the introduction of the controllable dose?

Recently, Professor R Clarke, ICRP Chairman, published his proposal for a renewal of the basic radiation protection concept (1999 J. Radiol. Prot. 19 107-15). The two main points of his proposed system are: (a) the term 'controllable dose' is introduced and (b) the protection philosophy is based on the individual. For practical use terms like 'action level', 'investigation level' etc are introduced. The outline of the new system promises a much less complex frame; no distinction between practices and interventions and unified treatment for occupational, medical and public exposures. There is, however, an inconsistency within the new system: though linearity is not assumed, the relations between the definitions of the new terms of the system of protection and the doses assigned to them are still based on the LNT hypothesis. To avoid this inconsistency a new definition of action level is recommended as a conservative estimate of the lowest dose where harmful effects have ever been demonstrated. Other levels should be defined by the action level and safety factors applied on the doses.

Caesium-137 migration in Hungarian soils.

Distribution of 137Cs activity concentration has been measured in soils from 19 Hungarian counties. The levels of soil contamination in Hungary after the Chernobyl fallout are sufficient to study the vertical distribution of the 137Cs under natural conditions. Samples were taken layer by layer on plain, grassy and uncultivated fields down to a depth of 20 cm. The results show that radiocaesium migrates very slowly in the soils investigated. The 137Cs concentration peaks in almost all of the soils remain in the top 5 cm layer, but the distribution patterns are different. Variations in diffusion, sorption-desorption, and complexing processes, plant uptake and plant- and animal-originated mechanical changes result in the formation of site-specific radionuclide distribution profiles. On the basis of the experimental data a model has been developed for the long-term prediction of the 137Cs migration. By application of an applied diffusion-convection model the results can be well fitted, the initial deposition can be calculated and the data can be used for the long-term prediction of the 137Cs distribution profile.

Structural differences between human and rat lungs: implications for Monte Carlo modeling of aerosol deposition.

The geometrical structure of the lung is one of the main factors governing inhaled particle deposition; structural differences among different species are, therefore, of great importance for extrapolation modeling. A statistical analysis of morphometric data for the human and rat tracheobronchial tree reveals significant interspecies differences in airway branching patterns: compared to the relatively dichotomous and symmetric structure of the human lung, the rat lung displays a more monopodial airway branching pattern. Thus, for the rat lung, we recommend characterizing the properties of a given airway, i.e., its size, physiologic function, and distance from the trachea, by its diameter rather than by a theoretically assigned generation number. A Monte Carlo method is used to construct an airway geometry along each inhaled particle's path by randomly selecting airway parameters from their frequency distributions and the correlations among them. While the airway geometry is selected randomly, particle deposition in individual airways is calculated analytically. These stochastic deposition calculations allow for structural differences between the human and rat lung, and the variability of the airway system within each species.

Stochastic morphological model of the rat lung.

The laboratory rat is often used as a human surrogate in aerosol inhalation studies. Here we present a new stochastic model for the rat lung analogous to that for the human lung. Morphometric data on the tracheobronchial geometry of the rat lung provided by the Lovelace Inhalation Toxicology Research Institute were analyzed. The results of this statistical analysis reveal significant differences in diameters and branching angles between major and minor daughter tubes starting from the same bifurcation. As a consequence of the more monopodial airway branching in the rat lung compared to the more dichotomous structure of the human lung, we recommend classifying the rat lung airways by their diameters and not by generation numbers. The distributions of the geometric airway parameters and the correlations among them will be used for Monte Carlo deposition calculations.

Calculations on the relationship between gamma source distributions in the soil and external doses.

Calculated activity concentration to free-in-air kerma conversion factors are given for natural emitters distributed differently in the soil. Dose modifying effects of snow and concrete covers are also studied. Finally, air-kerma to effective dose equivalent conversion factors are calculated for anthropomorphic phantoms.

Analysis of human lung morphometric data for stochastic aerosol deposition calculations.

A stochastic lung model is proposed for aerosol deposition calculations. Airway geometry is selected randomly to reflect intrasubject variations in the human airway system. This may also be adjusted to take intersubject variations into account. The statistical analysis of the human airway geometry used is based on morphometric data measured at the Lovelace Inhalation Toxicology Research Institute. Average values and distributions of airway diameters and lengths, distributions of branching angles and criteria for termination of the pathway (when the alveolar region is reached) are presented. Correlations between the cross sections of tubes of succeeding generations and those between diameters and lengths of the same generation are also given.

Depth-dose distribution measurements and calculations in an elliptical phantom.

Calculations and measurements are presented for the dose distribution in a water-filled elliptical phantom when the phantom was irradiated with neutrons from two different, unshielded light-water moderated reactors. The calculations were performed by a Monte Carlo code; for the measurements we used activation, TL and solid-state nuclear track detectors. We observed that the neutron spectra do not vary significantly inside the phantom and that not only the total absorbed dose but also the kerma value at a depth of approximately 2 cm can be higher than that on the front (in our case, by a factor of about 1.2). The measurements and calculations resulted in a kerma attenuation from the front to the back of the phantom a factor of about 5, less than was previously known through the literature.