BIOPHYSICAL ANALYSIS OF THE DOSE-DEPENDENT OVERDISPERSION AND THE RESTRICTED LINEAR-ENERGY-TRANSFER DEPENDENCE EXPRESSED IN DICENTRIC CHROMOSOME DATA FROM ALPHA-IRRADIATED HUMAN-LYMPHOCYTES

Experimental data for the induction of dicentric chromosomes in phytoh emagglutinin (PHA)-stimulated human T lymphocytes by Am-241 alpha-part icles obtained by Schmid et al. have been analyzed in the light of bio physical theory. As usual in experiments with alpha-particles, the rel ative variance of the intercellular distribution of the number of aber rations per cell exceeds unity, and the multiplicity of the aberration s per particle traversal through the cell is understood as the basic e ffect causing this overdispersion. However, the clearly expressed dose dependence of the relative variance differs from the dose-independent relative variance predicted by the multiplicity effect alone. Since s uch dose dependence is often observed in experiments with alpha-partic les, protons, and high-energy neutrons, the interpretation of the over dispersion needs to be supplemented. In a new, more general statistica l model, the distribution function of the number of aberrations is int erpreted as resulting from the convolution of a Poisson distribution f or the spontaneous aberrations with the overdispersed distributions fo r the aberrations caused by intratrack or intertrack lesion interactio n, and the fluctuation of the cross-sectional area of the cellular chr omatin must also be considered. Using a suitable mathematical formulat ion of the resulting dose-dependent overdispersion, the mean number la mbda(1) of the aberrations produced by a single particle traversal thr ough the cell nucleus and the mean number lambda(2) Of the aberrations per pairwise approach between two alpha-particle tracks could be esti mated. Coefficient alpha of the dose-proportional yield component, whe n compared between Am-241 alpha-particle irradiation and Cs-137 gamma- ray exposure, is found to increase approximately in proportion to dose -mean restricted linear energy transfer, which indicates an underlying pairwise molecular lesion interaction on the nanometer scale.