Dosimetry and clustering properties of energy deposition from beta particles emitted by H-3 and C-14 in the nucleus of a mammalian cell

Abstract

It has been suggested that the more densely ionizing terminal portion of an electron track could be responsible for the majority of biological damage as opposed to the less densely ionizing initial part of the track. To test this hypothesis, it is proposed to compare chromosome aberration yields when cell nuclei are irradiated by incorporated 3H and 14C. The rationale is that virtually all electron tracks from 3H would originate and terminate in the nucleus, whereas almost none of the β particles would stop inside the nucleus for 14C. Since the intention would be to determine RBE values, careful evaluation of dosimetry of β particles from decays of 3H and 14C in the nucleus of a cell is necessary. Three aspects of this dosimetry were addressed: 1) the intranuclear dose, 2) the internuclear dose, and 3) the fraction of dose deposited by electron track ends. A Monte Carlo electron track-structure code was used as the primary tool for solving these problems. Comparisons of results produced by three different Monte Carlo codes in terms of energy deposition in spherical volumes showed good agreement. Track-structure codes were also compared with other analytical methods for beta dosimetry for a variety of nuclear sizes under assumptions of spherical and ellipsoidal geometry for a cell nucleus. A method for estimating the internuclear dose for cells with nuclei of different geometries was described. For cells with flat nuclei, similar to those that will be used in the experiments, the internuclear dose was determined to be negligible for 3H and a few percent of the intranuclear dose for 14C. A K-means clustering algorithm was employed to characterize electron track ends. Criteria were developed and implemented in a scoring procedure to identify clusters associated with track ends of primary and secondary electrons and separate these from the less densely ionizing portion of electron tracks. The fraction of dose due to electron track ends was estimated as ~43% for 3H and ~30% for 14C. These values were independent of geometry for a cell nucleus.