Initial damage in human interphase chromosomes from alpha particles with linear energy transfers relevant to radon exposure.

To determine the efficiency at which alpha particles at LETs chosen to simulate exposure to radon progeny break chromosomes, the premature chromosome condensation technique was used to measure breaks soon after irradiation. Noncycling human fibroblasts were irradiated with graded doses of monoenergetic alpha particles accelerated to produce LETs of 90, 120, 150, 180 and 200 keV/microns at the midpoint of the cell nuclei. Premature chromosome condensation was initiated immediately after irradiation and cells were scored for the total number of prematurely condensed chromosomes and fragments per cell. Similar experiments were conducted with 250 kVp X rays for comparison. Irradiation with alpha particles produced 8.6 to 13.1 excess fragments per gray, while X rays produced 5.8 excess fragments, resulting in RBEs around 2. Calculations of the number of breaks produced on average by a single particle traversal of a cell nucleus indicated that at the LETs tested more than one break (1.5-2.8) was produced by each traversal, the maximum being that produced by 180 keV/microns alpha particles. When chromosome aberrations are scored at metaphase after high-LET irradiation, RBEs considerably greater than those recorded here (approximately 2) have been reported. These results showing relatively small differences in initial break levels for alpha particles in the LET range of the radon progeny relative to X rays indicate that the greater aberration frequencies are not due principally to an increase in breakage efficiency, but interactions between breaks along the same particle track are important.