Cellular and subcellular effects of very heavy ions.

The biological effects of irradiation with ions of masses larger than 40 and energies up to 20 MeV per atomic mass unit are reviewed. The objects are viruses, bacterial spores, yeast and mammalian cells. Experimental parameters include loss of colony forming ability, induction of mutants, chromosomal aberrations, cell cycle progression, inhibition of biochemical activities and the formation of strand breaks. Some of the pertinent physical questions--e.g. track structure--are also discussed. It is shown that with very heavy ions the biological effectiveness is no longer unambiguously related to a single parameter like l.e.t. or Z*2/beta 2 but depends strongly on ion energy. This points to the importance of far-reaching delta-electrons. The analysis indicates also that even with very high l.e.t., cells are not killed by the passage of a single particle through their nucleus. Possible implications of the findings for fundamental radiation biology are outlined.