Inactivation of haemopoietic stem cells by slow alpha-particles.

Inactivation of murine haemopoietic stem cells, assayed as spleen colony-forming units (CFU-S), has been determined after in vitro irradiation under well defined conditions with small numbers (0.15-3) of alpha-particles of incident energy 3.3 MeV (LET 121 ke V microns-1). Exponential survival curves were obtained with inactivation dose coefficients of 1.83 +/- 0.11 and 1.63 +/- 0.06 Gy-1 for 8 and 12-day CFU-S, respectively, corresponding to inactivation cross sections of 35.6 +/- 2.1 and 31.7 +/- 1.2 microns2. The results indicate that these radiosensitive stem cells have a significant probability of surviving the passage of a single alpha-particle track, estimated at 8 and 18% per particle passage, respectively, if the cells have a diameter of 7 microns. However, this estimation is strongly dependent on the assumed diameter. Conversely, it is demonstrated that the cells cannot have mean geometric diameters of less than 6.7 +/- 0.2 and 6.4 +/- 0.1 microns, respectively. Associated experiments have shown that some cells from the irradiated population do survive to form colonies that express delayed non-clonal chromosome aberrations.