IN VIVO SENSITIVITY OF MURINE HAEMOPOIETIC PROGENITOR CELL POPULATIONS TO MIXED GAMMA -RAYS-NEUTRON IRRADIATION AT DIFFERENT GAMMA /N RATIOS

: Changes in mice haemopoietic cellular populations and in the radiosensitivity of CFU-C and BFUe progenitors cells were determined in vivo for mixed field radiations composed of a gamma-ray component and a neutron component. Five Dgamma/Dtotal ratios (gamma-rays over total dose ratios, quoted as tau in this report) were obtained (tau = 0.95, 0.83, 0.67, 0.33 and 0.09). Myelogram changes were enlarged with the increase of the neutron component. Radiosensitivity of the two progenitor cell lineages were increased with lower tau values (excess of neutrons). The radiosensitivity of haemopoietic progenitor cells exposed in vivo varies with the ratio of the high- and low-LET components in the mixture. The D0 value varied from 3.3 +/- 0.22 to 0.85 +/- 0.04 Gy with the decrease of tau for CFU-C and from 2.08 +/- 0.22 to 0.64 +/- 0.07 Gy for BFUe. The obtained relative biologic efficiency (RBE) varied from 1.2 +/- 0.08 to 4.7 +/- 0.24 for CFU-C and from 1.1 +/- 0.1 to 3.6 +/- 0.16 for the BFUe. The relation between RBE and tau could be somewhat non-linear for CFU-C and seems to be close to linear for BFUe. The higher is the neutron component, the higher is the radiosensitivity. These results indicate that variations of the quality of the mixed field in the haemopoietic local territory are of great importance in terms of radiation damage and cell killing as well as in terms of the ability to restore the haemopoietic system.