The response of stromal progenitor cells in mouse marrow to graded repeated doses of X rays or neutrons.

After four repeated doses of 1.5 Gy X rays to mice (3-week intervals), the number of colonies containing fibroblasts that could be grown in vitro from femoral marrow recovered to control numbers by 80 days. After four higher doses, recovery was incomplete, and after four doses of 4.5 Gy, the number was half of the control level up to 1 year after irradiation. After four doses of 14.7 MeV neutrons, no threshold was detected for this late effect, and the RBE was about 2 at 4.5 Gy X rays and about 6 at 1.5 Gy X rays. The RBE for the acute effect on the stromal progenitor cells was also about 2 at 4.5 Gy X rays. The stromal progenitor cells could not be spared relative to hemopoietic stem cells (CFU-S) after neutrons, in contrast to the sparing observed after low-dose fractions of X rays. Experiments with local X irradiations of the leg, and with subcutaneous transplants of femora, revealed no significant recovery of stromal progenitor cells due to migration. However, recovery in situ could be enhanced slightly by marrow grafts i.v.