Short- and long-term effects of whole-body irradiation with fission neutrons or X rays on the thymus in CBA mice.

Young adult (6 weeks old) female CBA mice were exposed to whole-body irradiation with either 2.5-Gy fast fission neutrons of 1 MeV mean energy or 6.0-Gy 300 kVp X rays at centerline dose rates of 0.1 and 0.3 Gy/min, respectively. The weight of spleen and animal and the weight, cellularity, and histological structure of the thymus were studied at different times after irradiation. Thymic recovery after whole-body irradiation showed a biphasic pattern with minima at 5 and 21 days after irradiation and peaks of regeneration at Days 14 and 42 after X irradiation or at Days 14 and 70 after neutron irradiation. After the second phase of recovery, a marked decrease in relative thymus weight and cellularity was observed, which lasted up to at least 250 days after irradiation. Splenic recovery showed a monophasic pattern with an overshoot on Day 21 after irradiation. After neutron irradiation a late decrease in relative spleen and animal weight was observed. The observed late effects on thymus and spleen weight and thymus cellularity are discussed in terms of a persistent defect in the bone marrow.