Effects of Protracted Irradiation on the Blood-forming Organs of the Rat. Part II: Divided Doses

Studies are reported of the response of the blood and bone-marrow of rats to daily doses of whole-body X irradiation ranging from 200 rads to 15 rads per day and given at a dose-rate of about 30 rads/min. The results are compared with those obtained using continuous irradiation, reported in Part I of this paper. The response is similar in that a steady state in the peripheral blood count can be maintained for long periods of time at the lower dose-rates, but under divided doses of radiation the steady state is represented by a sub-normal level of peripheral blood count. Also the total radiation doses that can be accumulated before death at the various dose-rates are somewhat lower with divided than with continuous irradiation. The part played by the spleen in maintenance of mononuclear count is less marked with divided than with continuous irradiation, though in both cases splenectomy increases survival at the dose-rates investigated, probably a result of a reduced platelet fall. Some discussion of possible mechanisms involved in the maintenance of the steady state is given.