Studies on the Radiosensitivity of Bone Marrow : II. The Effect of Large, Repeated Whole Body Irradiation Exposure on Hematopoiesis

1. Six animals (cats) were subjected to four whole body irradiation exposures over a period of one and one-half years. Irradiation dosage was 200 r to the whole body, an amount producing severe hematopoietic abnormalities in the cat. Irradiation in each case was immediately preceded by a large phlebotomy, so that functional impairment of erythropoiesis resulting from irradiation could be assessed in terms of a defective regeneration of erythrocytes. The effect of multiple radiation exposures could also be evaluated for each animal in terms of its prior response to phlebotomy and irradiation. The effects of repeated exposures on the leukocytes could likewise be assessed by means of the comparative leukocyte regeneration curves.

2. Little evidence of permanent functional damage of erythropoietic tissue by repeated exposures under the conditions of this experiment were observed. Erythrocyte and hemoglobin regeneration patterns were very similar from one exposure to another. However, hemoglobin regeneration was slightly slower after the fourth and last irradiation. Whether this represents encroachment on marrow reserve by multiple irradiations or whether it was an artifact resulting from iron deficiency cannot be said with certainty. Multiple exposures exerted little apparent effect on the leukocyte regeneration curves.

3. After recovery from the fourth irradiation, the percentage of mononuclear leukocytes was somewhat less than that prior to the first.

4. It is concluded that while repeated irradiation insults under the conditions of our experiment probably result in hidden damage even in the presence of apparent complete recovery, this damage is so subtle that it is difficult to detect by the tests of marrow reserve employed in this study.