Development of a Myeloproliferative Disorder in Beagles Continuously Exposed to ⁹⁰Sr

In a continuing study of the toxicity of ⁹⁰Sr in beagles exposed continuously from mid-gestation to 1.5 years of age, a myeloproliferative disorder has arisen in dogs fed the two highest dose levels, 12 and 4 µCi ⁹⁰Sr/day, respectively. Ten cases have occurred thus far at the highest level and 4 at the second highest, the difference in incidence being approximately equal to the threefold difference in dose rate. Age at onset has ranged from 14 to 72 months, and cumulative dose rates up to the time of death have varied from about 1,000 to 10,000 rads.

The 14 cases of myeloproliferative disorder present a morphologic spectrum which manifest in the acute form as granulocytic leukemia and in the chronic form resemble myelofibrosis with myeloid metaplasia (MMM). Anemia, poikilocytosis, anisocytosis, hypochromasia and terminal thrombocytopenia were constant features. Terminal leukocyte counts ranged from 3,300 to 38,900 cells/ mm³, with various degrees of shift to the left. Splenomegaly was the salient gross finding in most dogs. Major histologic lesions were pronounced granulocytic proliferation in bone marrow and spleen, with concomitant erythroid and megakaryocytic depletion. Liver, lymph nodes and a variety of other organs had less extensive and less frequent involvement. In the most acute cases, bone marrow and spleen contained cell populations composed almost entirely of blast cells.

This experimental situation should serve to shed light on fundamental aspects of radiation-induced granulocytic leukemia and might help to clarify the relationship between granulocytic leukemia and MMM.