The toxicity of intravenously administered${}^{137}{\rm CsCl}$ in the beagle dog was investigated as part of a program to evaluate the biological effects of internally deposited fission-product radionuclides. The intravenous route of exposure was chosen for simplicity and accuracy because it was known that after intravenous injection, inhalation or ingestion, internally deposited${}^{137}{\rm CsCl}$ is rapidly absorbed and distributed throughout the body, exposing the whole body to β-particle and γ radiations. Fifty-four dogs were injected intravenously with137 Cs to provide one group of six dogs with mean initial body burdens of 141 MBq${}^{137}{\rm Cs}/{\rm kg}$ body mass and four groups of 12 dogs each with mean initial body burdens of 104, 72, 52 and 36 MBq${}^{137}{\rm Cs}/{\rm kg}$. Twelve dogs were injected with isotonic saline as study controls. Because the number of study control dogs was small, data from an additional 49 control dogs from other studies at the Inhalation Toxicology Research Institute that were performed over a similar span of years were also used. There was a significant, dose-dependent decrease in survival of the${}^{137}{\rm Cs}\text{-injected}$ dogs. Eleven${}^{137}{\rm Cs}\text{-injected}$ dogs, including all six in the highest initial body burden group, died within 81 days after injection, primarily due to hematopoietic cell damage resulting in severe pancytopenia. An additional 25 dogs had transient hematological dyscrasia but survived for long times. All${}^{137}{\rm Cs}\text{-injected}$ male dogs had marked damage to the germinal epithelium of the testicular seminiferous tubules with azoospermia in the long-term survivors. Benign and malignant neoplasms occurred in a variety of organs in${}^{137}{\rm Cs}\text{-injected}$ dogs, rather than in a single target organ. When individual organs were considered, the incidence of malignant neoplasms was increased in the liver and in the nasal cavity and paranasal sinuses of the${}^{137}{\rm Cs}\text{-injected}$ dogs. There was a137 Cs treatment effect in the incidence of malignant neoplasms (P < 0.001) in male dogs but no${}^{137}{\rm Cs}\text{-related}$ treatment effect in female dogs. However, when malignant mammary neoplasms were excluded from the analysis, there was no gender difference, and there was a dose-related response (P < 0.001) in both males and females for the incidence of malignant neoplasms.

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