Repeated inhalation exposure of Syrian hamsters to 144CeO2. III

Syrian hamsters have been repeatedly exposed to aerosols of 144CeO2 particles. The biological effects observed will be compared with those following a single inhalation exposure to 144CeO2. Lung burdens (LB) of 144Ce were reestablished at 0.4, 2.0, or 10 μCi at bimonthly intervals for one year beginning at 84 days of age. Other Syrian hamsters were exposed once at 84, 220, or 360 days of age. Control Syrian hamsters were exposed once or repeatedly to stable CeO2. Repeated inhalation exposure resulted in cumulative beta radiation dose rates 1.5 to 2.7 times those in Syrian hamsters exposed once. Reestablishment of the LB at 10 μCi significantly shortened the median survival time whereas reestablishment of the LB at 0.4 or 2 μCi had a minimal effect on median survival time. Three pulmonary carcinomas were observed in Syrian hamsters in which the LB was reestablished at 2 μCi. No pulmonary tumors were observed in Syrian hamsters exposed once to 144CeO2 or to stable CeO2. These results indicate that a maintained beta radiation dose rate to the lungs may be an important factor in the development of malignant pulmonary tumors