Cerium-144, either as the chloride or as the fluoride, was intratracheally injected into rats at dose levels that ranged from 0.5 to 50 microc. In the case of both substances, the radiocerium was deposited in the lungs as descrete focii of radiation. Lung cancer was found at every treatment level. Among 395 rats so treated that survived for 30 days or longer, a total of 101 malignant neoplasms was found. On the basis of pulmonary clearance studies, the organ mean radiation dose that had been absorbed by the lung, from the radioactive particulates, during the survival time of each rat was calculated. The cumulative tumor frequency, when plotted against the logarithm of the organ mean dose, resulted in the lower half of an ogiveshaped dose-response curve.
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