Six to eight- week-old Marshall-August rats were given a single iv injection of 4.5 μCi/kg soluble|${\rm Pu}({\rm NO}_{3})_{4}$|, a dose-level expected to give an approximately 80% yield of osteosarcomas. Localized dose-rates were measured at fixed reference distances between 5 and 20 μm from bone surfaces by counting α-flux entering small cylindrical targets located in a thin nuclear emulsion in contact with a bone section. Dose-rates 1 day after the injection ranged from 22-57, 12-37 and 7-26 rads/day at distances of 5, 12.5 and 20 μm, respectively, from a number of different endosteal surfaces selected for measurement. These local dose-rates changed with time by factors of the order of 2, in either direction, depending on the prevailing conditions of remodeling. Histological evidence of "preferred" sites for the development of osteosarcomas is discussed in relation to local α-dose. A cumulative α-dose of about 2000 rads (delivered to primitive cells at trabecular surfaces over a period of 36 wk) may be associated with a 10% probability of developing a tumor in an individual femoral epiphysis.

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