Using the method of${}^{86}\text{rubidium}$ extraction as developed by Sapirstein, we have studied the uptake of this isotope to measure changes in perfusion of normal mouse kidney, lung, liver, skin, and skeletal muscle after regional single doses of 240 kV X-rays estimated to be at or near the level of normal tissue tolerance for viscera. In the kidney, the extraction of isotope declines soon after irradiation and becomes significantly decreased from age controls by 2 months after 1100, 1500, or 1900 rads. This decrease in86 Rb extraction appears dose-dependent, continues to decline for at least 1 yr, and preceded histologic fibrosis by several months. The uptake of86 Rb in irradiated lung decreased significantly between 3 and 4 months after which it gradually returned to normal levels. No significant change in86 Rb extraction was seen in liver, skin or muscle after doses up to 1500 rads. Details of this isotopic technique and possible implications of perfusion changes in normal tissues for clinical radiation therapy are discussed.

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