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Biological Analysis and Dosimetry for ¹⁵O-Labeled O₂, CO₂, and CO GasesAdministered Continuously by Inhalation

Memorial Sloan-Kettering Cancer Center, Biophysics Laboratory, New York, New York

Correspondence: For reprints contact: R. E. Bigler, Biophysics Laboratory, Memorial Sloan-Kettering Cancer Center, 1275 York Ave., New York, NY 10021.

ABSTRACT

Estimates of radiation absorbed dose have been determined for the steady-state distribution of oxygen-15 (T_1/2 = 122 sec) from inhalation of molecular oxygen, ¹⁵O₂; carbon dioxide, C¹⁵O₂; and carbon monoxide, C¹⁵O. Biodistribution data for ¹⁵O-labeled water, produced by the metabolism of oxygen and from CO₂ by pulmonary carbonic anhydrase, were used. Lung gas and intravascular activities are also included. The total oxygen utilized was taken to be 14.4 l/hr. Seventeen tissues were included as source organs. The radiation dose is directly proportional to the duration of inhalation. Air containing a constant level of ¹⁵O is provided in excess of need to the patient, who breathes under his own control. The lung, which is known to be a particularly radiosensitive tissue, appears to be the dose-limiting or critical tissue. The radiation dose estimates for lung, based upon 1 hr of breathing air with an activity concentration of 1 mCi/l, are 3.6, 1.2, and 2.8 rads, respectively, for ¹⁵O₂, C¹⁵O₂, and C¹⁵O.