Biological analysis and dosimetry for 15O-labeled O2, CO2, and CO gases administered continuously by inhalation

Estimates of radiation absorbed dose have been determined for the steady-state distribution of oxygen-15 (T1/2 = 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.