DETERMINATION OF RADIATION DOSIMETRY FOR PET RADIOPHARMACEUTICALS

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The introduction of new positron emission tomography (PET) radiopharmaceuticals for human use requires appropriate radiation dose estimates. We have developed a convenient approach to dosimetry using PET imaging in the rhesus monkey. Animals (5-6 kg) are positioned in the gantry of a whole-body PET scanner with the long axis of the animal parallel to the imaging planes to obtain sagittal slices through the body. Thus most or all of the animal is within field of view of the scanner, providing simultaneous visualization of all organs and tissues without repositioning. Larger animals can be imaged using standard transaxial slices. Region-of-interest (RO1) placement in the emission images is aided by information from the transmission scan used for attenuation correction and from an oxygen-15 water scan that delineates high-flow organs. Since it is not possible to identify accurately the border of organs in the emission images to obtain their total activity, ROls are used to sample tissue activity. Summing ROI data over the sequence of non- decay corrected emission images provides the time-integral of radioactivity per cc of tissue. To calculate residence times, information on organ density (to obtain data per gram of tissue) and monkey organ weights from standard veterinary tables (to obtain whole organ data) are used. The organ residence times determined in the monkey are then extrapolated to humans using a method that adjusts for organ weight in relation to body weight. Dose estimates can then be obtained with standard MIRDOSE software. This approach was used to obtain radiation dose estimates for C-11 arachidonic acid, a radiotracer to study brain phospholipid metabolism. The liver was found to receive the largest dose (0.11 rad/mCi), followed by lung and kidney; the effective dose was 0.021 rem/mCi. This method is being applied to determine radiation dose estimates for other C-11 and F-18 labeled PET radiopharmaceuticals.