DOSIMETRY OF [O-15]WATER - A PHYSIOLOGICAL APPROACH

Earlier dosimetry estimates for [O-15]water assumed its instantaneous equilibrium with total body water. This assumption leads to an underes timation of the absorbed doses to organs with high blood flows, since the biodistribution of this short-lived radiopharmaceutical is depende nt upon blood flow to organs. We have developed a physiologically base d whole body blood flow model (WBBFM) using a commercially available i con-driven mathematical simulation software package and applied it to the reevaluation of [O-15]water dosimetry in humans. The WBBFM uses mu ltiple parallel compartments to represent organs, heart chambers, the injection site for [O-15]water, and blood sampling sites (arterial and venous). Input values to the WBBFM include organ blood flows, organ m asses, organ water volumes, organ:blood partition coefficients, inject ed activity and S-values of [O-15]. The WBBFM is based on the same ass umptions that are used in calculating regional blood flow using [O-15] water and simulates the human body closely in its physiologic response . The activity in each organ is derived from the simulation and is use d to calculate absorbed doses. The WBBFM calculated absorbed doses in mu Gy/MBq (mrad/mCi) to various organs are as follows: heart-2.66 (9.8 4), kidneys-2.20 (8.15), thyroid-1.83 (6.78), brain-1.66 (6.13), ovari es-1.25 (4.61), breast-1.24 (4.59), and small intestine-1.03 (3.83). T hese values are approximately two- to threefold higher than the earlie r estimates of Kearfott [J. Nucl. Med. 23, 1031-1037 (1982)] and simil ar to the recent findings of Herscovitch et al. [J. Nucl. Med. 34, 155 P (1983)]. We believe this approach yields more realistic dosimetry es timates for [O-15]water. Accordingly, we have revised the amount of [O -15]water administered during regional blood flow studies at our insti tution. The relative ease and accuracy of this approach suggests its u sefulness in dosimetry estimation for other freely diffusible radiopha rmaceuticals. (C) 1996 American Association of Physicists in Medicine.