QUANTITATION OF AT-211 IN SMALL VOLUMES FOR EVALUATION OF TARGETED RADIOTHERAPY IN ANIMAL-MODELS

We have evaluated SPECT and two planar imaging methods, geometric mean (GM) and buildup factor (BF), for their potential to quantitate in vi vo At-211 distributions in rat spinal subarachnoid spaces using phanto m studies. The use of medium-energy collimators and the small diameter (3 mm) of the subarachnoid space complicate quantitation. Net activit ies from distributions in various backgrounds were obtained using a la rge region of interest with background subtraction. Results showed qua ntitation accuracy within 10% for SPECT and BF in low backgrounds incr easing to 25% at higher background levels while GM errors ranged from 20 to 45%. We have also obtained images of [At-211]astatide distributi ons, administered intrathecally, in rats.