A method for imaging therapeutic doses of iodine-131 with a clinical gamma camera

Imaging therapeutic doses of 131I-labeled monoclonal antibody would provide valuable biodistribution data for dosimetry, but gamma cameras are unable to accurately handle the corresponding high counting rate. To image patients undergoing radioimmunotherapy, we attached 1.6- to 6.4-mm-thick Pb sheets to the front face of a high-energy parallel-hole collimator. With this method, we were able to acquire planar images of up to 700 mCi of radiolabeled antibody 1 hr after infusion. Monte Carlo simulations indicated that less than 7% of the events counted in the photopeak window were due to 364-keV photons that scattered in the Pb attenuator. Measurements indicated that the Pb sheets degraded system resolution by no more than 13%. A quantitative comparison of trace and therapy biodistribution data from planar images of the same patient was made using corrections for Pb sheet attenuation and camera deadtime.