Status and trends in the external counting of inhaled heavy elements deposited in vivo

External counting has been routinely used for estimation of plutonium in vivo for approx. 10 yr. However, this method is fraught with inherent uncertainties resulting from the few radiations and the severe attenuation of the radiations. Present counting capability allows detection of from 1/2 to 1 Maximum Permissible Lung Burden (16nCi). Current efforts in the development of counters and in intercalibration may lead to small improvements in detection limits and accuracy, but substantial improvements are not expected. Two troublesome areas in the in vivo counting area are the non-uniform distribution of material within the lungs and the influence of material translocated from the primary deposition site. New concepts such as induced nuclear fission of the deposited material can possibly lead to improvements in accuracy and in the detection limit. (author)