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H Hänscheid, Department of Nuclear Medicine, University of Würzburg, Würzburg, 97080, Germany
M Lassmann, Würzburg, Germany
M Luster, Ulm, Germany
R Kloos, Columbus, United States
C Reiners, Würzburg, Germany

Correspondence: Heribert Hänscheid, Email: haenscheid{at}nuklearmedizin.uni-wuerzburg.de

Abstract

A simple method is presented to estimate the radiation absorbed dose to the blood after radioiodine administration from a single external measurement of the whole body retention in patients suffering from differentiated thyroid cancer. The blood dose is calculated applying the formalism of the Medical International Radiation Dose Committee MIRD under the assumptions that whole body activity decays exponentially and that 14% of the whole body residence time can be attributed to the blood. Accuracy and applicability of the method were tested based on data from 29 assessments, 18 pre-therapeutic tracer studies and 11 ablation therapies, with whole body and blood retention measurements over at least 4 days. The mean of the absolute deviations between estimates and actual blood doses was found to be 14% if external whole body counting was performed on day 1 or 2 after radioiodine administration. This simple formalism is: 1) applicable to pre-therapeutic dosimetry for remnant ablation or treatment of metastases in a blood dose based treatment concept and 2) applicable to blood dose estimates after radioiodine therapy to determine radiation exposure. When combined with a measurement of the whole body retention 1 or 2 days after radioiodine administration this single time-point method closely approximates the classic, yet much more labor intensive multi-day dosimetry that measures both blood and whole body activities.