What is the best pre-therapeutic dosimetry for successful radioiodine therapy of multifocal autonomy?

M. Gotthardt, C. Rubner, A. Bauhofer, F. Berce, W. Oyen, J. Goecke, A. Pfestroff, A. Schlieck, F. Corstens, M. Béhé and T. Behr

Department of Nuclear Medicine, St. Radboud University Nijmegen Medical Center, PO Box 9101, 6500 HB Nijmergen, The Netherlands. M.Gotthardt@nucmed.umcn.nl


Dose calculation for radioiodine therapy (RIT) of multifocal autonomies (MFA) is a problem as therapeutic outcome may be worse than in other kinds of autonomies. We compared different dosimetric concepts in our patients.Data from 187 patients who had undergone RIT for MFA (Marinelli algorithm, volumetric compromise) were included in the study. For calculation, either a standard or a measured half-life had been used and the dosimetric compromise (150 Gy, total thyroid volume). Therapeutic activities were calculated by 2 alternative concepts and compared to therapeutic success achieved (concept of TcTUs-based calculation of autonomous volume with 300 Gy and TcTUs-based adaptation of target dose on total thyroid volume).If a standard half-life is used, therapeutic success was achieved in 90.2\% (hypothyroidism 23,1\%, n = 143). If a measured half-life was used the success rate was 93.1\% (13,6\% hypothyroidism, n = 44). These differences were statistically not significant, neither for all patients together nor for subgroups eu-, hypo-, or hyperthyroid after therapy (ANOVA, all p > 0.05). The alternative dosimetric concepts would have resulted either in significantly lower organ doses (TcTUs-based calculation of autonomous volume; 80.76 +/- 80.6 Gy versus 125.6 +/- 46.3 Gy; p < 0.0001) or in systematic over-treatment with significantly higher doses (TcTUs-adapted concept; 164.2 +/- 101.7 Gy versus 125.6 +/- 46.3 Gy; p = 0.0097).TcTUsbased determination of the autonomous volume should not be performed, the TcTUs-based adaptation of the target dose will only increase the rate of hypothyroidism. A standard half-life may be used in pre-therapeutic dosimetry for RIT of MFA. If so, individual therapeutic activities may be calculated based on thyroid size corrected to the 24h ITUs without using Marinelli's algorithm.