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

Purpose: 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. Patients, methods: 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). Results: 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). Conclusions: TcTUs-based 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. (orig.)