Negative correlation between therapeutic success in radioiodine therapy and TcTUs: are TcTUs-adapted dose concepts the only possible answer?

Calculation of iodine-131 activities for radioiodine treatment (RIT) in patients with disseminated thyroid autonomy may be difficult because of uncertainties in the determination of the autonomous volume (vol(aut)). The algorithm established by Emrich is used for calculation of the vol(aut) based on the TcTUs (technetium thyroid uptake under TSH suppression) (vol(aut)= 5xTcTUs+0.6). Clinical experience using this approach has shown that there is a negative correlation between increasing TcTUs and the results of RIT. Our aim was to identify the reasons for this observation as well as to assess the relation between TcTUs and sonographic vol(aut). Furthermore, we intended to find an alternative algorithm for the TcTUs-based calculation of the vol(aut). Data from 100 patients with unifocal autonomy who met strict inclusion criteria were used to evaluate the correlation between TcTUs and sonographic vol(aut). Using Marinelli's algorithm, we calculated the therapeutic activities for a standardised patient at a target dose of 300 Gy. The vol(aut) was determined based on the TcTUs using the four published algorithms [Emrich 1993 (vol(aut)= 5xTcTUs+0.6), Kreisig 1992 (vol(aut)=10xTcTUs-9.3), Joseph 1977 (vol(aut)=8.33xTcTUs-6.67) and 1994 (vol(aut)=2.88xTcTUs+0.09)]. We then compared the results of the calculation of therapeutic activities obtained using Emrich's algorithm (with known success rates) with those obtained by the other algorithms in order to determine which algorithm would lead to better results in RIT. Only a weak correlation was found between the TcTUs and the sonographic vol(aut) ( r(2)=0.39). The calculated therapeutic activities of (131)I were similar for all algorithms at a TcTUs of around 2% but Joseph's (1977) and Kreisig's (1992) algorithms resulted in clearly higher activities than Emrich's algorithm at a TcTUs above 2%. The need for target doses to increase with TcTUs in RIT may be overcome by the use of adequate algorithms for determination of the vol(aut). The algorithm published by Joseph and co-workers in 1977 probably offers the most reliable approach to the TcTUs-based calculation of vol(aut) in RIT. In contrast to the other algorithms, it is based on autoradiographic planimetric data. Thus, it takes into account the polyclonal origin of thyroid nodules as well as the presence of regressive or cystic changes. The well-established algorithm of Emrich underestimates the true vol(aut), which explains the decreasing success of RIT with increasing TcTUs.