Comparing pre-therapeutic ¹²⁴I and ¹³¹I uptake tests with intra-therapeutic ¹³¹I uptake in benign thyroid disorders

¹²⁴I-PET/CT can be used for pre-therapeutic assessment of radioactive iodine uptake in benign thyroid disorders, however systematic comparisons with intra-therapeutic uptake are still lacking for these disorders. The goals of this study were to compare ¹²⁴I RAIU and conventional ¹³¹I RAIU tests with each other; to compare both tests with intra-therapeutic uptake (reference); and to verify the time course of radioactive iodine uptake at three time points (30, 102, and 336 h [14 days] post administration; p.a.).

Thirteen patients with benign thyroid diseases underwent ¹³¹I RAIU test and ¹²⁴I RAIU test one after another before the intra-therapeutic ¹³¹I uptake (reference) was measured via short-range and long-range measurements. After correction for decay, relative uptake differences were calculated and subjected to the Bland-Altman method for the evaluation of levels of agreement.

Radioactive iodine uptake tests with ¹²⁴I-PET/CT and ¹³¹I probe did not show systematic deviations at any time point. Likewise, at 30 and 102 h p.a. there was no systematic discrepancy between pre-therapeutic and intra-therapeutic uptake levels. At 14 days p.a., however, both pre-therapeutic tests tended to overestimate the uptake compared to reference. Findings showed, for the first time with ¹²⁴I, that radioiodine therapy has some early radiobiological effects possibly limiting the accuracy of pre-therapeutic dosimetry.

¹²⁴I RAIU tests represent a feasible alternative to standard ¹³¹I RAIU tests. The additional benefits of ¹²⁴I-PET/CT (e.g., functional topography, inclusion of retrosternal areas, possibility to enable fusion imaging) may thus increase the scope of this technology in benign thyroid disorders.