Comparison of I-131 Biokinetics after Recombinant Human TSH Stimulation and Thyroid Hormone Withdrawal Measured by External Detector in Patients with Differentiated Thyroid Cancer.

The aim of this study was to compare radioactive iodine (I-131) biokinetics after recombinant human TSH stimulation (rhTSH) and thyroid hormone withdrawal (THW) in patients with differentiated thyroid cancer (DTC). External effective dose rates were measured using external detectors and imaged quantitatively at the time of discharge from the isolation wards. We retrospectively analyzed 32 patients who had been diagnosed with DTC, papillary or follicular, and underwent remnant ablation after either rhTSH stimulation (n=22) or THW (n=10). The uptake of I-131 by remnant thyroid tissue was measured from 20.0 cm, 100.0 cm and 200.0 cm distances using a handheld external detector. The remnant thyroid tissue measured by the whole body images two to five days from administration was 10.7+26.0% (range 0.5 to 60.0%). The values measured at 20 cm were best correlated to the thyroid residual uptake measured by SPECT/CT. The half-lives of I-131washout (T1/2) in rhTSH group measured by external detector were shorter than those of THW group. T1/2 becomes longer when it was measured over longer distances. They were 10.9, 12.3 and 13.1 hours at distances of 20, 100, and 200 cm in rhTSH group, respectively. The TWH group showed 12.8, 14.9 and 17.7 hours, respectively. We conclude that I-131 biokinetics can be measured by external detector after high dose I-131 therapy for DTC. It showed that washout of I-131 was faster after rhTSH stimulation than THW, and slower in patients with distant metastasis than those without metastasis.