Abstract
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Objectives The aim of this study was to assess I-131 biokinetics of remnant normal thyroid tissue and residual thyroid cancer in patients with differentiated thyroid cancer (DTC) using whole body scan (WBS) and SPECT acquired after I-131 therapy. Influence of TSH stimulation method to the kinetics was also evaluated.
Methods A total of 57 thyroidectomized DTC patients who received I-131 therapy from 2.96 to 7.4GBq were prospectively included. TSH stimulation was achieved by either recombinant human TSH (rhTSH, n=23) or by thyroid hormone withdrawal (THW, n=34). Every patient received 3 sequential WBSs on the 1, 2, and 4-5 day after I-131 administration and a SPECT/CT also acquired at last WBS. Lesion-based retention rate and absorbed dose of ThyR and mLN were calculated based on the following formula [A(n) / {A(1) × exp (-λ × Δt)}, n=1, 2, 3 (A(n) : observed γ-counts on the n-th scan, λ : decay constant, Δt : time duration from 1st to n-th scan)] using commercial dosimetric toolkit combined with OLINDA software.
Results A total of 126 lesions in 57 patients classified as thyroid remnant (ThyR) (n=118) or metastatic lymph node (mLN) (n=8) by considering the finding of SPECT/CT. The retention rate of mLN was lower than that of ThyR (p<0.001) and retention rate of ThyR in the rhTSH group was higher than that in the THW group (p<0.001). The absorbed doses (Gy/GBq) of ThyR and mLN in the rhTSH group were seemed to be higher than those in the THW group (61.3 and 21.9 vs. 51.7 and 9.5, respectively), but the difference was not statistically significant (p=0.662 and 0.064). Interestingly in one patient, mLNs were visualized on the first two scans, not on the third scan.
Conclusions Biokinetics of I-131 seems to be different by the biological nature of lesion and by the method of TSH stimulation. Optimal I-131 dose for thyroid ablation and therapy can be different by the method of TSH stimulation. Differentiation of ThyR and mLN might be possible with pattern of I-131 kinetics measured from the sequential planar scans.