I-131 biokinetics of remnant normal thyroid tissue and residual thyroid cancer in patients with differentiated thyroid cancer: comparison between recombinant human TSH administration and thyroid hormone withdrawal.

OBJECTIVE: The aim of this study was to assess I-131 biokinetics in thyroid cancer and remnant tissue in patients with differentiated thyroid cancer using whole-body scan (WBS) and SPECT images acquired after I-131 therapy. The influence of thyroid stimulating hormone (TSH) stimulation method on the kinetics was also evaluated.

METHODS: A total of 57 patients who received I-131 therapy (2.96-7.4 GBq) were retrospectively included. TSH stimulation was achieved by recombinant human thyrotropin (rhTSH) or by thyroid hormone withdrawal (THW). Each patient received three sequential WBSs on days 1, 2, and 4 (or 5) after I-131 administration. All lesions were classified either as thyroid remnant (ThyR) or as metastatic lymph nodes (mLN) after considering the SPECT/CT images acquired during the last WBS. The lesion-based retention rate and absorbed dose of ThyR and mLN were calculated using a commercial dosimetric toolkit combined with the OLINDA software.

RESULTS: The retention rate and the effective half-time of mLN were lower than that of ThyR (p < 0.001, p = 0.003). In addition, the retention rate and the effective half-time of ThyR in the rhTSH group were higher than those in the THW group (p < 0.001, p < 0.001). The differences in the retention rate and the effective half-time of mLN were not statistically significant between the THW group and rhTSH group (p = 0.549, p = 0.571).

CONCLUSIONS: Radioiodine therapy using rhTSH delivered an at least similar radiation dose to target lesions compared to using THW in thyroid remnants and metastatic lymph nodes.