Ablation of Thyroid Residues with 30 mCi 131I: A Comparison in Thyroid Cancer Patients Prepared with Recombinant Human TSH or Thyroid Hormone Withdrawal

Abstract

The aim of the study was to assess whether stimulation by recombinant human TSH (rhTSH) may be used in patients with differentiated thyroid carcinoma for postsurgical ablation of thyroid remnants using a 30-mCi standard dose of 131I during thyroid hormone therapy. The rate of ablation was prospectively compared in three groups of patients consecutively assigned to one of three treatment arms: in the first arm, patients (n = 50) were treated while hypothyroid (HYPO); in the second arm, patients (n = 42) were treated while HYPO and stimulated in addition with rhTSH (HYPO + rhTSH); in the third arm, patients (n = 70) were treated while euthyroid (EU) on thyroid hormone therapy and stimulated with rhTSH (EU + rhTSH). The outcome of thyroid ablation was assessed by conventional HYPO 131I scan performed in HYPO state 6–10 months after ablation. Basal serum TSH was elevated in the HYPO and HYPO + rhTSH groups. In the EU + rhTSH group, basal serum TSH was 1.3 ± 2.5 μU/ml (range, <0.005–11.9 μU/ml). After rhTSH, serum TSH significantly increased in the HYPO + rhTSH group and the EU + rhTSH group. Basal 24-h radioiodine thyroid bed uptake was 5.8 ± 5.7% (range, 0.2–21%) and 5.4 ± 5.7% (range, 0.2–26%) in the HYPO and HYPO + rhTSH groups, respectively. In the HYPO + rhTSH group, mean 24-h thyroid bed uptake rose to 9.4 ± 9.5% (range, 0.2–46%) after rhTSH (P < 0.0001). The 24-h uptake after rhTSH in the EU + rhTSH group was 2.5 ± 4.3% (range, 0.1–32%), significantly lower (P < 0.0001) than that found in the HYPO and HYPO + rhTSH groups. The rate of successful ablation was similar in the HYPO and HYPO + rhTSH groups (84% and 78.5%, respectively). A significantly lower rate of ablation (54%) was achieved in the EU + rhTSH group. Mean initial dose rate (the radiation dose delivered during the first hour after treatment) was significantly lower in the EU + rhTSH group (10.7 ± 12.6 Gy/h) compared with the HYPO + rhTSH group (48.5 ± 43 Gy/h) and the HYPO group (27.1 ± 42.5 Gy/h). In conclusion, our study indicates that by using stimulation with rhTSH, a 30-mCi standard dose of radioiodine is not sufficient for a satisfactory thyroid ablation rate. Possible reasons for this failure may be the low 24-h radioiodine uptake, the low initial dose rate delivered to the residues, and the accelerated iodine clearance observed in EU patients. Possible alternatives for obtaining a satisfactory rate of thyroid ablation with rhTSH may consist of increasing the dose of radioiodine or using different protocols of rhTSH administration producing more prolonged thyroid cells stimulation.