Recombinant Human TSH (rhTSH)-Aided Radioiodine (131) Therapy for Thyroid Remnant Ablation in Differentiated Thyroid Cancer Patients: a Comparison Between 1850 MBq and 3700 MBq Activities

Aim: Recently, a multicenter study in differentiated thyroid cancer (DTC) patients showed that 3700 MBq of 131I after rhTSH stimulation had successful post-surgical thyroid ablation rate similar to that obtained after thyroid hormone withdrawal. We investigated whether 1850 MBq of 131I have a rate of successful ablation similar to 3700 MBq in patients prepared with rhTSH.

Methods: Seventy-two patients with DTC were randomly assigned after surgery to receive 1850 MBq (group A, n: 36) or 3700 MBq (group B, n: 36) of 131I after rhTSH. The two groups were comparable for age, sex, histotype and tumor stage. One injection of 0.9 mg of rhTSH was administered for two consecutive days; 131I therapy was delivered 24 hours after the last injection. A post-therapy whole body scan (WBS) was performed 72 hours later. Successful ablation was assessed 6-8 months after therapy.

Results: Successful ablation, defined as no visible uptake in the diagnostic WBS after rhTSH stimulation, was achieved in 88.9% of group A and group B patients. Basal and rhTSH-stimulated serum Tg was undetectable (<1 ng/ml) in 88.6 % of group A and 84.8% of group B patients (p=0.65). Similar rates of ablation were obtained in both groups also in patients with lymph node metastases. Dosimetric data showed similar thyroid bed uptake, effective half-life and adsorbed dose in the two groups. Failure to ablate was not correlated with TNM staging, peak TSH levels, thyroid bed uptake and urinary iodine excretion at the time of ablation, but was influenced by the absorbed dose of 131I (< or >300 Gy).

Conclusion: Our results demonstrate that therapeutic 131I activities of 1850 MBq are equally effective as 3700 MBq for thyroid ablation in DTC patients prepared with rhTSH, even in the presence of lymph node metastases.