Thyroid Stunning After Diagnostic Dose of 185 MBq (5 mCi) Iodine-131 in Patients with Differentiated Thyroidal Cancer

Iodine-131 (131I) plays an important role in the management of patients with well-differentiated thyroid cancers. Post-thyroidectomy ablation therapy with 131I is performed to destroy the small amount of thyroid tissue remaining in the neck after surgery [8,14]. The use of radioiodine for ablation has been shown to decrease the risk of recurrence, increase the sensitivity of post-ablation whole-body scanning with radioiodine and increase the sensitivity of serum thyroglobulin testing [12]. In patients being considered for ablation therapy, a pre-treatment diagnostic 131I scan is performed 5 to 6 weeks following surgery to assess the presence of metastatic lesions. Iodine-131 given for a diagnostic scan can exert a negative effect on the uptake of the therapeutic dose by residual thyroid bed tissue and functioning metastases. This is referred to as (thyroid stunning). The (Stunned) thyroid tissue then loses its iodine trapping function partially or completely. This is a radiobiologic phenomenon and the degree of stunning depends on the absorbed radiation dose. In fact, the higher the diagnostic dose used, the greater the possible subsequent decrease in uptake of the therapeutic dose [25]. While some findings support the concept of stunning, other authors have found no evidence of stunning for diagnostic 131I activities in the range of 37-370 MBq [11,13,15,21,22,26]. Due to inherent difficulties in the exact volumetric determination of remnant or metastatic tissue, it is very difficult to quantify just how much of the diagnostically administered radioiodine is actually absorbed ABSTRACT