Commonly, a diagnostic scan using low activities of iodine-131 is performed prior to radioablation to avoid the controversial “stunning effect” [27] from any exposure to sublethal radiation in a diagnostic dose. In stunning, the diagnostic RAI dose decreases uptake of a subsequent therapeutic dose. Alternatively, we use [I-123] radioiodine at very low dose (1.4 mCi) in pre-ablation patients. Uptake in the thyroid bed occurs in 75% to 100% of patients, commonly due to remnant normal thyroid tissue [28].
The typical activity used for RAI ablative therapy is 30–100 mCi. The administration of high activities (150–200 mCi) of [I-131] radioiodine has been used to treat recurrent or metastatic disease. This treatment can be very effective, especially in young patients [29].
Side Effects and Contraindications
Common side effects of radioiodine treatment include sialadenitis, radiation thyroiditis, tumor hemorrhage or edema, nausea, transient oligospermia or amenorrhea and nasolacrimal duct obstruction. Moreover, patients treated with radioiodine have a modest increased risk of developing other malignancies [30].
[I-131]Radioiodine must be avoided in pregnancy and in breastfeeding [31]. Indeed, breast tissue has a strong tendency to uptake iodine so breastfeeding should be stopped 5 to 8 weeks before radioiodine treatment, otherwise it can lead to a false-positive radioiodine scan in the chest [32], or worse, deliver radioiodine to the baby with detrimental effects and potential ablation to the baby’s thyroid gland.
Patients treated with radioiodine are advised to drink abundant water after the treatment in order to increase its renal elimination. If no stool elimination occurs in 14 to 24 hours, laxatives may be indicated to eliminate radioiodine from the gastrointestinal track. In addition, patients are advised to avoid sexual contact, avoid sharing bed, utensils, towels, toothbrushes, razors, and avoid public transportation and public places among other measures to avoid exposing the population to radiation [33]. The duration of this restriction depends on the dose administered.
Adjuvant Treatment in this Patient
As the patient was at high risk for recurrence, he received TSH suppression therapy to levels < 0.1 mIU/L. He was referred to nuclear medicine for I-131 treatment. However, at 3 months following thyroidectomy, thyroglobulin measurement showed an elevation (40.5 ng/mL). Ultrasound showed enlarged lymph nodes at level II at the right and at level II at the left. A FNA of left neck node was positive for papillary thyroid cancer.
• How should the patient be treated now?
Treatment of Locoregional Metastatic Disease
The best treatment for residual disease or local recurrences is surgery. ATA guidelines recommend compartmental lateral and/or central neck dissection for patients with persistent or recurrent disease confined to the neck [6]. Radioiodine can be an alternative when recurrent disease is not visible on imaging. Other treatments that can be used for local recurrences or isolated metastases when surgery is not possible are radiofrequency ablation [34], chemo-embolization [35], or ethanol ablation [36]. External beam radiotherapy, which is discussed later, could also be used in selected cases.
Further Treatment
The patient underwent a bilateral modified radical neck dissection followed by adjunctive radioiodine therapy. His initial radioiodine scan showed mild uptake in the neck at the site of his prior surgery. He received treatment with 215 mCi, then 6 months later he was treated with 250 mCi, as his scan showed continued mild uptake. Eleven months later his radioiodine scan showed no uptake and thyroglobulin levels remained stable at 14.4 ng/mL.