High incidence of thyroid dysfunction despite prophylaxis with potassium iodide during ¹³¹I-meta-iodobenzylguanidine treatment in children with neuroblastoma

BACKGROUND. Treatment modalities like targeted radiotherapy with ¹³¹I-meta-iodobenzylguanidine (¹³¹I-MIBG) improve survival rates after neuroblastoma (NB). Radiation to the thyroid gland can lead to hypothyroidism and even malignancy. Because hypothyroidism after ¹³¹I-MIBG treatment was reported, the current KI prophylaxis against thyroidal radiation damage was evaluated. METHODS. The incidence, pathogenesis, and consequences of thyroid dysfunction among 42 NB patients treated with ¹³¹I-MIBG were evaluated retrospectively. Efficacy of KI prophylaxis was established by measuring thyroidal radioiodide uptake. Thyroid damage was expressed as thyrotropin elevation (TE, plasma concentration of thyroid stimulating hormone ≥4.5 mU/L). RESULTS. The mean followup was 2.3 years (range, 0.1-8.5). The mean number of treatments with ¹³¹I-MIBG was 3.3. Of 428 scintigrams, uptake of ¹³¹I in the thyroid was visible in 92 (21.0%). Twenty two patients (52.4 %) presented TE after a mean period of 1.4 years (range, 0.1-5.8). Clinical signs of hypothyroidism were not observed. Eight patients received suppletion therapy with thyroxine. Thyrotropin elevation was transient in four patients. Of 25 survivors, with a mean followup of 3.5 years, 16 (64%) developed TE. No correlation was found between TE and thyroid visualization after ¹³¹I-MIBG administration or the number of treatments. No abnormalities were seen by ultrasound imaging of the thyroid. CONCLUSIONS. Occurrence of thyroid dysfunction after treatment with ¹³¹I-MIBG for NB is high, in spite of KI prophylaxis. Close followup of thyroid function and structure is required in patients treated with ¹³¹I-MIBG. New ways of protecting the thyroid during exposure to radioiodine should be developed.