Thyroidal uptake and radiation dose after repetitive I‐131‐MIBG treatments: Influence of potassium iodide for thyroid blocking

Authors:
  • MMMyriam Monsieurs
    Myriam Monsieurs
  • HTHubert Thierens
    Hubert Thierens
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Abstract
Background: In I‐131‐MIBG therapy, I‐131‐iodide can be released from the I‐131‐MIBG molecule. Hypothyroidism might result from the undesirable irradiation of the thyroid gland. To prevent this, stable iodide such as potassium iodide (KI) is given to oversaturate the thyroid before I‐131‐MIBG is administered.Procedure: In the present study, the incidence of hypothyroidism (elevated TSH) was correlated with the thyroidal uptake of I‐131 and dose (MIRD dosimetry) after 35 individual treatments in ten patients. Iodine‐131‐MIBG therapy was performed using a modified dosage of 1.9–11.1 GBq (50–300 mCi) IV. Premedication with KI was done as recommended with a dose of 100 mg KI orally from 2 days before until 4 weeks after I‐131‐MIBG.Results: The absorbed thyroidal dose amounted to a very variable range of 0.2 (patient # 1) up to 30.0 (patient 3) Gy with 7.1 ± 7.9 Gy per treatment and 24.1 ± 19.2 Gy per patient (mean ± SD), despite the same and compliantly taken KI premedication protocol. Up to now, 4/10 or 40% of patients have developed hypothyroidism after a mean follow‐up period of 11 months and a mean total administered dose of 18.7 GBq (505 mCi). A trend towards higher thyroidal doses was seen in the hypothyroid patients.Conclusions: This study observes a general high inter‐ and intra‐individual variability in radio‐iodide uptake in the thyroid after I‐131‐MIBG therapy despite KI premedication, as well as possible occurrence of hypothyroidism. A dose‐response relationship needs confirmation on a larger cohort of patients to reach statistical value. An alternative thyroid cytoprotection strategy for possible long‐term survivors may be considered. Med Pediatr Oncol 2002;38:41–46.

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