Thyroid Stunning by Diagnostic Use of Radioiodine 1–131

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Thyroid stunning is a radiobiological phenomenon observed when a relatively high dose of 1–131 is used for diagnostic purposes before radioablation therapy for patients with well-differentiated thyroid cancers. The thyroid remnant or cancer cells are rendered functionally less active in trapping the iodide but are not killed. The ensuing radioablation therapy becomes less effective because the stunned cells cannot take up the therapeutic radioiodine to its fullest potential. The dilemma is that to detect metastases with higher diagnostic sensitivity, one needs to use a higher scanning dose of radioiodine, but doing so increases the risk of stunning the cells. This undesirable effect has been observed with scanning doses of as little as 3 mCi.

There appears to be no easy solution to this problem. One possibility is utilizing radioiodine I-123 as the scanning agent. It is, unfortunately, more costly and may not be available in certain parts of the world. Other alternatives include using a smaller dose of 1–131 and obtaining the scan earlier than at the usual 72 hours, or using non-iodine radiopharmaceuticals such as Tc99 m sestamibi, T1–201, or F-18 fluorodeoxyglucose. These have their own drawbacks. Further studies are indicated to find a cost-effective method of detecting metastases without stunning the cells before radioablation therapy.

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