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The role of 123I‐diagnostic imaging in the follow‐up of patients with differentiated thyroid carcinoma as compared to 131I‐scanning: avoidance of negative therapeutic uptake due to stunning

OBJECTIVE Some patients with relapsed differentiated thyroid cancer may show rising thyroglobulin (Tg) levels despite a lack of 131I uptake on routine whole body imaging. A significant proportion of these patients, after therapy doses of 131I, may demonstrate positive 131I uptake with a subsequent fall in serum Tg, implying a therapeutic effect. Attempts to identify such patients by increasing the dose of the diagnostic 131I tracer may lead to inhibition of subsequent uptake after the therapy dose, an effect referred to as ‘stunning’ and associated with a reduction in therapeutic effect. 123I is a short half‐life γ‐emitter, thought to be unlikely to cause stunning, which may thus be more suitable than 131I for diagnostic imaging of thyroid cancer.