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 gamma-emitter, thought to be unlikely to cause stunning, which may thus be more suitable than 131I for diagnostic imaging of thyroid cancer.
Design and patients: The efficacy of the 123I radionuclide was determined in a longitudinal study of 12 patients who were selected only because they showed elevated serum Tg and a negative diagnostic 131I whole body study prior to therapy with 131I.
Results: There was almost complete concordance in uptake between 123I diagnostic imaging and the final scans carried out after 131I therapy (hereafter known as therapy studies) in 11 out of 12 patients at their first evaluation, in each of four patients receiving 123I at their second evaluation and in a single patient receiving 123I at a third evaluation. One patient had a positive 123I study but a negative 131I therapy study: following therapy Tg declined from 5.5 pg/l to undetectable levels, implying a therapeutic effect, and suggesting that the negative uptake was not the result of stunning. Two negative diagnostic 123I studies were followed by negative therapy studies, and thus there were no false negatives. 123I correctly identified disease in the nine patients with metastases in the lungs, mediastinum and bone at the first evaluation, in all four patients at the second evaluation and in the single patient at the third evaluation. At the end of the study, patients had received up to three 131I therapy doses, Tg had risen in four patients, fallen in eight and become undetectable in one patient.
Conclusions: 123I is highly sensitive in diagnosing local recurrence and metastatic disease, and produces scintigraphic images which concord well with uptake following 131I therapy. It is proposed that 123I imaging, in combination with serum Tg measurements, should replace 131I tracer imaging as an indicator of the potential efficacy of 131I therapy. Stunning, with its detrimental effects on 131I therapy, may thus be avoided. The possibility of false negative images due to the stunning phenomenon must always be borne in mind if there is a discrepancy between positive 131I imaging studies and a surprisingly negative subsequent 131I therapy scan.