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

Objective Some patients with relapsed differentiated thyroid cancer may sho w rising thyroglobulin (Tg) levels despite a lack of I-131 uptake on routin e whole body imaging. A significant proportion of these patients, after the rapy doses of I-131, may demonstrate positive I-131 uptake with a subsequen t fall in serum Tg, implying a therapeutic effect. Attempts to identify suc h patients by increasing the dose of the diagnostic I-131 tracer may lead t o inhibition of subsequent uptake after the therapy dose, an effect referre d to as 'stunning' and associated with a reduction in therapeutic effect. I -123 is a short half-life gamma -emitter, thought to be unlikely to cause s tunning, which may thus be more suitable than I-131 for diagnostic imaging of thyroid cancer. Design and Patients The efficacy of the I-123 radionuclide was determined i n a longitudinal study of 12 patients who were selected only because they s howed elevated serum Tg and a negative diagnostic I-131 whole body study pr ior to therapy with I-131. Results There was almost complete concordance in uptake between I-123 diagn ostic imaging and the final scans carried out after I-131 therapy (hereafte r known as therapy studies) in 11 out of 12 patients at their first evaluat ion, in each of four patients receiving I-123 at their second evaluation an d in a single patient receiving I-123 at a third evaluation. One patient ha d a positive I-123 study but a negative I-131 therapy study: following ther apy Tg declined from 5.5 pg/l to undetectable levels, implying a therapeuti c effect, and suggesting that the negative uptake was not the result of stu nning. Two negative diagnostic I-123 studies were followed by negative ther apy studies, and thus there were no false negatives. I-123 correctly identi fied disease in the nine patients with metastases in the lungs, mediastinum and bone at the first evaluation, in all four patients at the second evalu ation and in the single patient at the third evaluation. At the end of the study, patients had received up to three I-131 therapy doses, Tg had risen in four patients, fallen in eight and become undetectable in one patient. Conclusions I-123 is highly sensitive in diagnosing local recurrence and me tastatic disease, and produces scintigraphic images which concord well with uptake following I-131 therapy. It is proposed that I-123 imaging, in comb ination with serum Tg measurements, should replace I-131 tracer imaging as an indicator of the potential efficacy of I-131 therapy. Stunning, with its detrimental effects on I-131 therapy, may thus be avoided. The possibility of false negative images due to the stunning phenomenon must always be bor ne in mind if there is a discrepancy between positive I-131 imaging studies and a surprisingly negative subsequent I-131 therapy scan.