Diagnostic I–131 scintigraphy in patients with differentiated thyroid cancer: No additional value of higher scan dose

Objective: After initial treatment with total thyroidectomy and radioiodine ablation, most follow-up protocols for patients with differentiated thyroid carcinoma contain cyclic diagnostic I–131 imaging and serum thyroglobulin measurements. The applied diagnostic I–131 doses vary between 37 and 370 MBq. The aim of this study was to determine the yield of a diagnostic scan with 370 MBq I-131 in patients with a negative diagnostic scan with 74 MBq I–131.Methods: Retrospective evaluation of 158 patients who received a high-dose diagnostic scan with 370 MBq I–131 because of a negative low-dose diagnostic scan with 74 MBq I–131. Special attention was paid to the patients with positive high-dose diagnostic scanning and undetectable serum thyroglobulin levels after thyroid hormone withdrawal.Results: In 127 (80%) of patients the 370 MBq I–131 scan was negative, just like the preceding low-dose scan. In 31 (20%) of patients abnormal uptake was present on the 370 MBq diagnostic scan. In 19 of these 31 patients serum thyroglobulin was undetectable. In 15/19 the high-dose diagnostic scan proved either false positive or demonstrated clinically irrelevant minor ablation rests. In only four patients (2.5%) did the high-dose diagnostic scans reveal possibly relevant uptake caused by residual differentiated thyroid cancer.Conclusion: In 98% of patients a 370 MBq dose of I–131 for diagnostic WBS had no additional value. The combination of a low-dose diagnostic I–131 scan using only 74 MBq combined with a serum Tg level measurement proved sufficient for correct clinical decision making regarding whether the patient requires additional I–131 therapy.