131I whole body scintigraphy in thyroid cancer patients

Iodine-131 is the most specific radionuclide to follow up patients with differentiated thyroid cancer (DTC). However there are some aspects that should be considered if 131I whole body scintigraphy (131I WBS) is performed. 1) Several prior conditions, including a bTSH above 30 mU/l and an urinary iodine excretion below 100-150 micrograms/g Crea, should be fulfilled. 2) Only about two thirds of metastases from DTC accumulate iodine. Therefore, in addition to 131I WBS, there is a need for other nonspecific tracers such as 99mTc Tetrofosmin WBS, 99mTc Sestamibi WBS or F-18 FDG PET to detect also iodine negative recurrences or metastases. There new tracers, especially F-18 FDG PET have demonstrated a very high detection rate of iodine negative metastases with mostly low differentiation. 3) The sensitivity of 131I WBS depends on the administered dose. Whereas the sensitivity of a diagnostic 131I WBS (up to 185 MBq) is below 60%, the value for a post-therapeutic 131I WBS (after 3700-7400 MBq) increases up to 75%. This means that in case of elevated serum thyroglobulin, iodine positive metastases cannot be excluded until WBS after 131I therapy is performed. 4) In patients with elevated serum thyroglobulin and/or known metastases, who are scheduled for 131I treatment, the question arises whether a diagnostic 131I WBS should be performed and if so, which dose should be administered to avoid thyroid stunning. There is evidence in the literature that the dose for a pre-therapeutic diagnostic 131I WBS should not exceed 74 MBq. 5) Despite the high specificity of 131I WBS, several pitfalls of iodine accumulation in non-malignant diseases and malignancies of other origin than thyroid cancer should be taken into account.