Determination of effective half-life of 131I in patients with differentiated thyroid carcinoma: comparison of cystatin C and creatinine-based estimation of renal function.

PURPOSE: Renal function and effective half-life (t1/2,eff) of I-131 have not been fully elucidated in patients undergoing radioiodine therapy (RAIT) for differentiated thyroid cancer (DTC). Aim of the present analysis was to evaluate the potential of cystatin C-based estimated glomerular filtration rate (eGFRCysC) in comparison to conventional creatinine (eGFRCrea) and to verify which methods to determine t1/2,eff are most accurate to predict t1/2,eff.
METHODS: Forty-eight patients receiving whole-body I-131-scintigraphy were included. eGFRCysC was compared to eGFRCrea with regard to accuracy of t1/2,eff prediction. Three different methods (i.e. blood-based, gamma camera-based and probe-based) and two protocols with either three (short period,SP; up to 42 h) or four (long period,LP; up to 114 h) time points were compared using the Akaike's information criterion.
RESULTS: The eGFRCysC measurement is more likely than eGFRCrea in predicting the t1/2,eff. High correlation coefficients were found between t1/2,eff assessed by gamma camera and probe measurements and blood-based determination revealed lower values. Patients with normal eGFR showed higher values of t1/2,eff of LP compared to SP.
CONCLUSIONS: eGFRCysC should be included in further study protocols. As camera and probe measurements lead to almost superimposable results, one of the methods is expendable. Blood-based results of t1/2,eff were lower, presumably due to unspecific iodine retention, whereas the lower correlation with renal function may be caused by individual differences in intestinal iodine resorption. SP-protocols up to 42 h after I-131 administration are sufficient to determine t1/2,eff. Further studies are necessary for specific recommendations regarding I-131 activity reduction during RAIT in patients with DTC and renal insufficiency.