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

Purpose Renal function and effective half-life ( t 1/2,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 (eGFR CysC ) in comparison to conventional creatinine (eGFR Crea ) and to verify which methods to determine t 1/2,eff are most accurate to predict t 1/2,eff . Methods Forty-eight patients receiving whole-body I-131-scintigraphy were included. eGFR CysC was compared to eGFR Crea with regard to accuracy of t 1/2,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 eGFR CysC measurement is more likely than eGFR Crea in predicting the t 1/2,eff . High correlation coefficients were found between t 1/2,eff assessed by gamma camera and probe measurements and blood-based determination revealed lower values. Patients with normal eGFR showed higher values of t 1/2,eff of LP compared to SP. Conclusions eGFR CysC 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 t 1/2,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 t 1/2,eff . Further studies are necessary for specific recommendations regarding I-131 activity reduction during RAIT in patients with DTC and renal insufficiency.