Whole body retention of I-131 at 24hr vs 48hr as a predictor of maximum tolerated activity (MTA)

Objectives To evaluate the reliability and robustness of a single measurement at 24hr post administration of I-131 of fractional whole body retention (WBR) as a predictor of MTA.

Methods Previously we reported on a bi-exponential model (Thyroid 2009 19(10):1093-98) for estimating the maximum tolerated activity (MTA) of I-131 for the treatment of metastatic, well-differentiated thyroid cancer (WDTC) based on a limit of 2 Gy to the blood. This model incorporates the patient’s body surface area (BSA) along with the fractional whole body retention (%WBR) of a diagnostic dosage of I-131 at 48 hrs post oral administration to estimate the MTA. The advantage of the 24hr time point, if reliable, is it might allow an extension of this model based on the %WBR at 48hr to instead be used with I-123 and an earlier time point, which is typically used for pre-therapy imaging. Since Mar 2011 all patients (n=99) undergoing dosimetry also had their %WBR calculated at 24hr post administration of approx 74 MBq of I-131. These patients were prepared for their anticipated radioiodine therapy for WDTC either by thyroid hormone withdrawal (n=41) or rhTSH (n=58).

Results A very good correlation exists between the WBR at 24hr and 48hr (r=0.96). When the iodine clearance is relatively rapid (%WBR @ 24hr < 35%) it can also be a good predictor of the MTA (r=0.88). Over this range of %WBR none of the patients (n=52) had a measured MTA which was < 80% of the estimated value based solely on the %WBR. This is more often the case for rhTSH compared to THW where the median values for the %WBR at 24hr were 28.9% and 41.8%, respectively. For patients with less rapid clearance the variability is too great and a second delayed measurement would be warranted.

Conclusions The WBR at 24hr using I-123 could potentially be used in a predictive model of the MTA for the majority of patients provided this value is < 35%. Otherwise a delayed measurement at 48hr might be necessary.