Validation of a Simple Thyroid Cancer Dosimetry Model Based on the Fractional Whole-Body Retention at 48 Hours Post-Administration of (131)I.

Abstract

Standard dosimetric methods to determine the maximum tolerated activity (MTA) of (131)I for the treatment of metastatic, well-differentiated thyroid cancer (DTC) are time-consuming and require complex analysis. As a result, reliable, accurate, and simplified methods are desirable. The objective of this study was to evaluate the validity of a simple regression dosimetry model. Previously, the authors reported a bi-exponential model for estimating the MTA of (131)I for the treatment of metastatic DTC based on a limit of 2 Gy to the blood. This model uses the patient's body surface area (BSA) along with the fractional whole-body retention (WBR) at 48 hours following oral administration of a diagnostic dosage of (131)I. A bi-exponential regression model was developed between the MTA normalized to the patient's BSA and the percent retention value at the 48-hour time point (R): MTA (GBq)/BSA (m(2)) = (13.91 · e(-0.0387R) + 42.33 · e(-0.8522R)). In this study, the same model was applied to a different set of adult patients referred for dosimetry and possible (131)I treatment of DTC under conditions of thyroid hormone withdrawal or recombinant human thyrotropin (rhTSH) stimulation. All patients (n = 170; 96 female) referred to the authors' clinic for dosimetry and possible (131)I treatment for metastatic DTC during the collection period were included in this study, apart from those undergoing renal dialysis. The MTA predicted (MTAp) using the model described above was compared to the measured MTA (MTAm), with statistical analysis performed using ProStat v4.5. In this group, the MTAm ranged from 2.3 to 41.1 GBq. The linear correlation between the MTAp and MTAm was excellent (r = 0.96), with an average deviation of only ± 1.2%. However, to avoid overdosing a patient on the basis of the MTAp, a weighting factor (<1.0) should be applied (e.g., using a value of 0.7 would result in only one patient receiving a prescribed activity of (131)I that exceeded the MTAm [<3%]). The % 48-hour WBR as determined by the bi-exponential function noted herein with reasonable restrictions has been validated as a reliable simplified dosimetry model.