Efficacy of Dosimetric Versus Empiric Prescribed Activity of 131I for Therapy of Differentiated Thyroid Cancer

Background: The optimal management of high-risk patients with differentiated thyroid cancer (DTC) consists of thyroidectomy followed by radioiodine (I-131) therapy. The prescribed activity of I-131 can be determined using two approaches: 1) empiric prescribed activity of I-131 (E-Rx); and 2) dosimetry-based prescribed activity of I-131 (D-Rx). Aim: The aim of the study was to compare the relative treatment efficacy and side effects of D-Rx vs. E-Rx. Methods: A retrospective analysis was performed of patients with distant metastases and/or locoregionally advanced radioiodine-avid DTC who were treated with either D-Rx or E-Rx. Response to treatment was based on RECIST (Response Evaluation Criteria in Solid Tumors) 1.1 criteria. Results: The study group consisted of 87 patients followed for 51 +/- 35 months, of whom 43 were treated with D-Rx and 44 with E-Rx. Multivariate analysis, controlling for age, gender, and status of metastases revealed that the D-Rx group tended to be 70% less likely to progress (odds ratio, 0.29; 95% confidence interval, 0.087-1.02; P = 0.052) and more likely to obtain complete response (CR) compared to the E-Rx group (odds ratio, 8.2; 95% confidence interval, 1.2-53.5; P = 0.029). There was an association in the D-Rx group between the observed CR and percentage of maximum tolerable activity given as a first treatment of I-131 (P = 0.030). The advantage of D-Rx was specifically apparent in the locoregionally advanced group because CR was significantly higher in D-Rx vs. E-Rx in this group of patients (35.7 vs. 3.3%; P = 0.009). The rates of partial response, stable disease, and progression-free survival, as well as the frequency of side effects, were not significantly different between the two groups. Conclusion: Higher efficacy of D-Rx with a similar safety profile compared to E-Rx supports the rationale for employing individually prescribed activity in high-risk patients with DTC. (J Clin Endocrinol Metab 96: 3217-3225, 2011)