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

<jats:title>Abstract</jats:title> <jats:sec> <jats:title>Background:</jats:title> <jats:p>The optimal management of high-risk patients with differentiated thyroid cancer (DTC) consists of thyroidectomy followed by radioiodine (131I) therapy. The prescribed activity of 131I can be determined using two approaches: 1) empiric prescribed activity of 131I (E-Rx); and 2) dosimetry-based prescribed activity of 131I (D-Rx).</jats:p> </jats:sec> <jats:sec> <jats:title>Aim:</jats:title> <jats:p>The aim of the study was to compare the relative treatment efficacy and side effects of D-Rx vs. E-Rx.</jats:p> </jats:sec> <jats:sec> <jats:title>Methods:</jats:title> <jats:p>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.</jats:p> </jats:sec> <jats:sec> <jats:title>Results:</jats:title> <jats:p>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 131I (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.</jats:p> </jats:sec> <jats:sec> <jats:title>Conclusion:</jats:title> <jats:p>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.</jats:p> </jats:sec>