Efficacy of dosimetric versus empiric prescribed activity of 131I for therapy of differentiated thyroid cancer

Abstract

Background: The optimal management of high-risk patients with differentiated thyroid cancer (DTC) consists of thyroidectomy followed by radioiodine ((131)I) therapy. The prescribed activity of (131)I can be determined using two approaches: 1) empiric prescribed activity of (131)I (E-Rx); and 2) dosimetry-based prescribed activity of (131)I (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 (131)I (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.

Fig. 1

Objective response to the treatment with ¹³¹I. A, Whole study group. Kaplan-Meier curves of PFS for patients treated with D-Rx vs. E-Rx (P = 0.34). B and C, Patients with DM. B, No statistically significant differences in the treatment efficacy in patients with DM treated with D-Rx vs. E-Rx [CR, two of 29 (6.9%) vs. one of 14 (7.1%), P = 0.704; PR, 0 of 29 vs. 0 of 14, P = 1.0; SD, 21 of 29 (72.4%) vs. eight of 14 (57.2%), P = 0.202; and PD, six of 29 (20.7%) vs. five of 14 (35.7%), P = 0.244]. C, No differences in PFS in the DM group treated with D-Rx vs. E-Rx (P = 0.61). D and E, Patients with LA disease. D, Higher rate of CR in D-Rx group compared with E-Rx [five of 14 (35.7%) vs. one of 30 (3.3%); P = 0.009]. No differences in PR, SD, PD [PR, 0 of 14 (0%) vs. 0 of 30 (0%), P = 1.0; SD, eight of 14 (57.2%) vs. 23 of 30 (76.7%), P = 0.951; PD, one of 14 (7.1%) vs. six of 30 (20.0%), P = 0.270]. E, No difference in PFS in the LA group of patients treated with D-Rx vs. E-Rx (P = 0.422). DM, Distant metastases; LA, locally advanced.

Fig. 2

No difference in the frequency of adverse side effects in patients treated with D-Rx vs. E-Rx. PLT, Platelets; RPD, restrictive pulmonary disease.