Optimal Time of Diagnostic and Posttherapeutic Iodine-131 Whole-body Scan in Post-Operative Pediatric and Young Adult Differentiated Thyroid Cancer Patients.

Abstract

Purpose of the study: The 2015 American Thyroid Association pediatric differentiated thyroid cancer (DTC) guidelines recommend posttherapy whole body scan (PTS) 4-7 days after (Iodine-131) I-131 activity administration. There is no recommendation of timing of performing a diagnostic whole-body scan (Dx-WBS). However, this 4-7 day time frame for PTS lacks a solid basis and is essentially arbitrary. This is especially crucial as it has the potential to significantly impact patient management. Our primary goal in this study was to establish the optimal timing for both Dx-WBS and PTS in pediatric and young adult patients with DTC.

Methods: The DTC patients aged ≤21 years underwent serial whole-body scan (WBS) at 24 h, 48 h, and/or 72 h or more after the administration of diagnostic and therapeutic activities of I-131. The utility of Dx-WBS and PTS was assessed based on the identification of new lesions that could potentially influence the prescribed therapeutic activity of I-131. The optimal timing for acquiring Dx-WBS and PTS was determined based on when the first lesion appeared in the I-131 WBS.

Results: Ninety-five patients (27 males and 68 females) with an average age of 17.9 ± 3 years received a 74 MBq I-131 for Dx-WBS. Ten patients (10.5%) showed no uptake in Dx-WBS, thus, no I-131 therapy was given. The remaining 85 patients received a therapeutic activity of 1.11-5.55 GBq I-131 based on the extent of their disease. The serial Dx-WBS or PTS showed no additional lesions in patients with thyroid remnants. However, additional nodes were detected in 2/32 patients in the ≥48 h Dx-WBS, and 1/32 patients in the known nodal disease patients, which were not clinically relevant. Importantly, 72 h PTS picked up pulmonary metastases in 17.6% (3 out of 17) of patients, which were missed in serial Dx-WBS. However, >72 h PTS did not have additional value.

Conclusion: I-131 Dx-WBS is best to be performed at 48 h, and PTS at 72 h in pediatric and young adult patients with DTC.