Clinical significance of discordant findings between pre-therapy ¹²³I and post-therapy ¹³¹I whole body scan in patients with thyroid cancer

Radioactive therapy with ¹³¹I (RAI) is commonly used during the management of patients with differentiated thyroid cancer (DTC). The aim of this study was to determine the clinical significance of discordant findings between pre-RAI whole body scan (WBS) with ¹²³I and post-RAI WBS in the management of DTC. We retrospectively evaluated 342 individuals between 2002 and 2008 who had a diagnosis of DTC and underwent RAI. All had WBS one day before RAI and WBS one week after RAI. Patients were divided into 3 groups: 1) RAI-naive subjects without known distant metastatic disease (M1); 2) patients with history of prior RAI and persistent disease (except M1); and 3) patients with known M1. In Group 1 (n=311), 7% of patients (n=22) had discordant scans, but in only 4 of these cases did this represent true disease (3 unsuspected lung and 1 mediastinal node metastasis). In the remaining 18 patients, discordant findings corresponded to physiologic or other benign causes. In group 2 (n=23), 7 subjects (30%) had discordant findings and all of the discrepant sites consisted of loco-regional nodal disease in the neck/ upper mediastinum (n=6) and M1 in lung (n=1). In group 3 (n=8), 5 patients (62%) showed discordant uptake in lung and bone which corresponded to the locations of known M1. A total of 12 patients with iodine-avid M1 were identified on post-RAI WBS (3.5% of entire cohort). Pre-RAI WBS was only concordant in 3 of these cases (25%). In conclusion, the significance of pre and post-RAI WBS is highly influenced by the clinical setting. Unsuspected distant metastatic disease is infrequent in RAI-naive patients without known M1, where most discordant findings are usually due to benign explanations, and represent false positive findings in this group. In contrast, in patients with history of previous RAI or known M1, discordant results likely correspond to true disease. In our study, pre-RAI scans showed a low yield to detect iodine-avid distant metastatic disease when compared to post-RAI scans.