Impact of <sup>131</sup>I SPECT/CT on Therapeutic Decision-Making in Differentiated Thyroid Carcinoma: A Single-Center Retrospective Study.

Abstract

Rationale and objectivesDifferentiated thyroid carcinoma (DTC) has a favorable prognosis when properly managed. Whole-body scanning (WBS) is the standard functional imaging method for post-surgical staging; however, its limited spatial resolution may lead to misclassification of iodine-131 (131I) uptake sites. Single-photon emission computed tomography/computed tomography (SPECT/CT) improves anatomical localization and diagnostic accuracy. We aimed to evaluate the incremental clinical value of SPECT/CT over WBS in determining the 131I therapeutic dose in patients with DTC.Materials and methodsThis retrospective study included 199 patients with DTC treated at a tertiary academic center between 2011 and 2017. WBS-based management decisions were compared with those incorporating SPECT/CT findings. Agreement was assessed using the intraclass correlation coefficient (ICC) and Kappa.ResultsSPECT/CT changed therapeutic management in 20.6% of patients (n = 41). The administered 131I activity was reduced in 27 of these patients (65.9%), preventing overtreatment, and increased in 14 (34.1%), preventing undertreatment. By staging, 26 patients (63.4%) were downstaged, and 15 (36.6%) were upstaged; in one upstaged patient, a surgically resected bone metastasis led to a dose reduction rather than an increase. Overall dose agreement was good (ICC = 0.833; 95% confidence interval [CI]: 0.785-0.871). Concordance was very high for T staging (κ = 0.977), M staging (κ = 0.931), and risk category (κ = 0.971), but only moderate for lymph node evaluation (κ = 0.589; 95% CI: 0.478-0.700). Predictors of management change included: cervical uptake (P 131I dose (P = 0.027), and undetectable antithyroglobulin antibodies (P = 0.032).ConclusionSPECT/CT after WBS has a significant clinical impact in DTC, enabling more personalized therapy and reducing both undertreatment and overtreatment in approximately one-fifth of patients, with its greatest contribution in lymph node characterization.