Incremental Value of Diagnostic I-131 SPECT/CT Fusion Imaging in the Evaluation of Differentiated Thyroid Carcinoma

OBJECTIVE. The purpose of this study was to determine the incremental value of I-131 SPECT/CT over traditional planar imaging of patients with differentiated thyroid carcinoma. MATERIALS AND METHODS. Fifty-six planar and SPECT/CT scans were obtained for 53 patients. Forty-eight scans were diagnostic I-131 studies before first radioiodine therapy, four were diagnostic I-131 studies with recombinant human thyroid-stimulating hormone stimulation, and four scans were posttherapy I-131 studies. Two nuclear physicians interpreted central neck and distant activity on planar scans and reviewed SPECT/CT images to assess the incremental diagnostic value with respect to localization and characterization of focal activity and to evaluate reader confidence. One of the readers was unblinded and had access to clinical, imaging, histologic, and biochemical information. RESULTS. Planar scans depicted 130 neck foci and 17 distant foci. At SPECT/CT these foci were further characterized as thyroglossal duct and thyroid bed remnant (n = 98), cervical nodal metastasis or local residual disease (n = 26), physiologic activity (n = 11), and distant metastasis (n = 12). Interobserver disagreement occurred on eight of 147 foci (5%). Because of superior lesion localization and additional anatomic information derived from the low-dose CT component, incremental diagnostic value with SPECT/CT over planar imaging was found for 70 of 147 foci (47.6%), including 53 of 130 neck foci (40.8%) and all 17 (100%) distant foci. Reader confidence increased regarding 104 of 147 foci (70.7%). CONCLUSION. Iodine-131 SPECT/CT is useful for accurate evaluation of regional and distant activity in characterization of foci as residual thyroid tissue or nodal, pulmonary, or osseous metastasis. Suspected physiologic mimics of disease can be confirmed with increased reader confidence.