[68Ga]Ga-DOTATATE PET/CT Versus [18F]F-FDG PET/CT in Tenis Syndrome - a Head-to-head Comparison With Elevated and Suppressed Tsh Levels in Papillary Thyroid Carcinoma – a Pilot Study

Abstract BACKGROUND TENIS syndrome (Thyroglobulin-Elevated Negative Iodine Scintigraphy) in differentiated thyroid carcinoma is characterized by reduced expression of sodium-iodine symporter, rising serum thyroglobulin levels (Tg), and negative whole-body [131I]Iodine scans. In such patients, somatostatin receptor imaging with [68Ga]Ga-DOTATATE PET/CT (SSR PET/CT) and [18F]F-FDG PET/CT (FDG PET/CT) can identify metastases. To compare the uptake pattern of SSR PET/CT and FDG PET/CT in papillary thyroid carcinoma (PTC) patients with TENIS Syndrome under two conditions: elevated (eTSH) and suppressed (sTSH) TSH serum levels. Based on imaging findings in these patients, we intend to identify potential candidates for peptide receptor radionuclide therapy (PRRNT). Fifteen patients with PTC and TENIS Syndrome were prospectively enrolled. All patients underwent both SSR PET/CT and FDG PET/CT with sTSH and eTSH. Images were blindly evaluated for differences in SUVmax values and lesion detectability. The reference standard consisted of neck ultrasound, CT, MRI, PET/CT, biopsy, and follow-up. Three patients were selected for PRRNT due to lesion uptake similar or higher than the liver. RESULTS On a per-patient and on a per-lesion based analysis, sTSH SSR PET/CT detected a greater number of cervical and distant LNs (both p = 0.0253 on per-patient and p = 0.0176 and p = 0.0391 on per-lesion, respectively) when compared to sTSH FDG PET/CT. Likewise, eTSH SSR PET/CT detected a greater number of patients with local recurrences (p = 0.0455) and distant LN metastases (p = 0.0143) and in a per-lesion based analysis greater number of cervical and distant LNs (p = 0.0337 and p = 0.0039, respectively) when compared to eTSH FDG PET/CT. No differences were found for lung and bone metastases. One of three patients submitted to 3 cycles of PRRNT presented with a visual partial response, a 20% reduction in quantitative analyses and stable disease regarding Tg and TgAb levels. CONCLUSIONS In patients with PTC and TENIS Syndrome, SSR PET/CT detected a greater number of loco-regional and distant LN metastases than FDG PET/CT with both sTSH and eTSH. One of 3 patients submitted to PRRNT presented a partial response to treatment. Our findings may impact in patient restaging, management and theranostics strategies with radiolabeled somatostatin analogs.