Diagnostic value of [Tc-99m]Tc-HYNIC-TOC scintigraphy in the management of differentiated thyroid cancer with elevated thyroglobulin and negative radioiodine whole-body scan

Introduction: Negative radioiodine (I-131) whole-body scan with elevated serum thyroglobulin (Tg) level are found in 20% of patients with differentiated thyroid cancer (DTC), which can be a diagnostic challenge. We evaluated the efficacy of Technetium-99m-Hydrazinonicotinyl-Tyr3-Octreotide ([(99)mTc]Tc-HYNIC-TOC) somatostatin receptor scintigraphy (SRS) for detection of non-iodine-avid metastases and its impact on staging and management of these patients. Methods: The study population consisted of 35 DTC patients (25 females; PTC = 88.2%, FTC = 11.8%) who had elevated serum Tg levels despite negative post-ablation radioiodine whole-body scan. All patients underwent whole body SRS 3-4 hours after intravenous injection of 20mCi (740 MBq) of [(99)mTc]Tc-HYNIC-TOC. Sites of suspected radiotracer accumulation were confirmed with anatomic imaging. Ultimately, corresponding changes in the staging and management were recorded. Results: SRS was positive in 27 (77.1%) cases. Patients with positive scan had significantly higher Tg levels at the time of scan, compared to those with negative scans (154.5 +/- 188.6 vs. 28.2 +/- 32.7 ng/mL, p-value = 0.005). Interestingly, previous history of neck external beam radiation therapy (EBRT) was significantly correlated with [(99)mTc]Tc-HYNIC-TOC avidity (Likelihood ratio = 11.2, p = 0.005). Addition of SSTR scintigraphy changed overall staging and management in 11% and 32.4% of the patients, respectively. Conclusion: SRS can be a useful diagnostic adjunct in DTC patients with highly elevated Tg and negative radioiodine whole-body scan. The likelihood of positive fmdings on [(99)mTc]Tc-HYNIC-TOC was higher in cases with previous history of EBRT or high Tg levels (i.e. suppressed-Tg >80 ng/mL) at the time of scan.