Postradioiodine treatment whole-body scan in the era of 18-fluorodeoxyglucose positron emission tomography for differentiated thyroid carcinoma with elevated serum thyroglobulin levels

Background: Patients with differentiated thyroid cancer (DTC) who have a suspicious recurrent or persistent disease based on an elevated serum thyroglobulin (Tg) or Tg antibodies (TgAb) are usually referred for empiric radioiodine (¹³¹I) administration to localize and treat the disease. The aim of this retrospective monocentric study was to assess the sensitivity of postempiric ¹³¹I whole-body scan (WBS) compared to 18-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) in such patients who had an initial normal postablation WBS. Methods: Among 47 consecutive patients with DTC who had a normal postablation WBS and were referred for empiric ¹³¹I administration, 34 patients (12M, 22F; mean age 53 years) underwent FDG PET/CT and form the basis of this report: 23 patients had persistently elevated serum Tg levels, 10 had elevated Tg levels observed during follow-up after they initially became under 1 ng/mL, and 1 had appearance of TgAb during follow-up. Postempiric ¹³¹I WBS and FDG PET/CT were analyzed by independent readers. Results: A total of 75 lesions were found in 23 patients, distributed in 36 organs. Lesions were located in the neck (30), lungs (28), mediastinum (11), and bones (6). The sensitivities for the detection of individual lesions and for the diagnosis of metastatic organs were 88% and 97% for PET/CT and 16% and 22% for WBS, respectively (p<0.01). PET/CT was abnormal in 22 patients, among which 5 also had an abnormal postempiric ¹³¹I WBS. There was only one patient with an abnormal postempiric ¹³¹I WBS and a normal FDG PET/CT. This patient underwent two further ¹³¹I administrations, with the last WBS being normal and the last stimulated Tg level being undetectable. Other patients were either treated with surgery, or classified as radioactive iodine refractory and treated with levothyroxine suppressive therapy or tyrosine kinase inhibitors. Conclusion: In patients with suspicious recurrence based on the Tg level after a normal postablation WBS, FDG PET/CT is the preferred scintigraphic method to localize disease rather than postempiric ¹³¹I WBS. Empiric ¹³¹I administration may be used only in patients who do not have a significant FDG uptake.