Impact of 18F-Fluoro-2-Deoxy-D-Glucose Positron Emission Tomography (FDG-PET) in Patients with Biochemical Evidence of Recurrent or Residual Medullary Thyroid Cancer

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Abstract
Background: Conventional imaging such as with 99mTc(V)dimercaptosunnic acid (DMSA), 111In-octreotide scintigraphy, computed tomography (CT), and magnetic resonance imaging (MRI) rarely localizes occult medullary thyroid cancer (MTC). The role of 18F-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) is not well defined. The aim of this study was to examine the usefulness of postoperative FDG-PET in localizing MTC metastases. Methods: FDG-PET was performed in 26 patients with elevated serum tumor markers after total thyroidectomy with central compartment dissection and additional neck dissection on indication. Patient- and lesion-based results were compared with the findings of conventional nuclear imaging and validated by morphological imaging (CT, MRI, ultrasonography), including bone scintigraphy and pathology when possible. Clinical impact was evaluated. Results: FDG-PET detected foci in 50% of patients with lesion-based sensitivity of 96%. 111In-octreotide detected foci in 19% with sensitivity of 41%, and 99mTc(V)DMSA scintigraphy and morphological imaging detected foci in 21% and 40%, respectively, with sensitivity of 57% and 87%. No lesions were found in 11 patients (42%). Positive FDG-PET findings led to surgical intervention in nine patients (35%). They all underwent surgery for removal of residual tumor or metastases. One patient achieved disease-free status. In all patients who underwent surgery, serum calcitonin levels were reduced by an average of 58 ± 31%. Conclusions: For detection of occult MTC lesions, FDG-PET is superior to conventional nuclear imaging and is the best detection method yet available. FDG-PET in postoperative follow-up has clinical value and may be used for guiding reoperation and additional morphological imaging preoperatively.