Localization of medullary thyroid carcinoma after surgery using ¹¹C-methionine PET/CT: comparison with ¹⁸F-FDG PET/CT

Abstract

Tumor localization is difficult in patients with medullary thyroid carcinoma (MTC) that have persistent hypercalcitoninemia after thyroidectomy. In this study, the ¹¹C-methionine positron emission tomography/computed tomography (PET/CT) was compared with the ¹⁸F-FDG PET/CT for diagnostic sensitivity in detecting residual or metastatic disease. ¹¹C-methionine PET/CT and ¹⁸F-FDG PET/CT were performed on 16 consecutive patients with MTC that had persistent hypercalcitoninemia after surgery in this prospective, single-center study. Patient- and lesion-based analyses were performed using a composite reference standard which was the sum of the lesions confirmed by all combined modalities, including neck ultrasonography (US) with or without fine needle aspiration cytology, CT, bone scan, magnetic resonance imaging (MRI), and surgery. By patient-based analysis, the sensitivities of ¹¹C-methionine PET/CT and ¹⁸F-FDG PET/CT were both 63%. By lesion-based analysis, the sensitivity of ¹¹C-methionine PET/CT was similar to ¹⁸F-FDG PET/CT (73% vs. 80%). Excluding hepatic lesions, which could not be detected because of physiological uptake of methionine by the liver, the sensitivity of ¹¹C-methionine PET/CT was better than ¹⁸F-FDG PET/CT especially for detecting cervical lymph node lesions; however, it was not superior to US. All patients with serum calcitonin levels ≥370 pg/mL showed uptake by ¹¹C-methionine PET/CT and ¹⁸F-FDG PET/CT. This preliminary data showed that despite its similar sensitivity to ¹⁸F-FDG PET/CT for detecting residual or metastatic MTC, ¹¹C-methionine PET/CT provided minimal additional information compared to combined ¹⁸F-FDG PET/CT and neck US.