[18F]FDG PET/CT outperforms [18F]FDG PET/MRI in differentiated thyroid cancer

Purpose To evaluate the diagnostic potential of PET/MRI with [F-18]FDG in comparison to PET/CT in patients with differentiated thyroid cancer suspected or known to have dedifferentiated. Methods The study included 31 thyroidectomized and remnant-ablated patients who underwent a scheduled [F-18]FDG PET/CT scan and were then enrolled for a PET/MRI scan of the neck and thorax. The datasets (PET/CT, PET/MRI) were rated regarding lesion count, conspicuity, diameter and characterization. Standardized uptake values were determined for all [F-18]FDG-positive lesions. Histology, cytology, and examinations before and after treatment served as the standards of reference. Results Of 26 patients with a dedifferentiated tumour burden, 25 were correctly identified by both [F-18]FDG PET/CT and PET/MRI. Detection rates by PET/CT and PET/MRI were 97 % (113 of 116 lesions) and 85 % (99 of 113 lesions) for malignant lesions, and 100 % (48 of 48 lesions) and 77 % (37 of 48 lesions) for benign lesions, respectively. Lesion conspicuity was higher on PET/CT for both malignant and benign pulmonary lesions and in the overall rating for malignant lesions (p < 0.001). There was a difference between PET/CT and PET/MRI in overall evaluation of malignant lesions (p < 0.01) and detection of pulmonary metastases (p < 0.001). Surgical evaluation revealed three malignant lesions missed by both modalities. PET/MRI additionally failed to detect 14 pulmonary metastases and 11 benign lesions. Conclusion In patients with thyroid cancer and suspected or known dedifferentiation, [F-18]FDG PET/MRI was inferior to low-dose [F-18]FDG PET/CT for the assessment of pulmonary status. However, for the assessment of cervical status, [F-18]FDG PET/MRI was equal to contrast-enhanced neck [F-18]FDG PET/CT. Therefore, [F-18]FDG PET/MRI combined with a low-dose CT scan of the thorax may provide an imaging solution when high-quality imaging is needed and high-energy CT is undesirable or the use of a contrast agent is contraindicated.