Background: Differentiated (papillary/follicular) thyroid cancers retain the sodium iodide symporter and hence concentrate radioiodine (I-131). Conventional treatment comprises near-total thyroidectomy followed by whole body scintigraphy and ablation of residual/metastatic thyroid tissue using I-131. However, the ability to concentrate radioiodine is lost on de-differentiation, leaving only biomarkers like thyroglobulin to monitor disease progression in post-thyroidectomy patients. This study evaluated the utility of F-18 fluorodeoxyglucose positron emission tomography-computed tomography (FDG PET-CT) in detecting such de-differentiated tumours, based on their metabolic ability to concentrate glucose.
Methods: We retrospectively analysed our records of whole body FDG PET-CT imaging for the last 2 years and assessed the scans of patients with I-131 scan negative thyroid cancer along with their thyroglobulin levels.
Results: Fourteen I-131 scan negative patients (8M, 6F) aged 19-77 years with thyroglobulin levels from 2.8 to > 300 (118.2 ± 110.8) IU/ml underwent FDG PET-CT. Twelve patients had papillary, one follicular and one Hurthle cell carcinoma. Eight patients (66%) showed FDG avid cervical/supraclavicular lymph nodes, with one patient each showing additional FDG avid lung nodules and an osteolytic vertebral lesion. These patients were all treated empirically with high dose (100 mCi) I-131 therapy. Four patients were negative on FDG PET-CT, while two showed minimal FDG avidity in cervical lymph nodes; these six patients are being followed up with regular thyroglobulin assay and neck ultrasonography.
Conclusion: FDG PET-CT detects metabolically active metastasis in a significant proportion of patients with I-131 scan negative thyroid cancer with raised thyroglobulin.