Postsurgical diagnostic evaluation of patients with differentiated thyroid carcinoma: comparison of ultrasound, iodine-131 scintigraphy and PET

Purpose. The aim of this study was to compare the results of ultrasound (US), whole-body scintigraphy with iodine- 131 (I-131 WBS) and positron emission tomography with fluorine-18 deoxyglucose (FDG-PET) in the follow-up of patients after thyroidectomy for differentiated thyroid carcinoma (DTC). Materials and methods. Thirteen patients (3 men, 10 women) were evaluated by neck US, I-131 WBS and FDG-PET. In each patient six anatomical regions (right and left thyroid bed, right and left cervical region, right and left supraclavicular region) were investigated, for a total of 78 regions. Distant metastases were investigated by I-131 WBS and FDG-PET and considered separately in the analysis. Imaging findings were compared with the reference standards, such as fine-needle aspiration cytology (2), biopsy (4) or clinical-radiological studies (7). Results. US, FDG-PET and I-131 WBS showed concordant negative results in most (70, 90%) of the anatomical sites considered. In one patient with left cervical lymph node metastasis, the imaging techniques showed concordant positive results (1%). In the remaining 7 regions (9%), the imaging results were discordant; in particular, tumour lesions, nodal metastases (4) and thyroid bed recurrences (3) were detected by US only (3), by US and I-131 WBS (1) and by FDG-PET only (3). With regard to distant metastases, FDG-PET and I-131 WBS yielded concordant negative results in the majority (77%) of patients (9); in one patient only were the two imaging techniques concordant in their positive result. In the last three patients, the results were discordant; in particular, distant metastases were detected by I-131 WBS only in two patients and by FDG-PET only in one patient. Conclusions. Our work indicates a fundamental role for US in evaluation of the neck after surgery for DTC. WBS is useful to determine differentiation of tumour lesions, to identify thyroid remnants and to look for distant metastases. FDG-PET has an important role in cases of dedifferentiated thyroid carcinoma in which WBS and thyroglobulin measurements are unable to detect tumour lesions.