The Role of Fluorodeoxyglucose-Positron-Emission Tomography Imaging (18F-FDG PET/CT) in The Management of Differentiated Thyroid Cancer.

Objectives To evaluate the impact of 18F-FDG PET/CT (PET-CT) in the management of differentiated thyroid cancer (DTC) patients with negative 131I post-therapy whole body scan (WBS) and elevated serum thyroglobulin (Tg).

Methods This retrospective study included 55 patients with DTC (papillary ( 76%) and follicular (24%) carcinoma). All patients were submitted to total thyroidectomy and to several radio 131I treatments (RIT) for thyroid remnant ablation or loco-regional and distant metastases. WBS was performed 5-7 days after RIT. Inclusion criteria were negative WBS, discordance between WBS results and clinical suspicion, elevated serum Tg levels, and negative anti-Tg auto-antibodies (AbTg) observed during the long-term follow-up. PET/CT scan was performed in fasting patients 60 minutes following i.v. administration of 18F-FDG.

Results In total 55 patients (37 females and 18 males) underwent 87 PET imaging (median number of PET per patient was 1 with a maximum of 5). The median age of the population was 54-year-old (std. dev. 15.6). Twelve patients had positive iodine scanning prior to PET with a clinical suspicion of more extended disease. All the rest had negative results. Among the former group the major indication for PET/CT was clinical suspicion of lymph nodes not detected by scintigraphy. However, rising of thyroglobulin alone with negative scintigraphy was the most frequent presentation. Fifty-four percent of PET/CTs detected at least one lesion, mainly cervical lymph nodes (49.9%), mediastinal lymph nodes (40.4%) followed by local recurrence (34%) and bone or tissue metastasis (36.2%). The three major sites of metastasis detected by mean of PET were lung, bone and liver.

Conclusions PET/CT identified recurrent and metastatic neoplastic tissue in 45% of patients with high levels of Tg and negative WBS. PET/CT modified the therapeutic management in 50% of patients suggesting suitable modality of treatment (surgery versus thyrosine kinase inhibitor). In addition, Almost 55% of the patients were FDG negative. Further studies are needed to assess the prognostic/clinical value of negative PET examination.