Prognostic value of 18F-fluorodeoxyglucose-positron emission tomography in patients with differentiated thyroid carcinoma and circulating antithyroglobulin autoantibodies - Introduction

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Nuclear Medicine Communication 
ISSN: 0143-3636 • Frequency: 12/year •  Impact Factor 1.315 • Subscribe Now

 By Trond Velde Bogsrud & Val J. Lowe

Thyroglobulin (Tg) is produced exclusively by thyroid follicular cells, including cells of differentiated thyroid cancer (DTC). Tg is a sensitive and specific tumor marker and has a central role in the follow-up of patients with DTC. After total thyroidectomy, with or without radioiodine ablation, an undetectable Tg level indicates successful ablation. A persistent low but stable Tg level indicates residual normal thyroid tissue or indolent residual disease.  A high, or especially an increasing, level of Tg indicates more aggressive residual or recurrent disease. In these high-risk patients, radioiodine therapy, external radiation therapy, surgery, or novel targeted anticancer agents can be considered.

An 18F FDG-PET scan is indicated in this patient group as residual or metastatic disease not detected by other imaging modalities may be found on PET, and the findings on PET may influence patient management. The presence of FDG-avid metastatic lesions indicates unfavourable prognosis with increased risk of cancer-associated mortality, while a negative FDG-PET result indicates good prognosis despite increased level of Tg.

In patients with circulating autoantibodies to Tg (TgAb), Tg measurements are unreliable because of analytic interference. Tg is commonly undetectable in TgAb-positive patients despite leakage of Tg from residual thyroid tissue and the Tg response to TSH stimulation is commonly blunted or completely absent. This problem with TgAb is common in patients with DTC (prevalence 10-25%), and TgAb is reported to be more common in patients with DTC than in the general population (10%). After successful total thyroidectomy with or without radioiodine treatment or after surgical removal of metastatic lymph nodes, TgAb levels become negative within a few years. Persistent TgAb levels indicate residual or recurrent disease and TgAb measured serially may be used as a surrogate tumor marker in TgAb-positive patients. The clinical value of 18FDG-PET in this subgroup of TgAb positive patients with DTC has not previously been studied.

We studied the prognostic value of  18F FDG-PET in 17 post-therapy radioiodine negative, TgAb positive patients with DTC. A negative FDG PET result was associated with the absence of active disease and disappearing TgAb over time. FDG-avid residual lesions were associated with persistently increased TgAb levels, aggressive disease, and poor outcome. Thus, FDG PET appears to have the same prognostic value in radioiodine negative, TgAb positive patients with DTC as it has for TgAb negative, radioiodine negative DTC patients with increased Tg levels.

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