Follow-up of thyroid cancer patients with "poor prognosis".

High-risk differentiated thyroid carcinoma is the most frequent thyroid tumor of "poor prognosis": this mainly includes patients with extra-thyroidal invasion, or distant metastases, younger patients (<16 years old), and older patients (>45 years old). Among them, metastatic patients with multiple organ involvement at the time of initial diagnosis have the higher risk of cancer death. Additionally, certain histological subtypes are classically more aggressive, and bilateral cervical lymph-nodes metastases or mediastinal involvement may also impart a poorer overall prognosis. More aggressive therapy to produce undetectable thyrotropin levels is usually recommended, although the benefit of such therapy and how long to maintain thyrotropin suppression has not been definitively established. As about two-thirds of the recurrences occur within the first decade after initial treatment, this first decade seems particularly critical, even if follow-up is necessary throughout the patient's life as recurrences may also occur over several decades. Coupled thyroglobulin (Tg) and Tg antibody (TgAb) assay is the first-line tool in their follow-up. Tg measurement obtained either after LThyroxine withdrawal or rhTSH stimulation may permit the selection of patients for scanning with a high dose of 131-I. When either basal Tg level is high or TgAb increases, it appears preferable to schedule patients directly for 131-I therapy followed by a post-therapy WBS. Therefore, the discovery of foci of 131-I uptake is possible in 60 to 80% of such patients. 131-I therapy is proposed as long as metastases trap 131-I without any limit to the cumulative dose of 131-I, although the risk of leukemia rises slightly above a 500 mCi (18,500 MBq) cumulative dose. But when 131-I post therapeutic WBS is negative, any further administration of 131-I is not justified. Alternative imaging procedure is thus required to detect metastases that have lost their capacity to concentrate 131-I. Conventional imaging with ultrasonography of the neck, a CT scan or an MRI of the neck and the chest and bone imaging, and even non-conventional imaging with other isotope procedures, such as 18-FDG whole-body scanning, are nowadays indicated. The goal is to localize those metastases in order to propose the more adequate therapeutic options.