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The value of positron emission tomography (PET) in the management of patients with thyroid cancer

https://doi.org/10.1016/j.beem.2008.10.001Get rights and content

For more than a decade, positron emission tomography (PET) has had an important role in the management of thyroid cancer patients. It may be involved in initial, sometimes inadvertent, diagnosis; in postoperative evaluation; in detection of occult metastases; in the evaluation of thyroid nodules; and in prognostication of metastatic disease. In this review we will update the advances in the application of PET scanning to optimal patient management. The majority of the published studies to date have used 18F-fluoro-deoxyglucose (FDG) as the PET isotope, and unless specifically noted, all references to PET scanning will imply that this tracer has been used.

Section snippets

History of FDG-PET and thyroid cancer

The first studies to clearly demonstrate increased glucose consumption by neoplastic tumors were those of Warburg et al.1 Subsequent investigations discovered this phenomenon to be due to both elevated membrane glucose transport2 and to increased intracellular glycolytic pathway activity.3 Sokoloff et al4 were the first to quantify this biological activity using deoxyglucose and autoradiography. These initial studies were followed by the development of 18F-fluoro-deoxyglucose (FDG) which could

Use of FDG-PET to detect occult metastases

Eight years after the initial observations by Joensuu and Ahonen, Feine et al*7, 8 and Grunwald et al9 confirmed that there was a reciprocal or ‘flip-flop’ relationship between FDG avidity and radio-iodine avidity in metastatic lesions. Well-differentiated thyroid cancers tended to concentrate RAI (but not FDG), while poorly differentiated or anaplastic tumors were highly FDG-avid, but frequently did not concentrate RAI. Some disagreement has arisen regarding the value of FDG scanning for

FDG-PET evaluation of thyroid nodules and incidentalomas

Adler and Bloom were the first to study thyroid nodules with PET scanning.27 They evaluated nine individuals and found increased FDG uptake in seven. Pathology showed that in only three of the seven were the lesions malignant. The malignant lesions all had higher SUVs than the benign ones. Importantly, they were unable to distinguish the malignant nodules based on the visual pattern of uptake. A somewhat larger series of 19 patients who all had surgical resection was reported by Bloom and Adler.

Comparison of PET/CT to PET OR CT in detecting metastatic lesions

The advent of PET/CT fusion technology has markedly increased the ability of radiologists to verify and validate PET findings. Zoller et al35 have compared the diagnostic accuracy of combined PET/CT with that of PET alone or CT alone. They found that PET/CT not only improved the diagnostic accuracy in patients with elevated serum thyroglobulin and negative diagnostic 131I scans, but it also had a dramatic effect on treatment decisions options. Freudenberg et al36 did a similar retrospective

Does the TSH level affect FDG-PET outcome?

Sisson at al39 was the first to observe that the uptake of FDG was apparently higher in thyroid cancer metastases when the level of thyroid-stimulating hormone (TSH) was elevated. The authors attributed this to the known induction of GLUT expression in thyroid cells by TSH.40

Wang et al18 subsequently confirmed that an elevated TSH often led to finding more metastatic lesions in the same patient compared with a suppressed TSH. However, in their series the new sites detected in the high TSH state

Does FDG-PET provide information about the biology of the tumor?

It is well established that metastases from thyroid carcinoma which do not concentrate radio-iodine are more clinically aggressive45, 46, and this has been confirmed histologically by the studies of Ghossein and colleagues.47 If FDG scans can detect thyroid cancer metastases which do not concentrate radio-iodine, one would hypothesize that the patients who harbored such lesions would have a less favorable prognosis. Wang et al48 directly tested this proposal by retrospectively evaluating

Resistance of FDG-AVID lesions to radioactive iodine therapy

The evidence to date clearly suggests a positive correlation between the FDG avidity of a metastatic lesion and the level of de-differentiation. There is ample evidence from the ‘flip-flop’ pattern that the less-differentiated lesions do not concentrate radio-iodine very effectively. How then are clinicians to approach the therapy of RAI-negative, FDG-positive lesions? A recent report from our unit examined the value of using very-high-dose 131I therapy for such lesions.50 We compared the

124I-iodine PET scanning comparison of FDG-PET to FDG-PET scanning

At present, many centers that treat patients with metastatic disease are using high-dose radio-iodine therapy, with minimal therapeutic effect but with all of the potential side-effects. There is currently no convenient means to accurately predict the radiation dose to an individual metastatic lesion. For the past 50 years we have based our treatments on not exceeding the maximum safe dose to blood or lung, rather than on the amount of radiation necessary to destroy the lesion. Based on studies

Summary

Despite the relatively good overall prognosis, approximately 1500 patients are expected to die of thyroid cancer in the US alone in 2008. More than 70% of the deaths occur in individuals whose initial histology was well-differentiated thyroid carcinoma. The current prognostic scoring systems (e.g. MACIS, TNM, AGES), which utilize clinical and pathological information obtained at the time of initial diagnosis, have served us well and have stood the test of time. Their major limitation is their

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