الفهرس 
EpidemiologyOnly 14 pages are availabe for public view
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Abstract
Thyroid cancer is the 6th common cancer in women and accounts for approximately 1% of all cancer cases(1).
Differentiated thyroid cancer is generally characterized by long term survival, good prognosis and low aggressiveness. Its prognosis is related to the age at diagnosis, tumor dimension, extra capsular extension and presence of distant metastases. Distant metastases is relatively rare with incidence ranging from 4- 27%(2).
Most people diagnosed with thyroid cancer have a total thyroidectomy followed by radio-active iodine ablation. After treatment has finished patients will have regular check-ups. At some visits, patients may have blood tests, US, CT, MRI and scans(3).
The thyroglobulin (Tg) is primarily used as a tumor marker to evaluate the effectiveness of treatment for thyroid cancer and to monitor for recurrence. Not every thyroid cancer will produce thyroglobulin, but the most common types, papillary and follicular thyroid cancer, frequently do, resulting in increased levels of thyroglobulin in the blood. The magnitude of Tg levels may be related to tumor mass, degree of differentiation & location of metastasis. Low level of Tg(1ng/ml or less); provide a sensitive test, whereas higher cut-off levels result in a greater specificity at the expense of decreasing sensitivity in detection of recurrent cancer. Thyroid suppression & withdrawal of suppression influence the serum Tg levels. Tg surveillance is the sole screening test for recurrent thyroid cancer patients who have low risk & no evidence of recurrence on prior I-131 scan(4).
I-131 whole body scan (WBS) had been at the center of recurrent thyroid cancer detection; it detects iodine-avid cancers & is ineffective in undifferentiated tumors. One distinct advantage of I-131 WBS over other imaging modalities is its ability to identify distant metastasis. Similar to Tg, I-131 becomes more sensitive after thyroid suppression withdrawal & thyrotropin stimulation(4).
The differentiated thyroid cancer cells after total thyroidectomy & radioiodine ablation may undergo a process of transformation thus losing some or all their ability to take up & retain 131-iodine, but they still retain the ability to absorb FDG(4).
Recently reported that the loss of I-131 uptake in recurrences depends not only on a decrease in energy-dependent transport mediated by the Na+/I- symporter (NIS) gene but possibly on a reduction in the molecules regulating its intracellular metabolism. Moreover, high glucose transporter type1 (GLUT-1) gene expression supports the use of PET with specific tracers in clinical management of such cancers(2).
The role of F-18 FDG PET/CT in differentiated thyroid cancer (DTC) is well established, particularly in patients presenting with elevated thyroglobulin (Tg) levels and negative radioactive iodine whole body scan (WBS ). It has been demonstrated that F-18 FDG uptake represents less differentiated thyroid cancer cells or dedifferentiated cells and PET positive lesions are more likely to be resistant to 131-Iodine treatment. The uptake of F-18 FDG is related to tumor size, thyroid capsule invasion and histological variants with a poor prognosis(5).
Combination between positron emission tomography (PET) and computed tomography (CT) allow anatomic, functional & molecular information. The advantages of this combined technique over PET alone have become obvious. There is increasing evidence to suggest that PET/CT adds complementary information in staging, re-staging and follow-up in post-thyroidectomy patients, leading to changes in management plans(6).
The sensitivity of using FDG PET/CT in the detection of cancer thyroid is very high and more accurate than the other imaging modalities as it is capable of differentiating among tumors, scars, fibrosis and necrosis(7).
Also PET/CT images from survey of the body could reveal abnormal areas of uptake indicating the spread of the thyroid cancer to lymph nodes, lungs, bones or central nervous system(8).
The fusion of the metabolic and morphologic information in PET/CT was able to increase the diagnostic accuracy, reduces pitfalls and changes therapeutic strategies in a considerable number of patients(9).
Studies on the value of 18F-FDG PET for DTC have concentrated on patients in whom radioiodine scintigraphy is negative associated with an increase of TG level. It has been shown that 18F-FDG PET is the most accurate method in this situation, and sensitivities and specificities range between 85% and 94%(9-11). For the treatment of iodine-negative tumor tissue, surgery is the only curative therapy option; hence exact localization of 18F-FDG tumor foci is mandatory for successful resection of cancer in these cases.