الفهرس 
Anatomy of the thyroid glandOnly 14 pages are availabe for public view
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Abstract
Thyroid cancer is the sixth most common cancer in women and accounts for approximately 1% of all cancer cases.Thyroid cancer had a prevalence of >425,000 accounting for approximately 1,600 deaths.
Patients with thyroid cancer are often treated with total thyroidectomy followed by radio-iodine ablation. After treatment, measurement of thyroglobulin levels, ultrasonography and (131) I whole-body scintigraphy examinations are commonly used to monitor patients for cancer especially to detect local recurrence or distant metastases.
Ultrasonography is characterized by a relatively high sensitivity in these patients but it is highly operator-dependent and, moreover, it can be used to explore neck alone. Computed tomography (CT) scan and magnetic resonance (MR) imaging are characterized by a relatively low sensitivity even if they are useful to provide the surgeon with anatomical information of the operating basin. Various tumor-seeking radiotracers have been proposed, mainly using SPECT as 201Thallium, 99mTc-Sestamibi and 99mTc-Tetrofosmin with good results. But the most favorable results have been reported with some positron radiotracers, mainly the 18F-FDG with PET/CT tomographs.
Imaging with a combination of positron emission tomography (PET) and computed tomography (CT) can detect recurrent or metastatic thyroid cancer especially in patients who have an elevated thyroglobulin level and a negative (131)I whole-body scintigraphy examination (WBS).
A CT scan uses computer-controlled X-rays to create a three-dimensional image that helps the physician estimate the extent of the disease. A PET scan is another imaging technique where the cancer cells take the radiotracer attached isotope and the energy radiation emitted by these cells helps physicians locate cancer.
Primary thyroid cancers detectable by FDG-PET tend to have more aggressive histological features and denote a poorer prognosis. The intensity of FDG uptake, as expressed by the standard uptake value (SUV), does seem to be higher in malignant lesions vs. benign.
Conventional radioiodine imaging for thyroid cancer has typically been done with gamma cameras using either 123I or 131I. 131I is inexpensive and widely available, but it has the disadvantages of poor image resolution and of possibly causing “stunning. On the other hand, 123I produces superior image quality, but it is expensive, has a relatively short half-life, and has limited availability.
Currently, FDG PET-CT in thyroid cancer is most commonly used in cases where conventional imaging modalities are negative or inconclusive in the presence of elevating tumor markers. FDG PET-CT is superior to conventional modalities, such as ultrasound, contrast-enhanced CT in the detection of recurrent thyroid cancer especially among patients who have no symptoms or only mild ones and with elevated tumor marker. PET-CT imaging with FDG detects all primary tumor and nodal metastases, as well as distant metastases.
The most fundamental difference between PET and the various conventional radiologic imaging techniques, such as computed tomography (CT) and conventional magnetic resonance imaging, is that the former assesses functional or metabolic characteristics of the tumor, whereas the latter predominantly assess the tumor’s anatomical or morphologic features — for example, density, size, and shape. Furthermore, PET detects clinically relevant changes even when no changes or minimal ones are detected by morphologic imaging which permits a more accurate assessment after treatment and enables early detection of cancerous lesions.
FDG-PET has been advocated as a monitoring procedure for patients with:
1. High-risk disease.
2. Adverse histology (e.g., columnar cell, tall cell, and insular variants).
3. Rising Tg levels with no known anatomic source.
4. Hurthle cell carcinoma.
FDG-PET has also been recommended for post-treatment response assessment and lesion dosimetry.
PET measurement of glucose metabolism provides biologic information, as noted in the standardised uptake value (SUV). Patients whose cancers take up FDG well are not likely to respond to radioactive iodine. Furthermore, the PET-FDG SUV is a strong predictor of adverse prognosis, with higher SUV’s indicating worse overall prognosis.
Selected patients with thyroid cancer can benefit from the use of PET /CT scan which impacts on management by providing:
(1) More accurate information about staging of patients in terms of extent of tumor for better treatment planning, especially in patients who do not concentrate radioactive I-131 and to distinguish operable from non operable patients.
(2) The relationship of tumor involvement to vital structures, especially in the neck and central nervous system.
(3) Prognostic information (an SUV > 10 and extensive PET positive disease connotes a poor prognosis in advanced patients).
(4) Information about the efficacy of therapy after the start of treatment.
(5) Appropriate metabolic indicator of the final result of the treatment when comparing between pre- and post treatment scans.
(6) Ability to distinguish between viable tumors and necrosis or fibrosis in residual masses.
PET and PET/CT have emerged as powerful imaging tools in clinical oncology for the accurate staging and restaging of established disease, for the detection of occult tumors, and for the reliable prediction of the nature of residual masses that are difficult to evaluate with conventional imaging after therapy. PET/CT is also being evaluated for its ability to predict response or lack of response at a very early stage in the course of treatment. The favorable experience to date is beginning to support the use of PET/CT as a surrogate end point in trials that are aimed at testing or comparing the efficacy of new drugs or treatments. This innovation could shorten the time required to evaluate the efficacy of drugs or to determine the optimal therapeutic intervention. The indications for the use of PET/ CT in clinical oncology and cancer research are likely to expand with a move toward an assessment that is both functional and anatomical, so conclusively, in the post treatment follow up of thyroid carcinoma, PET/CT in conjunction with other conventional imaging modalities play an important role in making decision for initial and subsequent treatment strategy.