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Scintigraphy in benign and malignant thyroid disorders (CROSBI ID 537656)

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Mihaljević, Ivan Scintigraphy in benign and malignant thyroid disorders. 2007

Podaci o odgovornosti

Mihaljević, Ivan

engleski

Scintigraphy in benign and malignant thyroid disorders

Thyroid scintigraphy: Thyroid scintigraphy with 99mTc pertehnetate (99mTcO4) as a two-dimensional imaging method shows the mechanism of accumulation, and the iodine radionuclides (131I, 123I) scintigraphy the mechanism of accumulation and organification of iodine. We can distinguish hyperfunctional from hypofunctional nodules, which are more suspicious to malignancy, with scintigraphy. The reasons for decreased metabolism are regressive changes in adenomas, cystic degeneration and inflammatory destruction of thyrocytes or malignant growth with functional dedifferentiation of thyroid tissue. The incidence of differentiated thyroid carcinoma (DTC) in hypofunctional nodules is proportional to the quantity of iodine intake by diet in specific population. The better the iodine intake is the higher the possibility of the DTC appearance in the hypofunctional nodule is (5 - 15%). The nodules with the normal function or hyperfunctional ones cannot be considered benign without clinical evaluation, especially if they are shown with 99mTcO4 scintigraphy or if the nodule is very small (< 1 cm). Due to the low specificity of scintigraphy for DTC, in the assessment of malignancy of the thyroid nodules different radiopharmacs with affinity of accumulating in malignant lesions (201Tl-chloride, 99mTc-sestamibi or 99mTc-tetrofosmin), are used. The lack of accumulation of 201Tl inside the hypofunctional nodule is associated with a low probability of malignancy (7%), while the increased, or the 201Tl accumulation equal to the surrounding thyroid tissue, except in the malignant nodules, is also found in the follicular and Hürthle adenomas, and therefore the clear distinction between DTC and 201Tl thyroid adenoma in not possible. It is recommended to use two-phase scintigraphy when using 99mTc (sestamibi and tetrofosmin) marked cationic complexes, because the initial accumulation of radiopharmacs (5&#8211; 30 minutes after the intravenous injection) is dependant on both the perfusion of the nodules and the high concentration of radiopharmacs, which is also seen in the benign lesions. The retention of markers (30&#8211; 120 minutes after application), due to the binding in the multitude of mitochondria and intense metabolism, point to the TC. In the preoperative diagnostic evaluation, the 99mTc-MIBI thyroid scintigraphy can point to, but it cannot differentiate with certainty the DTC from the thyroid adenoma. Similar results were also found in the preoperative evaluations of hypofunctional nodules studies in which the scintigraphy was done with the 99mTc-tetrofosmin. It was found out that with 99mTc-tetrofosmin benign thyroid nodules cannot be distinguished from the malignant ones. Nevertheless, increased accumulation of 99mTc-tetrofosmine on the early scintigram with retention of radiopharmacs on the late scintigram points to thyroid adenoma (especially to the microfollicular and Hürthle adenoma). Fluorescent method: In the preoperative assessment of the nodular goiter, along with the scintigraphy and the ultrasound guided targeted cytological fine-needle aspiration biopsy, sometimes fluorescent method, which makes it possible to determine the quantity of the stable iodine (127I), usually found in thyroid gland in vivo, and the description of its distribution, is used. Stable iodine in the thyroid gland is activated by 241Am radiation that creates fluorescent radiation typical to the iodine and whose intensity is directly dependant on the quantity of the iodine, and is registered with semiconductor detector. The examination shows the toxic adenoma suppressed lobe and the display of thyroid gland during the suppression treatment, and, by measuring the quantity of stable iodine, it is possible to evaluate the course of the subacute thyreoiditis or the hyperthyreosis treatment and to differentiate the iodine caused hyperthyreosis from other forms of hyperthyreosis. The radiation dose administered to the thyroid gland is minimal, and as the radionuclide is not administered to the examinees, this examination is suitable for children and pregnant women. Whole-body scintigraphy (WBS): For a period of years, the 131I or 123I-WBS has been the standard procedure for discovering the DTC metastases. The sensitivity of the diagnostic 131I- WBS is relatively low (45 &#8211; 75%), it depends on applied activity and the TC histological type, and it is lower for papillar TC than for the follicular TC. Due to the fact that only two-thirds of the recidives and TC metastases accumulate iodine, WBS with unspecific markers (201Tl, 99mTc-MIBI or 99mTc-tetrofosmin) is introduced for detection of early (iodine-negative) recidives and metastases. The research of comparisons of WBS with 99mTc-MIBI and 99mTc-tetrofosmin, conducted during the follow-up of patients with DTC that have elevated levels of Tg, has determined that 99mTc-tetrofosmin has significantly higher in vivo tumour/background ratio and a higher metastases detection rate than 99mTc-MIBI. The in vivo tumour/background ratio for 201Tl, in comparison to 99mTc-tetrofosmin, is slightly, but not significantly, higher. Positron emission tomography (PET): During the course of preoperative evaluation of hypofunctional and/or hypoechogenic thyroid nodules, the 18F-fluorodeoxyglucose (18F-FDG) PET level has been researched, as well as during the TC follow-up. Visibly elevated accumulation of 18F-FDG has been established in patients with Hürthle cell and anaplastic TC, while in patients with DTC, the accumulation was moderately elevated. Other researches have indicated that malignant thyroid nodules can be differentiated from the benign ones by standardized uptake value (SUV) cut-off of 5 and time-activity curves. Further research of the afore-mentioned nodules have indicated that all of the patients with DTC and Hürthle adenoma have elevated 18F-FDG accumulation and that, using a SUV threshold of 2 for differentiating benign and malignant nodules, sensitivity, specificity and accuracy were 100%, 63% and 100%. Likewise, 18F-FDG can differentiate follicular adenoma from TC in patients with follicular proliferation cytological diagnosis. We can conclude that, with radiopharmacs known today, which accumulate in malignant lesions (including 18F-FDG), in the preoperative diagnostics, it is not possible to differentiate the benign from malignant thyroid nodules with 100% certainty. Nevertheless, in case of the impossibility of cytological diagnosis or with the follicular proliferation cytological findings, it seems that the 18F-FDG-PET is the preferred method of choice when it comes to deciding the way of treatment (operation or further follow-up). When Tg is elevated, the use of 18F-FDG has shown that radioiodine WBS underestimates the TC stage. Due to the limited resolution of a single-photon emitter, it is not possible to discover about 20 &#8211; 25% DTC metastases using conventional radionuclides and radipharmacs. To solve this problem, in the course of follow-up of the TC patients, the 18F-FDG-PET has been classified as 1a indication (according to the German consensus Conference 2000). Comparative research has shown that the sensitivity of 18F-FDG-PET, 131I-WBS and 201Tl/99mTc-MIBI-WBS was 75%, 50% and 53%, while specificity was 90%, 99% and 92%. Similar results were yielded comparing 18F-FDG-PET, 99mTc-tetrofosmin-WBS and postablation 131I-WBS, during the follow-up of TC patients. According to the German consensus Conference 2000, sensitivity and specificity of 18F-FDG-PET in 131I-negative metastases detection, in the case of elevated Tg was 85 &#8211; 94%, i. e. 90 &#8211; 95%. We can conclude that the 18F-FDG-PET is a valuable method for detecting recidives and DTC metastases, particularly when Tg is elevated and 131I-WBS is negative. Somatostatin receptor scintigraphy (SRS): During the previous years, it has become clear that thyroid carcinoma dedifferentiation can lead to excessive SR expression. The researches have shown that 99mTc and 111In-labeled somatostatin analogues scintigraphy can be useful in the detection of the iodine-negative metastases with the DTC patients. The researches have also shown that the evidence of SR expression in metastasic lesions can enable new therapy approaches for multiple 131I-negative metastases, for which there is no adequate therapy so far.

thyroid disorders; Scintigraphy

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Podaci o prilogu

2007.

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objavljeno

Podaci o matičnoj publikaciji

Podaci o skupu

Seccond Annual Meeting of the Croatian Society of Nuclear Medicine Technologists with International Participation

poster

25.05.2007-27.05.2007

Omiš, Hrvatska

Povezanost rada

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