Functional Imaging of Thyroid Cancer

Abstract

Due to their relatively unaggressive biological behavior, most differentiated thyroid carcinomas have a good prognosis [7, 28]. This holds true even for patients with lung metastases, particularly in cases with disseminated metastatic sites that are radioiodine-positive. Therefore, thyroid cancer is overall a rare cause of cancer-associated death. Serum thyroglobulin measurement is the most sensitive method to detect recurrence of differentiated thyroid carcinoma during follow-up [39]. Radioiodine scintigraphy can be used as a highly specific method to visualize tumor tissue. But in many cases, particularly in poorly differentiated cancer and in Hürthle cell carcinoma, radioiodine uptake is decreased or absent, owing to several mechanisms. Particularly DNA changes, encoding the Na+/I^- symporter, have to be considered. Therefore, the sensitivity of radioiodine scintigraphy is decreased from about 70% to less than 50% during the clinical course [37, 46]. Although therapeutic options are often limited to some extent in patients with radioiodine-negative metastases, correct staging is very important to plan further diagnostic and therapeutic steps. But also in cases with known radioiodine-positive tumor tissue, other functional techniques are clinically useful to prove or exclude additional radioiodine-negative tumor sites, which cannot be influenced by further radioiodine treatments. In some cases, recurrence or metastases are suspected during follow-up, even if no increased thyroglobulin values are observed. The reason might be pathologic thyroglobulin recovery values or the existence of very poorly differentiated cell lines which have lost the capability of Tg synthesis. Tumor-specific functional imaging techniques are necessary to evaluate equivocal morphologic alterations in these patients.