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Thyroid disease affects 27 million Americans, more than the number diagnosed with diabetes and cancer combined. The American Cancer Society estimates that there are approximately 48,000 new cases of thyroid cancer diagnosed in the United States annually, and of these new cases, over 70% will be women.
Thyroid cancers can occur at all ages but are relatively common in younger women, a group not typically at high risk for cancers. The notable risk factor that has been identified for thyroid cancer is radiation exposure at a young age. Data from the Chernobyl nuclear accident, atomic bomb survivors, and patients who have received therapeutic radiation to the head and neck indicate that radiation exposure is a strong risk factor for thyroid cancer.
Source: American Cancer Society. Cancer Facts & Figures 2011. Atlanta: American Cancer Society; 2011
There are four types of thyroid cancer: papillary, follicular, medullary and anaplastic, and most are highly curable with proper treatment. Papillary or follicular tumors are differentiated tumors and make up ninety percent of thyroid cancers. Medullary and anaplastic cancers are poorly differentiated tumors and make up the remainder.
Many patients in the early stages of thyroid cancer have no symptoms. As the cancer progresses, symptoms may include a lump or nodule in the front of the neck, hoarseness, neck or throat pain, enlarged lymph nodes, and/or difficulty swallowing, breathing, or speaking may occur.
Early stage thyroid cancer is nearly 100 percent curable with local therapies. The patient’s outlook or prognosis declines a little with lymph node involvement or lung metastases at presentation.
Primary diagnosis and staging of thyroid cancer is generally determined on patient history, physical examination, laboratory testing, ultrasound, thyroid scintigraphy and biopsy.
PET/CT imaging with the radiopharmaceutical 2-Deoxy-2-[ 18F ]fluoro-D-Glucose (FDG) is not normally used in the diagnosis or characterization of primary thyroid masses, but if increased FDG uptake is noted in a thyroid nodule as part of a whole-body study for cancer imaging, there is a moderately high risk that the nodule is malignant and should be evaluated further. Other diseases that could also cause increased uptake of FDG include Graves, disease and thyroiditis.
PET scanning may help by evaluating the cancer and determining the stage of the cancer, so the most appropriate therapy and treatment can be prescribed.
The prognosis for most thyroid cancer patients is very good, and depends on the patient’s age and overall health, the aggressiveness of cell type, and the stage of disease at presentation, whether disease is contained to the thyroid gland, or if it has spread to regional lymph nodes or metastasized to other sites.
Surgery effectively cures most thyroid cancer, but it cannot control advanced disease, which is systemic. Radioactive iodine scans are the most commonly used method of treating and monitoring thyroid cancer. There is, however, an occurrence called “flip flop phenomenon” where differentiated thyroid cancer cells may transform over time and lose some or all of their ability to absorb radioactive iodine. In these cases radioiodine therapy has limited or no value in treating non-iodine-avid disease.
When thyroid cancer cells lose their ability to concentrate radioactive iodine they may exhibit increased metabolic activity which results in increased glucose uptake. FDG PET/CT imaging can then aid the physician in the detection of metastatic disease, and the selection of the most appropriate treatment, which could include surgical resection or external radiotherapy.
A PET/CT scan can help show where tumor cells are growing, and helps the doctor determine the best course of treatment.
After treatment, physicians will schedule routine follow-up visits and depending on the type of the cancer, may use blood tests to monitor serial serum thyroglobulin levels. Doctors may also order follow-up imaging tests such as radioactive whole-body scintigraphy to make sure the cancer has not returned.
Functional imaging studies such as PET/CT may be recommended for patients where the results of radioactive whole-body scintigraphy were normal, but the patient’s thyroglobulin levels are rising. This could indicate that a phenomenon called “flip flop phenomenon” has occurred, where differentiated thyroid cancer cells transform over time and lose some or all of their ability to absorb radioactive iodine. When tumors become non-iodine-avid they may be detectable by FDG PET/CT scans which can help localize suspected recurrent disease.
It is in this role that PET/CT imaging has the greatest impact, by helping the physician determine if thyroglobulin producing, iodine-negative metastatic thyroid cancer has returned. When iodine-negative metastatic disease is detected and localized, appropriate treatment of surgical resection or external radiotherapy can be started.
If cancer does return the ability to detect and treat thyroid cancer recurrence early is critically important.
PET/CT is a noninvasive test that physicians utilize to stage the body for the presence or absence of active tumor, localize the tumor, assess the tumor response to treatment and detect recurrence in treated lesions.
Thyroid Cancer Indications:
Source: Atlas of Clinical Positron Emission Tomography by Sallie F. Barrington, Michael N. Maisey and Richard R. Wahl. Oxford University Press, Inc. New York, NY. 2006.
Source: Positron Emission Tomography: Basic Science and Clinical Practice. Peter E. Valk, Dale L. Bailey, David W. Townsend, Michael N. Maisey. Springer-Verlag London Limited. 2003.
Source: Practical FDG Imaging: A Teaching File by Dominique Delbeke, William H. Martin, James A Patton and Martin P. Sandler. Springer-Verlag New York, Inc. 2002.