Thyroid and Parathyroid Tumors

Abstract

Thyroid cancer occurred in approximately 45,000 patients, in the USA in 2010. There is a 3:1 ratio of women to men. Histologic types are divided into categories of differentiated thyroid cancer (DTC): papillary, mixed papillary and follicular, and follicular—including Hurthle cell variant, undifferentiated (anaplastic), and medullary cancer (arising from parafollicular C-cells). Other rare thyroid carcinoma accounts non-epithelial tumors, lymphoma and carcinomas characterized by the presence of mucin-producing cells and keratin. Differentiated thyroid cancer usually presents as a thyroid nodule. Thyroid ultrasonography is useful to detect and characterize thyroid nodules, as well as guide fine needle aspiration (FNA) biopsy. Radioiodide or 99mTc-pertechnetate thyroid scan has a low diagnostic specificity and sensitivity for characterizing thyroid nodules. X-ray of the neck is useful to disclose a deviation of the trachea or lumen restriction, in large nodules and in multinodular goiter. CT or MRI are generally reserved for mediastinal thyroid masses, or the identification of regional or distant metastasis. The most widely used staging system for thyroid carcinoma is the TNM classification system defined jointly by the UICC and by AJCC. 131I-iodide thyroid remnant ablation is indicated in differentiated thyroid cancer patients with a moderate to high likelihood of recurrence. 131I-iodide therapy is usually administered in the amount of 1.85 to 3.7 GBq for ablation. Patients are prepared with rhTSH and low iodine diet. Whole body scan (preferably with SPECT/CT of the neck) is performed 4–7 days after radioiodine therapy to detect lymph node involvement or unexpected metastases. The major diagnostic modalities employed to follow patients with differentiated thyroid cancer treated with remnant ablation is measurement of serum Tg, 131I-WBS, and neck US examination. Neck US examination is an integral component of follow-up evaluation in all DTC patients. If a lymph node metastasis is suspected, an FNA should be performed. Serum Tg levels that become detectable upon TSH stimulation indicate the need for further evaluation, possibly with additional radioiodine therapy. Although CT and MRI can in principle localize very small lesions in the neck, chest, and bones, the features of such lesions are rarely specific for recurrent/metastatic DTC. Patients with recurrent thyroid cancer may develop lesions which cannot concentrate radioiodide. [18F]FDG PET/CT is useful in these patients to determine the sites and extent of these metastases. The anaplastic thyroid carcinoma (ATC) is a rare tumor (<3% of all thyroid cancers) with poor prognosis derived from follicular cells. The most clinical presentation of an ATC is a new, large, firm thyroid nodule, often associated with signs/symptoms of local compression/invasion. Multimodality treatment of ATC includes surgery, EBRT, and combination chemotherapy. Therapy with 131I-iodide is not useful, since these tumors rarely concentrate radioiodide. Preoperative imaging with US, CT, MRI play an important role, and [18F]FDG PET is useful. Medullary thyroid carcinoma (MTC) is a well-differentiated thyroid tumor arising from the parafollicular, calcitonin-producing C cells. Its prevalence is 5–10% in all thyroid malignancies. Sporadic and familial forms are recognized. Elevated baseline serum levels of calcitonin (above 10 ng/mL) are diagnostic for MTC. Following surgery, MTC patients are monitored with serum calcitonin and CEA levels, and serial neck US examinations are performed. Calcitonin doubling time in serum is the most sensitive biomarker for MTC progression. Scintigraphy with 123I-MIBG has very high sensitivity for staging patients with MEN II and familial MTC. However, it has a low sensitivity in patients with increased serum calcitonin but no clinical site of disease. [18F]FDG PET is accurate in detecting lymph node involvement. Radionuclide therapy with the radiolabeled somatostatin analog 90Y-DOTA-Tyr3-octreotide (90Y-DOTA-TOC) has been tested in metastatic MTC. Parathyroid carcinoma is a very rare endocrine malignancy that occurs in <1% of primary HPTH. The initial clinical manifestations of parathyroid carcinoma are primarily linked to the effects of markedly elevated serum PTH levels. At initial presentation, very few patients have metastasis at regional lymph nodes or at distant sites. Parathyroid carcinoma tends to infiltrate adjacent structures in the neck. US, CT, and MRI have been used to localize parathyroid carcinomas and to detect mediastinal and thoracic recurrences or distant metastases. 99mTc-Sestamibi scintigraphy can be successful for preoperative localization of the neoplasia and can identify metastases in lymph nodes and at distant sites. PET with [18F]FDG can also detect metastatic parathyroid cancers. Parathyroid carcinoma recurs in more than 50% of the cases and imaging studies should be performed in all patients before reoperation.