Carcinoma of the thyroid gland is the most common endocrine malignancy, accounting for 1.6% of all new malignant disease (1). It has a prevalence rate of 335,000, and approximately 37,200 new cases of thyroid carcinoma were diagnosed in the United States in 2009 alone (1,2). Despite the generally good prognosis of thyroid carcinoma, 5 to 10% of patients will die of the disease (3,4). Differentiated thyroid carcinomas, those that derive from the follicular epithelial cells (papillary and follicular), account for 94% of these malignancies; 5% are medullary thyroid cancers, a neuroendocrine tumor derived from C cells in the thyroid gland; and the remaining 1% are anaplastic thyroid carcinoma.
Papillary carcinoma is the most common type of thyroid carcinoma, except that in regions where there is an iodine insufficiency, the follicular type is more common. Follicular thyroid carcinoma occurs in older people, with peak incidence in the fifth decade of life. Follicular thyroid malignancies have a worse prognosis than papillary tumors, especially in patients with fixed/invasive lesions. In one report from the Surveillance, Epidemiology, and Endocrine Points database, of approximately 15,700 patients in the United States, overall survival rates corrected for age and sex were 98% for papillary, 92% for follicular, 80% for medullary, and 13% for anaplastic carcinoma (5). Worse prognoses are associated with increasing age at diagnosis and metastatic disease at presentation.
Women are affected twice as often as men, although the latter gender dies of cancer twice as often as women (1). Although the median age of diagnosis is 45 years, thyroid carcinoma does affect children. In our institution, fewer than 10% of all patients with thyroid cancer were diagnosed before 20 years of age (6).
Diagnostic Evaluation of the Solitary Thyroid Nodule
The most common presentation of a patient with thyroid carcinoma is the presence of a solitary thyroid nodule found either on physical examination or discovered as an incidental nodule on imaging studies performed for other purposes. Cytologic examination of a fine-needle aspirate of a nodule more than 1cm in diameter is the most appropriate first diagnostic procedure. Papillary, medullary, and anaplastic carcinomas can be readily diagnosed by fine-needle aspiration (FNA), but distinguishing benign from malignant follicular lesions proves more difficult. Histologic examination showing capsular or vascular invasion is necessary to classify a lesion as malignant. Because follicular adenoma and carcinoma cannot be differentiated cytologically, they are grouped as "indeterminate or suspicious follicular neoplasms." The false-positive and false-negative rates for all nodules categorized as malignant or benign, respectively, is less than 5% (7). The rate of carcinoma for suspicious follicular neoplasms is about 20%. The incidence of malignancy increases with larger nodule size, male sex, and increasing age. Fifteen to 25% of the time, the FNA will yield "inadequate diagnostic material," and this necessitates repeat aspiration. The availability of ultrasound guidance ...