Although thyroid nodules are extremely common (in countries without iodine deficiency, thyroid nodules are clinically detectable in about 4% to 7% of the general population while this number can rise up to 30% of the population in areas with iodine deficiency) (5), malignant lesions derived from thyroid cells are relatively rare. Less than 1% of all malignant tumors are clinically recognized thyroid carcinomas. However, it is the most common endocrine malignant lesion comprising 90% of all endocrine malignant tumors. It is responsible for more deaths than all the other endocrine cancers combined (6).
Thyroid cancer occurs at all ages but is most common among middle-aged and postmenopausal women and in older men (7). According to reproductive status, the female to male incidence ratio varies, from being almost one in childhood, increasing to three from puberty to menopause, and declining to 1.5 by 65 years of age. Overall death rate is less than 10% (7).
Carcinoma of the thyroid is usually of follicular cell origin with four distinct subtypes (Table 1) or it can arise from the parafollicular or C-cells. Other malignant nonepithelial tumors of the thyroid are extremely rare (8,9).
Table 1 Classification of Thyroid Tumors
Epithelial thyroid carcinoma Papillary carcinoma
Oxyphilic or Hiirthle cell carcinoma Poorly differentiated carcinoma
Undifferentiated or anaplastic carcinoma
C-cell-derived thyroid carcinoma or medullary carcinoma
80% of the thyroid carcinomas are of the follicular variant subtype 20% of thyroid carcinomas with higher incidence rates in iodine-deficient areas With morphological and biologic attributes intermediate between differentiated and anaplastic carcinomas Highly malignant cancer with undifferentiated cell populations Arises from the parafollicular cells that secrete calcitonin. Sporadic or familial
Nonepithelial thyroid carcinoma Malignant lymphoma Sarcoma
Hemangioendothelioma Secondary tumors
Source: From Ref. 20.
Papillary thyroid cancer (PTC) and follicular thyroid cancer (FTC) are together referred to as differentiated thyroid cancer (DTC). These cancers synthesize thyroglobulin (Tg) and are usually characterized by radioiodine uptake. They tend to be sporadic although occasionally PTC is familial.
Anaplastic and medullary carcinomas and lymphomas comprise about 10% of all thyroid cancers (10) and they differ substantially from DTC relative to initial mode of spread, subsequent pattern of recurrence, and metastatic involvement. They have different prognosis and treatment.
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