For the past several decades thyroid cancer has been the most common endocrine tumor, with a ~ 5% increase in incidence each year in the USA. Thyroid cancer affects women more often than men and has been increasing over the last decade. Thyroid cancer is commonly first detected as a palpable thyroid gland during a physical exam.
Overall there is estimated to be 56,000 individuals diagnosed with thyroid cancer in the United States each year with only 2,000 dying from their disease. The vast majority of thyroid cancers arise from thyroid follicular cells (93%) and are well-differentiated (DTC). Most of these are categorized on histologic grounds as being papillary thyroid cancers (PTC), or less commonly as follicular thyroid cancers (FTC).1,2,3,4
The thyroid gland produces thyroid hormones, which regulate metabolism, growth, and development. The thyroid gland is located in the front of the neck and is attached to the lower part of the voice box (larynx) and to the upper part of the windpipe (trachea). Thyroid gland tissue envelops the upper trachea and usually four parathyroid glands lie posteriorly. Thyroid cancer is suspected if a small abnormal growth or “nodule” is found protruding from the thyroid gland. Most thyroid nodules are not cancer so diagnostic tests must be performed to determine if the nodule is benign or cancerous.
The main initial diagnostic test of the thyroid is evaluation with an iodine (I 131) scan. If this test shows that the I 131 is not taken up in an area of the gland, the nodule is said to be “cold” and cancer is suspected. The overall incidence of cancer in a cold nodule is ~15% and is higher in people younger than 40 years of age and those with calcifications.1,2,3,4
Types of Thyroid Cancer
Cancer may arise from different cells of the thyroid gland. By evaluating a sample of the cancer under a microscope, doctors can determine the type of thyroid cancer. There are four main types of thyroid cancer. The thyroid gland may occasionally be the site of other primary tumors, including sarcomas, lymphomas, epidermoid carcinomas, and teratomas. The thyroid may also be the site of metastasis from other cancers, particularly of the lung, breast, and kidney.5,6,7
Papillary: Papillary tumors are the most common form of thyroid cancer, accounting for more than 80% of all cases. Papillary cancers are typically irregular or solid masses that arise from otherwise normal thyroid tissue. More than half of papillary cancers have spread to lymph nodes in the neck. However, papillary cancers rarely spread to distant locations in the body. Papillary cancers typically occur in younger patients (30-50 years) and are commonly associated with a prior exposure to radiation. Patients with papillary cancer are highly curable with currently available treatment techniques.8
Follicular: Follicular cancers account for a smaller percentage of all thyroid cancers (approximately 15%) and rarely occur after radiation exposure. Follicular cancers are more aggressive; they tend to invade blood vessels rather than lymph nodes, and distant spread is therefore more common. Potential sites of distant spread include the lung, bone, brain, liver, bladder, and skin. Patients over 40 have more aggressive disease that is more difficult to treat. Nonetheless, most follicular cancers are very curable.
Medullary: There are two subtypes of medullary thyroid cancer: sporadic and familial. Sporadic almost always occurs on both sides of the thyroid gland. Familial tumors may be malignant or benign and may be associated with a variety of symptoms.
Approximately half of medullary thyroid cancers have spread to lymph nodes. Prognosis depends on the extent of disease at diagnosis—especially spread to lymph nodes—and the ability to completely remove the cancer with surgery.
Anaplastic: Anaplastic thyroid cancer is a rare disease that may also be called undifferentiated cancer. This type of thyroid cancer is very aggressive, grows rapidly, and commonly extends beyond the thyroid gland. It typically occurs in older patients and is characterized by extensive spread in the neck area and rapid progression. Patients typically die of their disease within months of diagnosis.
Well-differentiated tumors are highly treatable and usually curable. Poorly differentiated tumors are less common, aggressive, metastasize early, and have a poorer prognosis.
2 American Cancer Society: Cancer Facts and Figures 2017. Atlanta, Ga: American Cancer Society, 2017.
3 Chen AY, Jemal A, Ward EM. Increasing incidence of differentiated thyroid cancer in the United States, 1988-2005. Cancer. 2009;115(16):3801–3807. doi: 10.1002/cncr.24416.
4 Pellegriti G, et al. Worldwide increasing incidence of thyroid cancer: update on epidemiology and risk factors. J Cancer Epidemiol. 2013;2013:10. doi: 10.1155/2013/965212.
5 Liska J, et al. Thyroid tumors: histological classification and genetic factors involved in the development of thyroid cancer. Endocr Regul. 2005;39(3):73–83.
6 Tennvall J, Biörklund A, Möller T, et al.: Is the EORTC prognostic index of thyroid cancer valid in differentiated thyroid carcinoma? Retrospective multivariate analysis of differentiated thyroid carcinoma with long follow-up. Cancer 57 (7): 1405-14, 1986.
7 Khoo ML, Asa SL, Witterick IJ, et al.: Thyroid calcification and its association with thyroid carcinoma. Head Neck 24 (7): 651-5, 2002.
8 Lubitz CC, Sosa JA. The changing landscape of papillary thyroid cancer: Epidemiology, management, and the implications for patients. Cancer. 2016;122(24):3754-59. doi: 10.1002/cncr.30201.