Thyroid Cancer

thyroid cancer Malignant growths that develop within the tissues of the THYROID GLAND. Thyroid CANCER may be primary (originating in the thyroid gland) or secondary (metastasizing from cancer that originates elsewhere in the body). Thyroid cancer is uncommon in the United States, with about 11,000 cases diagnosed each year, and occurs primarily in people who are over age 70. There are four kinds of thyroid cancer: papillary, follicular, medullary, and anaplastic.

Papillary thyroid cancer About 75 percent of people who have thyroid cancer have papillary CARCINOMA, which is highly curable when detected and removed while the tumor is still encapsulated and clearly defined. Papillary thyroid cancer generally begins as a painless, single lump (nodule)

arising from the follicular cells and tends to grow slowly. This form of thyroid cancer is more common in people who have had previous RADIATION THERAPY to the neck, lower face, or upper chest, and in people who have autoimmune (Hashimoto's) THYROIDITIS. When papillary thyroid cancer metastasizes, it does so through the lymphatic system and usually only to adjacent lymph nodes.

Follicular thyroid cancer About 15 percent of thyroid cancers are follicular carcinomas, which also develop in the follicular cells. Follicular carcinoma does not correlate to previous radiation therapy or benign thyroid conditions. This form of thyroid cancer tends to metastasize through the BLOOD circulation, causing secondary tumors remote from the original tumor. With early detection and treatment (before METASTASIS), follicular thyroid cancer is highly curable. After metastasis, however, the prognosis declines considerably.

Medullary thyroid cancer About 8 percent of thyroid cancers are medullary carcinomas, which develop in the parafollicular cells which synthesize CALCITONIN. Consequently, excessive blood levels of calcitonin in the absence of other health conditions strongly suggest this thyroid cancer. Because these cells are less organized structurally, cancer that arises from them is less clearly delineated and accordingly more difficult to see or feel. This form of thyroid cancer often occurs in the inherited genetic disorder MULTIPLE ENDOCRINE NEO-PLASIA (MEN). Rarely, medullary thyroid cancer is inherited without MEN. Medullary thyroid cancer tends to metastasize early to adjacent lymph nodes. Distant metastasis to remote sites significantly worsens the prognosis.

Anaplastic thyroid cancer About 2 percent of people who develop thyroid cancer have anaplastic carcinoma, which is the most lethal of the cancers that involve the thyroid gland. It grows rapidly and spreads aggressively. Anaplastic thyroid cancer is more likely to develop in men over the age of 70 and is very rare in people under age 50.

Symptoms and Diagnostic Path

The typical symptom of any form of thyroid cancer is a painless lump or swelling in the neck. The person may see the swelling in the mirror or feel

the lump. Many times a doctor discovers a thyroid cancer during a routine medical examination or when examining the neck for other reasons. Some people experience difficulty swallowing or talking, depending on the location and size of the tumor.

The diagnostic path typically includes blood tests to measure the levels of the thyroid hormones, including thyroid-stimulating hormone (tsh), though the results may be normal. Ultrasound and a radioisotope iodine reuptake test can identify the tumor and provide some clues as to whether it is cancerous, though fine-needle aspiration (FNA) biopsy provides the definitive diagnosis. In FNA the endocrinologist uses a small needle and syringe to withdraw a sample of cells from the growth. Laboratory examination then determines whether the cells are cancerous, and what type of cancer is present.

Treatment Options and Outlook

For nearly all thyroid cancers, treatment is surgery to remove the thyroid gland (thyroidectomy) followed by radioactive iodine to kill any remaining thyroid cells. The radioactive iodine acts as a form of chemotherapy and invades thyroid cells no matter where they are in the body. The exception is for medullary thyroid cancer, which arises from the parafollicular cells that do not take in iodine. The doctor may choose conventional chemotherapy or radiation therapy to follow surgery for medullary thyroid cancer. When a papillary thyroid cancer is small and contained, the doctor may feel lobectomy (removal of the involved lobe of the thyroid gland) is adequate. The decision must consider numerous factors, however, and the person who has thyroid cancer should make an informed choice based on full consideration of those factors.

Thyroid cancers detected and removed early in their development have the highest treatment success rate, and can be up to 90 percent curable (papillary and follicular). Medullary and anaplastic thyroid cancers are more difficult to diagnose in their early stages, and thus tend to be more advanced and often have metastasized by the time treatment begins. After thyroidectomy, it is necessary to take lifelong thyroid hormone supplements.

Risk Factors and Preventive Measures

The primary risk factor for papillary and anaplastic thyroid cancers is previous radiation therapy to the neck, lower face, or upper chest. Radiation exposure causes about 80 percent of papillary thyroid cancers. Family history may establish a risk for medullary thyroid cancer, as does having MEN. Risk factors for anaplastic thyroid cancer are unknown. Early diagnosis is the most effective measure for successful treatment.

See also CANCER TREATMENT OPTIONS AND DECISIONS; GOITER; SURGERY BENEFIT AND RISK ASSESSMENT; THYROID NODULE; THYROID STORM.

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