Thyroid cancer overview

Overview
Thyroid cancer refers to any of four kinds of tumors of the thyroid gland: papillary, follicular, medullary and anaplastic. Papillary and follicular tumors are the most common and usually are benign: they grow slowly, may recur, but are generally not fatal in patients under 45 years of age. Medullary and anaplastic tumors are malignant. Medullary tumors have a good prognosis if restricted to the thyroid gland and a poorer prognosis if metastasis occurs. Anaplastic tumors are fast-growing and respond poorly to therapy. Thyroid nodule are diagnosed by ultrasound guided fine needle aspiration (USG/FNA) or frequently by thyroidectomy (surgical removal and subsequent histological examination). As the thyroid cancer can uptake iodine, radioactive iodine is a commonly used modality in thyroid carcinomas. However, it is followed by TSH suppression by thyroxine therapy.

Classification
Thyroid cancers can be classified according to their pathological characteristics. The following variants can be distinguished (distribution over various subtypes may show regional variation):
 * Papillary thyroid cancer (75%, incl. mixed papillary/follicular)
 * Follicular thyroid cancer (16%)
 * Medullary thyroid cancer (5-7%)
 * Anaplastic thyroid cancer (3%)
 * Lymphoma (1%)
 * Squamous cell carcinoma, sarcoma (0.5 - 2%)

Papillary thyroid cancer
This is the most common type of thyroid cancer. It occurs more frequently in women and presents in the 30-40 year age group. It is also the predominant cancer type in children with thyroid cancer, and in patients with thyroid cancer who have had previous radiation to the head and neck (in this group, the cancer tends to be multifocal with early lymphatic spread, and portends a relatively poor prognosis). Thyroglobulin can be used as a tumor marker for well-differentiated papillary thyroid cancer.

Follicular thyroid cancer
This occurs more commonly in women of over 50 years old. Thyroglobulin (Tg) can be used as a tumor marker for well-differentiated follicular thyroid cancer.

It is not possible to distinguish between follicular adenoma and carcinoma on cytological grounds. If fine needle aspiration cytology (FNAC) suggests follicular neoplasm, thyroid lobectomy should be performed to establish the histopathological diagnosis. Follicular carcinoma tends to metastasize to lung and bone via the bloodstream, while papillary thyroid carcinoma commonly metastasizes to cervical lymph nodes.

Hurthle cell variant
Hurthle cell thyroid cancer is a variant of follicular cell carcinoma. Hurthle cell forms are more likely than follicular carcinomas to be bilateral and multifocal and to metastasize to lymph nodes. Like follicular carcinoma, unilateral hemithyroidectomy is performed for non-invasive disease, and total thyroidectomy for invasive disease.

Medullary thyroid cancer (MTC)
This form of thyroid carcinoma originates from the parafollicular cells (C cells), which produce the hormone calcitonin. While the increased serum concentration of calcitonin is not harmful, it is useful as a marker which can be tested in blood. A second marker, carcinoembryonic antigen (CEA), also produced by medullary thyroid carcinoma, is released into the blood and it is useful as a serum or blood tumor marker. In general measurement of serum CEA is less sensitive than serum calcitonin for detecting the presence of a tumor, but has less minute to minute variability and is therefore useful as an indicator of tumor mass.

The prognosis of MTC is poorer than that of follicular and papillary thyroid cancer when it has metastasized (spread) beyond the thyroid gland. Approximately 25% the cancer develops in families. When MTC occurs by itself it is termed familial MTC; when it coexists with tumors of the parathyroid gland and medullary component of the adrenal glands (pheochromocytoma) it is called multiple endocrine neoplasia type 2 (MEN2).

Anaplastic thyroid cancer
This form of thyroid cancer has a very poor prognosis (near 100% mortality) due to its aggressive behavior and resistance to cancer treatments. It rapidly invades surrounding tissues (such as the trachea). The presence of regional lymphadenopathy in older patients in whom FNA reveals characteristic vesicular appearance of the nuclei would support a diagnosis of anaplastic carcinoma.