Thyroid Cancers

Papillary carcinoma accounts for 80 to 90% of all thyroid cancers. The female:male ratio is 3:1. It may be familial in 3 to 9% of patients. Most patients present between ages 30 and 60 years. The tumor is often more aggressive in older patients (1).

The most common genetic mutation that gives rise to these tumors is BRAF V600E.

The incidence of papillary thyroid carcinoma has risen over the last several decades, primarily due to the incidental discovery of small cancers in patients undergoing neck ultrasonography, MRI, CT scans, or PET scans that include the neck in the imaging field.

Papillary thyroid cancer can spread via lymphatics to regional lymph nodes in one third of patients and may metastasize to the lungs. Patients < 55 years old with small tumors confined to the thyroid have an excellent prognosis.

Active surveillance for increasing size or appearance of cervical lymph nodes metastases using ultrasonography may be an alternative to surgery for appropriately selected patients with papillary carcinomas < 1.5 cm since only 15% enlarge by 3 mm at 10 years and only 3.4% develop cervical lymph node metastases at 10 years (2).

For small tumors (between 1.5 and 4 cm), hemithyroidectomy is an appropriate treatment option and almost always curative. Tumors > 4 cm or multifocal tumors require total or near-total thyroidectomy.

The American Thyroid Association risk stratification system divides patients into 3-tiers and is used to estimate the risk of recurrence and mortality (3):

• Low-risk: Intrathyroidal disease, unifocal or multifocal, and involvement of < 5 lymph nodes

• Intermediate-risk: More extensive lymphadenopathy, extrathyroidal extension, and aggressive histologic characteristics or vascular invasion

• High-risk: Large lymphadenopathy (> 3 cm), gross extrathyroidal extension, vascular invasion in follicular thyroid carcinomas, and distant metastases

Postoperative radioactive iodine is used to ablate residual thyroid tissue for one or more of the following reasons:

• To enable interpretation of thyroglobulin levels during surveillance

• As adjuvant therapy if residual thyroid cancer is suspected or the risk of recurrence is intermediate or high

• To aid in cancer staging if distant metastatic disease is suspected in patients with high-risk thyroid cancer

Postoperative radioactive iodine has not proven to decrease recurrence in patients with low-risk thyroid cancer and it is therefore not recommended (4).

Transition to a cancer survivorship program or follow-up by a primary care physician is typically recommended after 5 years of surveillance in patients without evidence of disease, since the risk of recurrence falls below 1%. A thyroid ultrasound and a thyroglobulin and thyroglobulin antibody panel every 5 years can detect late recurrences, which are infrequent.

Follicular carcinoma accounts for approximately 4% of thyroid cancers. It is more common among older patients and in regions of iodine deficiency. It is more aggressive than papillary carcinoma since it spreads hematogenously and carries a higher risk of distant metastases. The most common genetic mutation that gives rise to these tumors is a RAS mutation.

Oncocytic thyroid carcinomas have been historically considered a subgroup of follicular thyroid carcinomas, but genomic data confirm that these tumors are a distinct entity. Loss of heterozygosity of most chromosomes and chromosomal duplications of chromosomes 5 and 7 as well as mitochondrial DNA mutations give these tumors a cytoplasm rich in mitochondria and their "oncocytic" designation.

These tumors can be encapsulated or widely invasive. Widely invasive oncocytic carcinomas respond less well to radioactive iodine, are FDG-avid on PET scans, and tend to metastasize hematogenously outside of the neck (1, 2).

Anaplastic carcinoma is an undifferentiated cancer that accounts for approximately 1% of thyroid cancers, and it contributes to 20% of the annual thyroid cancer-related mortality (1). It occurs mostly in older patients and slightly more often in females. The tumor is characterized by rapid, painful enlargement. Rapid enlargement of the thyroid may also suggest thyroid lymphoma, particularly if found in association with Hashimoto thyroiditis.

Approximately 45% of anaplastic tumors harbor a BRAF V600E2, 3).

Medullary carcinoma constitutes approximately 4% of thyroid cancers and arises from parafollicular C-cells that produce calcitonin. It may be sporadic (75%, usually unilateral); however, 25% of the cases are familial, caused by a mutation of the RET proto-oncogene. The familial form may occur in isolation (familial medullary thyroid carcinoma) or as a component of multiple endocrine neoplasia (MEN) syndrome type 2A (medullary thyroid carcinoma, primary hyperparathyroidism, and pheochromocytoma) or MEN 2B (medullary thyroid carcinoma, pheochromocytoma, and gastrointestinal ganglioneuromas). Although calcitonin can lower serum calcium and phosphate levels, serum calcium is normal because the high level of calcitonin ultimately down-regulates its receptors. Characteristic amyloid deposits that stain with Congo red are also present in the tumor.

Metastases spread is via the lymphatic system to cervical and mediastinal nodes and sometimes to liver, lungs, and bone.

Patients typically present with an asymptomatic thyroid nodule, although many cases are now diagnosed before a palpable tumor develops during routine screening of affected kindreds with MEN 2A or MEN 2B.

Medullary carcinoma may have a dramatic biochemical presentation when associated with ectopic production of other hormones or peptides (eg, adrenocorticotropic hormone [ACTH], vasoactive intestinal polypeptide, prostaglandins, kallikreins, serotonin).

The best test is measurement of serum calcitonin, which is greatly elevated. A challenge with calcium (15 mg/kg IV over 4 hours) provokes excessive secretion of calcitonin.

Radiography or ultrasonography may show a dense, homogenous, conglomerate calcification within the primary tumor.

All patients with medullary thyroid carcinoma should have genetic testing for mutations of the RET proto-oncogene; relatives of those with mutations should have genetic testing and measurement of basal and stimulated calcitonin levels. In hereditary forms, germline mutations of the RET proto-oncogene, which occur in 98% of cases, correlate with phenotype; that is, specific mutations in certain codons of the RET gene are associated with distinct syndromes or with more aggressive disease.

Total thyroidectomy and bilateral central compartment lymph node dissection are indicated even if bilateral involvement is not obvious (1). If hyperparathyroidism is present, removal of hyperplastic or adenomatous parathyroids is required.

Pheochromocytoma, if present, is usually bilateral. Pheochromocytomas should be ruled out by measuring plasma metanephrines and, if identified, removed before thyroidectomy because of the danger of provoking a hypertensive crisis during the operation.

Long-term survival is common in patients with sporadic medullary carcinoma and MEN 2A; more than two thirds of affected patients are alive at 10 years. Medullary carcinoma as part of MEN 2B has a worse prognosis.

Relatives with an elevated calcitonin level without a palpable thyroid abnormality should undergo thyroidectomy because there is a greater chance of cure at this stage. Some experts recommend surgery in relatives who have normal basal and stimulated serum calcitonin levels and a high-risk RET proto-oncogene codon mutation.

Thyroid tumors develop in people whose thyroid is exposed to large amounts of environmental radiation, as occurs as a result of exposure to nuclear weapons, nuclear reactor accidents, or incidental thyroid irradiation due to external beam radiation therapy for adjacent malignancies. Tumors may be detected 10 years after exposure, but risk remains increased for 30 to 40 years. Such tumors are usually benign; however, approximately 10% are papillary thyroid carcinoma. These thyroid cancers are frequently multicentric or diffuse, and the most common genetic mutations that occur in radiation-induced tumors are gene fusions.

Patients who had thyroid irradiation should undergo yearly thyroid palpation and selectively ultrasonography if a nodule if found on physical examination. A thyroid scan does not always reflect areas of involvement.

If ultrasonography reveals a nodule, fine-needle aspiration biopsy should be done.

Surgery is required if fine-needle aspiration biopsy suggests cancer. Near-total or total thyroidectomy is the treatment of choice with or without radioiodine ablation, depending on risk of recurrence (1).