– Papillary carcinomas are the most common type of thyroid cancer (60% to 70%). Follicular carcinomas account for 15% to 20% of diagnosed cases, anaplastic carcinomas for 10%, and medullary carcinomas for less than 5%.
Causes and Incidence
There is a strong link between radiation therapy to the neck region (a popular childhood treatment to shrink tonsils, adenoids, and thymus glands in the 1950s) and papillary cancer. Other suspected precursors of thyroid cancer include prolonged secretion of thyroid-stimulating hormone (TSH), iodine deficiencies, and chronic goiter. Familial predisposition (autosomal dominant trait) is strongly suspected in medullary cancer. About 10,000 cases of thyroid cancer are diagnosed in the United States each year. It can occur at any age, although anaplastic carcinoma is seen almost exclusively in the elderly. Thyroid cancer is two to three times more common in women than in men.
Papillary and follicular carcinomas begin in the epithelial cells of the thyroid, growing slowly and forming nodules in the gland. Papillary tumors are usually nonencapsulated, extend to adjacent tissue beyond the thyroid, and metastasize to local cervical lymph nodes. Distant metastasis is rare. Follicular tumors are encapsulated, invade local tissue and cervical nodes, and metastasize to distant sites (e.g., lungs, bone) through the bloodstream.
Anaplastic carcinomas arise from the epithelium of the thyroid and are characterized by rapid, painful invasive growth to the trachea and major blood vessels, with metastasis to the bones and liver. Medullary carcinoma arises from the parafollicular cells of the thyroid and causes excessive secretion of calcitonin, lowering serum calcium and phosphate levels. Amyloid and calcium deposits are common. The tumor grows rapidly and metastasizes through lymphatics to cervical and mediastinal nodes and to the liver, lungs, and bone, leaving dense calcifications in its wake.
The most common presenting sign is a palpable, symptomless lump in the neck.
The prognosis is excellent for papillary and follicular cancers if they are treated before distant metastasis occurs. Medullary and anaplastic cancers have a much higher death rate. Medullary tumors are treated successfully only if detected very early, before any tissue invasion is evident; anaplastic tumors are resistant to treatment and spread so rapidly they often cause death within 6 months of diagnosis. Complications include dysphagia, stridor, and tracheal obstruction.
X-ray examination, thyroid scans, ultrasound and computed tomography scans, and magnetic resonance imaging are used to visualize the size and extent of the tumor and calcifications. A calcitonin assay for elevated levels of calcitonin is a reliable indicator for medullary carcinoma. The definitive diagnosis is made through fine needle aspiration biopsy.
Thyroidectomy with or without lymph node dissection as primary treatment; modified radical neck resection for recurrence or metastasis.
Palliative treatment in widespread disease; thyroid hormone as replacement therapy and to suppress TSH production.
Radioactive iodine ablation as adjuvant to surgery or alone for palliation; instruction about lifelong use of thyroid replacement hormones.