The PDQ editorial boards use a ranking system of levels of evidence to help the reader judge the strength of evidence linked to the reported results of a therapeutic strategy. For any given therapy, results of prevention and treatment studies can be ranked on each of the following two scales:
Strength of the study design.
Strength of the endpoints.
Together, the two rankings provide a measure of the overall level of evidence. Screening studies are ranked on strength of study design alone...
Carcinoma of the thyroid gland is an uncommon cancer but is the most common malignancy of the endocrine system. Differentiated tumors (papillary or follicular) are highly treatable and usually curable. Poorly differentiated tumors (medullary or anaplastic) are much less common, are aggressive, metastasize early, and have a much poorer prognosis. Thyroid cancer affects women more often than men and usually occurs in people between the ages of 25 and 65 years. The incidence of this malignancy has been increasing over the last decade. Thyroid cancer commonly presents as a cold nodule. The overall incidence of cancer in a cold nodule is 12% to 15%, but it is higher in people younger than 40 years and in people with calcifications present on preoperative ultrasonography.[3,4]
Patients with a history of radiation administered in infancy and childhood for benign conditions of the head and neck, such as enlarged thymus, acne, or tonsillar or adenoidal enlargement, have an increased risk of cancer as well as other abnormalities of the thyroid gland. In this group of patients, malignancies of the thyroid gland first appear beginning as early as 5 years following radiation and may appear 20 or more years later. Radiation exposure as a consequence of nuclear fallout has also been associated with a high risk of thyroid cancer, especially in children.[6,7,8] Other risk factors for the development of thyroid cancer include the following:
A history of goiter.
Family history of thyroid disease.
The prognosis for differentiated carcinoma is better for patients younger than 40 years without extracapsular extension or vascular invasion.[10,11,12,13,14] Age appears to be the single most important prognostic factor. The prognostic significance of lymph node status is controversial. One retrospective surgical series of 931 previously untreated patients with differentiated thyroidcancer found that female gender, multifocality, and regional node involvement are favorable prognostic factors. Adverse factors included age older than 45 years, follicular histology, primary tumor larger than 4 cm (T2–T3), extrathyroid extension (T4), and distant metastases.[15,16] Other studies, however, have shown that regional lymph node involvement had no effect [17,18] or even an adverse effect on survival.[13,14,19] Use of sentinel lymph node biopsy may aid in identifying patients with occult metastases who might benefit from central neck dissection.