Adenomas are the most common benign tumors of the thyroid gland. They are of follicular cell origin, encapsulated with varying histology. They have been subdivided according to their histology, but no additional information is gained by this practice.

Thyroid adenomas are probably common because of the inability of pathologists to separate consistently cellular adenomatous nodules in nodular goiters from adenomas. The majority of "adenomas" are most likely adenomatous nodules.

The atypical adenoma is a troublesome lesion.2 It is a lesion characterized by solid architecture with disorderly arrangements of follicular components and is troublesome when there is moderate or marked cellular atypia, typical of "angioinvasive adenomas" or microangioinvasive well-differentiated follicular carcinomas. Capsular invasion, invasion of capsular blood vessels, or both identify the malignant lesions. Otherwise, the tumors can be histologically identical. The problem is compounded by the inability of any preoperative procedure to demonstrate consistently evidence of invasive growth.

The cytologic smears of these lesions can be problems. Cytologic reports are descriptive and often conclude by suggesting the presence of a follicular neoplasm or follicular neoplasia, the connotation being that a malignant tumor is not totally excluded. It is well recognized that well-differentiated angioinvasive carcinomas cannot usually be distinguished from atypical adenomas and cellular adenomatous nodules by fine-needle aspiration. Such tumors should be surgically removed and evaluated histologically for evidence of invasive growth, and treatment is dictated by the final histologic diagnoses.

The extent of thyroidectomies may be determined by protocols developed for the treatment of well-differentiated carcinomas of the thyroid gland at different institutions. The surgical procedure may vary from lobectomy to subtotal or total thyroidectomy. Frozen sections of the offending nodules may be requested when thyroidectomies are preceded by lobectomies. The nodules are sent for frozen sections. However, frozen sections are not useful in follicular and also Hurthle cell neoplasms because capsular or vascular invasion cannot be identified consistently with the small number of sections taken for histologic examination.3 An opposing opinion states that frozen sections are cost effective. However, a different procedure is used for frozen sections at that medical center.4

The histologic criteria to establish the diagnosis of an adenoma are not well defined. Adenomas are solitary nodules that are well encapsulated and histologically distinct from adjacent thyroid parenchyma. Goitrous or adenomatous nodules may satisfy these criteria, and when atypical histologic features are present, they are difficult to separate from minimally invasive follicular carcinomas.

The hyalinizing trabecular adenoma may present problems.5 These tumors are either single dominant nodules or one nodule in a nodular goiter. They are usually small (<2 cm) and are encapsulated or circumscribed with pseudofollicles, trabecular or alveolar arrangements of cells, or both. Hyalinization of stroma can be seen, which may be confused with amyloid. Immunohistochemical stains for thyroglobulin are positive and stains for calcitonin are negative. No cytoplasmic secretory granules are seen by electron microscopy. Erroneous diagnoses of medullary carcinoma and papillary carcinoma have been made by mistaking the hyalinized stroma for amyloid and the papillae for those of papillary carcinomas.

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