Immunohistochemical Distinction of Follicular Thyroid Adenomas and Follicular Carcinomas

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Abstract
The American Cancer Society estimates that there will be 33 550 new diagnoses of thyroid carcinoma in 2007.1 The incidence of thyroid cancer has been reported to be increasing at a rate of 3% per year.2 These patients are identified from a much larger group who present with thyroid nodules, most of which are benign. Fine-needle aspiration (FNA) biopsy is the recommended initial test for the evaluation of solitary thyroid nodules because it is safe, inexpensive, accurate, and office based.3 Although FNA biopsy excels at identifying papillary thyroid carcinoma, a major deficiency of this diagnostic procedure is that follicular thyroid carcinoma (FTC), follicular variant papillary thyroid carcinoma (FVTC), and follicular thyroid adenoma (FA) cannot be differentiated cytopathologically.3 The diagnosis of follicular carcinoma is dependent on the presence of capsular or vascular invasion on formal pathologic evaluation. In roughly 20% of all FNA biopsy specimens, the diagnosis is follicular neoplasm or follicular lesion and is regarded as indeterminate or suspicious.4 Of these indeterminate neoplasms, approximately 80% are benign and 20% are malignant.5,6 As a result, patients with this diagnosis are typically taken to the operating room for a thyroid lobectomy. If the final pathologic reading is carcinoma, most patients return to the operating room for completion thyroidectomy in anticipation of radioactive iodine ablation. The accurate, preoperative diagnosis of follicular thyroid lesions represents a clear diagnostic void that results in many unnecessary thyroid surgeries.