Abstract
Over the past 3 decades, fine needle aspiration (FNA) has developed as the most accurate and cost-effective initial method for guiding the clinical management of patients with thyroid nodules. Thyroid FNA specimens containing follicular-patterned lesions are the most commonly encountered and include various forms of benign thyroid nodules, follicular carcinomas, and the follicular variant of papillary thyroid carcinoma. Based primarily upon the cytoarchitectural pattern, FNA is used as a screening test for follicular-patterned lesions to identify the majority of patients with benign nodules who can be managed without surgical intervention. The terminology and reporting of thyroid FNA results have been problematic due to significant variation between laboratories, but the recent multidisciplinary NCI Thyroid FNA State of the Science Conference has provided a seven-tiered diagnostic solution. A key element of this approach is the category "atypical cells of undetermined significance" (ACUS) which is used for those aspirates which cannot be easily classified as benign, suspicious, or malignant. Lesions in this category represent approximately 3-6% of thyroid FNAs and have a risk of malignancy intermediate between the "benign" category and the "suspicious for a follicular neoplasm" category. The recommended follow-up for an ACUS diagnosis is clinical correlation and in most cases, repeat FNA sampling.