Head and Neck Pathol (2009) 3:82–85 DOI 10.1007/s12105-009-0104-7 P R O C E E D I N G S O F T H E 2 0 0 9 N O R T H A M E R I C A N S O C I E T Y O F H E A D A N D N E C K P A T H O L O G Y C O M P A N I O N M E E T I N G ( B O S T O N , M A ) Diagnosis and Reporting of Follicular-Patterned Thyroid Lesions by Fine Needle Aspiration William C. Faquin Received: 27 December 2008 / Accepted: 21 January 2009 / Published online: 22 February 2009 Humana 2009 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 manage- ment 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 iden- tify the majority of patients with benign nodules who can be managed without surgical intervention. The terminology and reporting of thyroid FNA results have been problem- atic 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. Keywords Thyroid FNA Follicular Bethesda W. C. Faquin (&) Department of Pathology, WRN 219, Massachusetts General Hospital and Harvard Medical School, 55 Fruit Street, Boston 02114, MA, USA e-mail: WFaquin@Partners.org Introduction In many pathology practices, thyroid FNAs comprise a signiﬁcant proportion of non-gynecologic cytology speci- mens reﬂecting the fact that nodules of the thyroid gland are very common. It is estimated that as many as 4–7% of the adult population has a palpable thyroid enlargement, and up to 10 times this number of individuals have subclinical nodules [1–4]. The majority of these thyroid nodules are benign with only a fraction representing malignant disease which accounts for approximately 1.1% of all cancers annually [5]. The large number of benign thyroid nodules relative to the small number of malignant ones creates a clinical dilemma—how to manage patients with a detect- able thyroid enlargement that statistically is more likely to be benign? Over the past three decades, FNA has developed as the most accurate and cost-effective initial method for evaluating thyroid nodules [6]. In experienced hands, the diagnostic accuracy of thyroid FNA for technically satis- factory specimens ranges from 80 to [95% [7]. FNA can be used as either a screening test for follicular carcinoma, or as a diagnostic test for other thyroid carci- nomas including papillary carcinoma, medullary carcinoma, undifferentiated carcinoma, and lymphoma. The follicular-patterned lesion is the most commonly encountered type of thyroid FNA specimen, and it will be the focus of this discussion. Follicular-patterned lesions include benign thyroid nodules (adenomatous nodules, hyperplastic nodules, follicular adenomas), follicular car- cinomas, and the follicular variant of papillary thyroid carcinoma. The application of thyroid FNA in the evalua- tion of the various follicular-patterned lesions, including key diagnostic pitfalls, and recent advances by the NCI Thyroid FNA State of the Science Conference in the reporting of these lesions will be reviewed.