Head and Neck Pathol (2009) 3:82–85 85 Fig. 4 This thyroid FNA demonstrates an indeterminate architectural pattern and is placed into the category ‘‘Atypical cells of undeter- mined signiﬁcance (ACUS).’’ (Papanicolaou stain) Fig. 5 This thyroid FNA contains occasional follicular cells with enlarged, pale, grooved nuclei in an otherwise benign aspirate, and is therefore diagnosed as ‘‘Atypical cells of undetermined signiﬁcance (ACUS).’’ (Papanicoloau stain) FNA. The latter will often result in a more deﬁnitive ‘‘benign,’’ ‘‘suspicious,’’ or ‘‘malignant’’ designation; how- ever, 20–25% of cases will result in a second ACUS diagnosis [13, 18]. The risk of malignancy for an ACUS diagnosis is difﬁcult to assess because only a subset of ACUS nodules will be surgically excised. However, it is estimated that the overall risk of malignancy will be approximately 5–10% which is intermediate between the ‘‘benign’’ and ‘‘suspicious for a follicular neoplasm’’ categories [18]. Conclusions Follicular-patterned thyroid lesions are a common and sometimes challenging area of cytopathology. FNA has developed as an effective means for evaluating these lesions particularly when combined with the recently pro- posed Bethesda System for reporting of thyroid FNAs. An important advance in this regard is the use of diagnostic categories with an associated risk of malignancy upon which patient management decisions can be based. References 1. Stoffer RP, Welch JW, Hellwig CA, Chesky VE, McCusker EN. Nodular goiter: incidence, morphology before and after iodine prophylaxis, and clinical diagnosis. Arch Intern Med. 1960;106: 10–4. 2. Klonoff DC, Greenspan FS. The thyroid nodule. Adv Intern Med. 1982;27:101–26. 3. Vander JB, Gaston EA, Dawber TR. The signiﬁcance of nontoxic thyroid nodules. Final report of a 15-year study of the incidence of thyroid malignancy. Ann Intern Med. 1968;69:537–40. 4. Rojeski MT, Gharib H. Nodular thyroid disease. Evaluation and management. N Engl J Med. 1985;313:428–36. 5. DeLellis RA, Lloyd RV, Heitz PU, Eng C. World health orga- nization classiﬁcation of tumors: tumours of endocrine organs. Lyon: IARC Press; 2004. 6. The American Thyroid Association Guidelines Taskforce. Man- agement guidelines for patients with thyroid nodules and differentiated thyroid cancer. Thyroid. 2006;16:1–33. 7. Yang J, Schnadig V, Logrono R, Wasserman PG. Fine-needle aspiration of thyroid nodules: a study of 4703 patients with his- tologic and clinical correlations. Cancer Cytopathol. 2007;111: 306–15. 8. Clark DP, Faquin WC. Thyroid cytopathology. New York: Springer; 2005. 9. Gardner HA, Ducatman BS, Wang HH. Predictive value of ﬁne needle aspiration of the thyroid in the classiﬁcation of follicular lesions. Cancer. 1993;71:2598–603. 10. Cibas ES, Ducatman BS. Cytology: diagnostic principles and clinical correlates. New York: Saunders; 2009. 11. Gharib H, Goellner JR. Fine-needle aspiration biopsy of the thyroid: an appraisal [see comments]. Ann Intern Med. 1993;118: 282–9. 12. Baloch ZW, Fleisher S, et al. Diagnosis of ‘‘follicular neoplasm’’: a gray zone in thyroid ﬁne-needle aspiration cytology. Diagn Cytopathol. 2002;26:41–4. 13. Yassa L, Cibas ES, Benson CB, et al. Long-term assessment of multidisciplinary approach to thyroid nodule diagnostic evalua- tion. Cancer Cytopathol. 2007;111:508–16. 14. Deveci MS, Deveci G, LiVolsi VA, Baloch ZW. Fine-needle aspiration of follicular lesions of the thyroid. Diagnosis and fol- low-up. Cytojournal. 2006;3:1–9. 15. Nikiforov YE. Thyroid carcinoma: molecular pathways and therapeutic targets. Mod Pathol. 2008;21:S37–43. 16. Cibas ES, Ali SZ. The Bethesda system for reporting thyroid cytopathology. New York: Springer; 2009. 17. Baloch ZW, LiVolsi VA. Diagnostic terminology and morpho- logic criteria for cytologic diagnosis of thyroid lesions: a synopsis of the national cancer institute thyroid ﬁne-needle aspiration state of the science conference. Diagn Cytopathol. 2008;36:425–37. 18. Krane JF, Nayar R, Renshaw AA. Atypical cells of undetermined signiﬁcance. In: Ali SZ, Cibas ES, editors. The Bethesda system for reporting thyroid cytopathology. New York: Springer; 2009.