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Atypia of Undetermined Significance/Follicular Lesion of Undetermined Significance

Chang, Sue MD; Krane, Jeffrey F. MD, PhD

doi: 10.1097/PCR.0000000000000086
Case Reviews

Atypia of undetermined significance/follicular lesion of undetermined significance (AUS/FLUS) is the most controversial category in The Bethesda System for Reporting Thyroid Cytopathology. The intent of this category is to recognize a heterogeneous group of cytologic findings that exceed what is acceptable in a benign aspirate while being insufficient to warrant any of the diagnoses associated with a higher risk of malignancy. An AUS/FLUS diagnosis may be prompted by quantitatively or qualitatively restricted features that raise concern for a follicular neoplasm (including a Hürthle cell neoplasm), papillary carcinoma, or other less common malignancies. These aspirates are often limited by factors such as sparse cellularity or preparation artifact. The AUS/FLUS category is anticipated to have a 5% to 15% risk of malignancy, but in practice has been shown to often pose a greater risk, particularly when the atypia is due to cytologic features that raise concern for papillary carcinoma. Recommended initial management for an AUS/FLUS diagnosis is typically repeat fine-needle aspiration with molecular testing presenting a potentially viable alternative for determining whether surgery is appropriate. The AUS/FLUS rate is recommended to not exceed 7%, but this target has proved difficult to achieve for many practitioners. A concerted effort to use alternative diagnostic categories wherever possible may help minimize overuse of AUS/FLUS.

From the Department of Pathology, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA

Reprints: Jeffrey F. Krane, MD, PhD, Department of Pathology, Brigham and Women’s Hospital, Boston, MA 02115. E-mail:

The authors have no funding or conflicts to declare.

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