Atypia of Undetermined Significance/Follicular Lesion of Undetermined Significance

Abstract

Atypia of undetermined significance/follicular lesion of undetermined significance (AUS/FLUS) was introduced in 2008 as one of three “indeterminate” (not clearly benign or malignant) diagnostic reporting categories for thyroid fine needle aspirations. The two terms are alternatives and equally acceptable; it is not recommended that both be used by a laboratory to imply distinctly different interpretations. Each of the indeterminate categories has an elevated risk of malignancy (ROM) compared to a benign aspirate. The AUS/FLUS category is reserved for cases with atypia, cytologic and/or architectural in nature, that is insufficient for either of the other two indeterminate categories. Of the three indeterminate categories, AUS/FLUS has the lowest ROM, meriting its distinction from the other two. Follow-up studies since the introduction of the AUS/FLUS category indicate a ROM that is higher than predicted initially (~10–30% rather than ~5–15%). Furthermore, the risk differs according to the nature of the atypia prompting the AUS/FLUS interpretation. Specifically, AUS/FLUS with cytologic atypia raising concern for papillary carcinoma has a higher ROM than AUS/FLUS associated with architectural atypia alone or Hurthle cells. The introduction of noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) terminology in 2016 will decrease the overall ROM for AUS/FLUS. The clinical approach to a nodule with an initial AUS/FLUS interpretation is a repeat FNA or molecular testing, although patient preference and clinical risk factors may also impact management.