The impact of noninvasive follicular thyroid neoplasm with papillary-like nuclear features on the rate of malignancy for atypia of undetermined significance subcategories.

BACKGROUND: The recent revision in terminology, with noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) replacing noninvasive follicular variant of papillary thyroid carcinoma, has reclassified the clinically indolent tumor as nonmalignant. The objective of this study was to evaluate the impact of this change on the rate of malignancy (ROM) for subcategories of an atypia of undetermined significance (AUS) diagnosis on fine-needle aspiration (FNA) cytology.

METHODS: Consecutive thyroid FNAs interpreted as AUS over a period of 4 years were retrospectively analyzed. The ROM for AUS subcategories, including atypia of undetermined significance with nuclear atypia (AUS-N), atypia of undetermined significance with a microfollicular pattern (AUS-F), atypia of undetermined significance with nuclear atypia and a microfollicular pattern (AUS-N/F), atypia of undetermined significance with Hürthle cells (AUS-H), and atypia of undetermined significance, not otherwise specified (AUS-NOS), were analyzed.

RESULTS: Of the 426 nodules interpreted as AUS, 244 were surgically excised. The incidence of NIFTP in each subcategory was as follows: 18% for AUS-N, 18% for AUS-F, 9% for AUS-N/F, 3% for AUS-H, and 0% for AUS-NOS. After the reclassification of NIFTP as nonmalignant, the ROM based on histologic follow-up significantly decreased from 43% to 26% for AUS-N (P < .001) and from 29% to 10% for AUS-F (P = .008). The ROM for AUS-N remained significantly higher than the ROM for AUS-F (P = .030).

CONCLUSIONS: A subset of resected AUS nodules can be reclassified as NIFTP, and that significantly decreases the ROM, especially for AUS-N and AUS-F. Nonetheless, AUS-N still harbors a substantially higher ROM than AUS-F. Cancer Cytopathol 2018;126:309-16. © 2018 American Cancer Society.