Sonographic and cytologic differences of NIFTP from infiltrative or invasive encapsulated follicular variant of papillary thyroid carcinoma

We reclassified 179 cases of the follicular variant (FV) of papillary thyroid carcinoma (PTC) into 72 (40.2%) noninvasive follicular thyroid neoplasm with papillary‐like nuclear features (NIFTP), 37 (20.7%) encapsulated FVPTC with invasion (EFVPTC), and 70 (39.1%) infiltrative FVPTC (IFVPTC) without a capsule. In the NIFTP group, 5.6% cytology were hypercellular and the remainder low to moderate cellularity. PTC nuclei were absent in 18%, focally present in 37.5%, and diffusely present in 44.4%. In the EFVPTC group, 8.1% cytology were hypercellular and the reminder low to moderate cellularity. PTC nuclei were absent in 24.3%, focally present in 29.7% and diffusely present in 45.9%. In IFVPTC group, 24.3% cytology were hypercellular and the reminder low to moderate cellularity. PTC nuclei were diffusely present in 88.6% and focally present in 11.4%. The ultrasound findings for NIFTP and minimally invasive EFVPTC typically demonstrated a circumscribed oval/round nodule with a hypoechoic rim, and the Doppler was mostly hypervascular. The ultrasound findings for overtly invasive EFVPTC typically showed a round/oval nodule with irregular margins and the Doppler was mostly hypervascular. The ultrasound findings for IFVPTC group showed at least one of the malignant gray‐scale features: markedly hypoechoic, taller‐than‐wide, microcalcifications or blurred margins. The Doppler in this group was mostly avascular. An algorithm is proposed to triage thyroid FNA based on different scenarios of the sampled cells, interpreted in the context of ultrasound features with histology outcomes. Five composite ultrasound‐cytology‐histology figures illustrate NIFTP, IFVPTC, and IFVPTC with intratumoral fibrosis, microfollicular EFVPTC and normofollicular EFVPTC. Diagn. Cytopathol. 2017;45:533–541. © 2017 Wiley Periodicals, Inc.