Fine-Needle Aspiration Cytology of Noninvasive Follicular Variant of Papillary Thyroid Carcinoma Is Cytomorphologically Distinct From the Invasive Counterpart.

OBJECTIVES: To review a series of noninvasive encapsulated follicular variant of papillary thyroid carcinomas (FVPTCs) in an attempt to further define the role of cytopathology in the diagnosis of noninvasive follicular thyroid neoplasm with papillary-like nuclear features and invasive FVPTC.

METHODS: Surgical pathology cases diagnosed as FVPTC with correlating thyroid fine-needle aspiration (FNA) were identified and divided into two FVPTC groups: noninvasive and invasive. Cytologic diagnoses were compared between them.

RESULTS: We identified 23 cases that met the criteria for noninvasive FVPTC and 27 cases that were typical infiltrative FVPTC (n = 16) or encapsulated FVPTC with either capsular and/or lymphovascular invasion (n = 11). Of the noninvasive FVPTC cases, there were four benign lesions, 14 follicular lesions of undetermined significance (FLUS), four follicular neoplasms (FNs), one suspicious case, and no papillary thyroid carcinomas (PTCs). In the invasive FVPTC group, there were no benign cases, four FLUS, three FNs, 12 suspicious cases, and eight PTCs.

CONCLUSIONS: There is a distinction in the cytologic diagnosis between noninvasive and invasive FVPTC. The invasive subtype was diagnosed by FNA as suspicious for PTC or PTC in nearly 75% of cases, while only one (4%) case for the noninvasive subtype was diagnosed as suspicious for PTC (P < .05).