Sonographic differences between conventional and follicular variant papillary thyroid carcinoma

Abstract

Follicular variant papillary thyroid carcinoma (FVPTC) may pose a diagnostic challenge due to higher likelihood of lower risk cytology compared to conventional papillary thyroid carcinoma (CPTC). Recent guidelines have recommended the use of sonographic features to guide decisions to biopsy thyroid nodules. The purpose of this study was to evaluate the sonographic features of CPTC and FVPTC. This is a retrospective study design done in an Academic teaching hospital setting. Preoperative ultrasounds of 79 patients with conventional CPTC (48) and FVPTC (31) were reviewed by a radiologist blinded to histological diagnosis. Sonographic features of nodules were classified according to the British Thyroid Association (BTA) U-classification system as normal (U1), benign (U2), indeterminate (U3), suspicious (U4), and malignant (U5). Pathology slides of patients with FVPTC were reviewed by two pathologists and subclassified into encapsulated, well circumscribed/partly encapsulated, and infiltrative subtypes. FVPTC had a significantly lower incidence of any calcifications (p = 0.0005), microcalcifications (p = 0.002), and irregular or lobulated margins (p = 0.03) than CPTC. Differences in hypoechogenicity (p = 0.06), taller > wide shape (p = 0.17) and presence of halo (p = 0.07) were not significant. FVPTC was significantly less likely to be classified sonographically as malignant (U5) (p = 0.006) or suspicious/malignant (U4/5) (p = 0.009) than conventional PTC. Among FVPTC cases, infiltrative FVPTC were more likely to be sonographically classified as suspicious/malignant (U4/5) than non-infiltrative FVPTC. FVPTC nodules are less likely to show sonographic features of malignancy than conventional PTC. Reliance solely on sonographic features for thyroid nodule evaluation may not be sufficient to exclude FVPTC.

Keywords: Follicular variant; Infiltrative; Microcalcifications; Papillary; Sonography; Thyroid carcinoma.