Comparison of Ultrasound, Pathologic and Prognostic Characteristics of the Follicular Variant of Papillary Thyroid Cancer According to Fine-Needle Aspiration Cytology

Ultrasound findings of noninvasive follicular thyroid neoplasm with papillary-like nuclear features compared with those of follicular variant of papillary carcinoma and encapsulated papillary carcinoma: a single-institution study in Japan

Noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) is a new entity adopted by the newest World Health Organization classification. It is differentiated from follicular variant papillary thyroid carcinoma (FVPTC) and regarded as non-malignant disease. Here, we compared the ultrasound findings of NIFTP (n = 40) with those of FVPTC (n = 94) and encapsulated PTC (encap-PTC) (n = 157). The NIFTP group showed benign findings on ultrasound significantly more frequently than the FVPTC group based on the Japan Society of Ultrasonics in Medicine criteria: a regular shape (p < 0.001), well edge definition (p = 0.007), smooth character (p < 0.001), isoechoic and homogeneous internal echoes (p < 0.001), lack of punctate microcalcification (p = 0.027), and a regular marginal hypoechoic zone (p < 0.001). Compared to encap-PTC, NIFTP has a significantly higher incidence of benign findings: isoechoic and homogeneous internal echoes (p < 0.001), lack of punctate microcalcification (p < 0.001), and a regular marginal hypoechoic zone (p = 0.004). Based on the ultrasound classification (USC) system at Kuma Hospital, no cases were classified as malignant (USC ≥3.5), but 55.4% of the FVPTCs and 53.5% of the encap-PTCs were diagnosed as malignancy. However, on cytology, the incidence of NIFTP classified as Bethesda-V or -VI (PTC) was very high at 86.9%. All patients underwent surgical treatment, but none of the NIFTP patients showed postoperative recurrence. Although avoiding surgery might be difficult because of the high incidence of malignant cytology, overtreatment (including extensive surgery) for NIFTP can be avoided by paying close attention to the lack of malignant findings on ultrasound.