Differences between Follicular Variant and Conventional Papillary Thyroid Carcinoma According to Fine-Needle Aspiration Cytology

Objectives : This paper was aimed to study the differences between follicular variant papillary thyroid carcinoma (FVPTC) and conventional papillary thyroid carcinoma (PTC) according to fine-needle aspiration cytology and to propose how to distinguish them cytologically. Methods : We re-evaluated cytological smears of FVPTCs and conventional PTCs that were histologically confirmed after a surgical procedure. Results : Nuclear characteristics, including intranuclear cytoplasmic inclusions, nuclear grooves, and powdery chromatin, are important signs of papillary carcinoma, but they are not useful for differentiation of FVPTC from conventional PTC. Aside from nuclear findings, papillary cell clusters, monolayered cell sheets, ropy colloids, and multinucleate giant cells are characteristic features of conventional PTC. On the other hand, microfollicular cell clusters are a characteristic feature of both invasive and noninvasive FVPTC. In case that nuclear findings are likely to be papillary carcinoma, the data on cell clusters and on the smear background need to be considered. If the following parameters are scored - papillary cell clusters, predominance of monolayered cell sheets, predominance of microfollicular cell clusters, ropy colloids, and multinucleate giant cells - then FVPTC can be distinguished from conventional PTC. Cytological characteristics of noninvasive FVPTC, recently renamed NIFTP (noninvasive follicular thyroid neoplasm with papillary-like nuclear features), are nearly identical to those of invasive FVPTC, except for nuclear inclusions. Although nuclear inclusions are more frequently seen in invasive FVPTC, it is difficult to distinguish noninvasive FVPTC from invasive FVPTC by fine-needle aspiration cytology. Conclusions : For discrimination of FVPTC from conventional PTC, papillary cell clusters, predominance of monolayered cell sheets, predominance of microfollicular cell clusters, ropy colloids, and multinucleate giant cells are effective characteristics. If these parameters are scored, then FVPTC as well as NIFTP can be clearly distinguished from conventional PTC.