Encapsulated follicular thyroid tumors of uncertain malignant potential in the new international histological classification

The new fourth edition of the WHO classification of tumors of endocrine organs published by the International agency for research of cancer (IARC) in 2017 as the previous third edition of 2004 pays substantial attention to the thyroid gland tumors. Experts revised criteria of malignancy in category of encapsulated non-invasive follicular thyroid neopasm with papillary-like nuclear features. The previous edition classified these tumors as malignant follicular variant of papillary thyroid carcinoma (PTC). The new edition considers their behavior status as borderline or tumors of uncertain malignant potential. An estimation of malignancy of follicular thyroid tumors is impossible in preoperative cytological smears due to unknown cytological markers of malignancy and in 10% tumors after surgery due to equivocal histological features of capsular/vascular invasion and/or doubtful PTC-like nuclear features. Criteria of non-invasive follicular thyroid neoplasm with papillary-like nuclear features as well as criteria of two classification categories of follicular and well differentiated tumors of uncertain malignant potential (UMP) are presented in this paper. A special attention is paid to diagnostic significance of invasive growth as a basic feature of malignancy. In connection with the revised criteria of international classification we discuss an importance of adequate histological examination and influence of new borderline categories on surgical treatment of encapsulated follicular thyroid tumors in the paper.