Classification of thyroid follicular cell tumors: with special reference to borderline lesions [Review]

Abstract

Abstract. We propose a new classification of thyroid follicular cell tumors which is correlated with patient's prognosis. It is unique as to two new categories: borderline malignancy between benign and malignant, and moderately differentiated adenocarcinoma (MDA) as a differentiation classification to stratify tumor aggressiveness. As to diagnostic criteria, we recommend invasiveness (capsular and vascular invasion) to separate benign and malignant and it should not be based on presence or absence of papillary thyroid carcinoma (PTC) type nuclear features (PTC-N). Thus borderline malignancy in our new classification includes some of the formerly malignant tumors and they are 1) papillary microcarcinoma, 2) encapsulated conventional PTC (EncPTC), 3) encapsulated follicular variant PTC (EnFVPTC), 4) well differentiated tumor of uncertain malignant potential (WDT-UMP), 5) follicular tumors of uncertain malignant potential (FT-UMP), and 6) capsular invasion only follicular thyroid carcinoma (FTC). Review of the literature revealed that those thyroid tumors have consistently excellent outcome. Well differentiated follicular cell adenocarcinoma (WDA) in our classification includes common type PTC and low-risk follicular carcinoma (FTC). They are invasive (diffuse infiltrative) common type PTC and minimally invasive type FTC with less than 4 foci of angioinvasion. Moderately differentiated follicular cell adenocarcinoma (MDA) includes FTC with angioinvasion (more than 4), aggressive variants of PTC, such as tall cell, columnar cell, solid, loss of cellular polarity/cohesiveness (hobnail) variants and encapsulated carcinoma with high grade histology. Poorly differentiated carcinoma (PDC) includes PDC of WHO definition, insular carcinoma, tumors with minor anaplastic transformation and tumors with distant metastasis at presentation.

Introducing the reporting system for thyroid fine-needle aspiration cytology according to the new guidelines of the Japan Thyroid Association [Review]

“…The first group, follicular neoplasm, is composed of AN, FA, follicular tumor of uncertain malignant potential (FT-UMP), FTC, FV-PTC and PDC, which provide follicular pattern cytological samples without PTC-type nuclear features [53][54][55][56][57][58]. The risk of malignancy of follicular neoplasm was reported to be about 15-30% in surgically treated patients [1-25, 26-36, 48-52].…”

Cytological features of well-differentiated tumors of uncertain malignant potential: Indeterminate cytology and WDT-UMP

“…These results suggest that AG/FTA, WDT-UMP and PTC are continuous lesions, and WDT-UMP occupies between benign AG/FTA and malignant PTC cytologically, suggesting it is so-called borderline malignancy [24,25].…”

“…Probability values less than 0.05 were considered statistically significant. cal, immunohistochemical and molecular features with PTC [4,24]. WDT-UMP may be one of the major reasons why PTC can be found in the indeterminate, because both WDT-UMP and PTC have overlapping morphological characteristics and PTC and follicular adenoma are unfortunately continuous lesions cytologically.…”

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

“…Справедливости ради следует заметить, что предложение перенести часть инкап-сулированных опухолей, в том числе ранее класси-фицируемых как злокачественные (фолликулярный вариант папиллярного рака) без признаков инвазив-ного роста (без капсулярной и сосудистой инвазии), в категорию пограничных было высказано несколько ранее, в 2012 г. [9]. Основанием для пересмотра крите риев диагностики пограничных опухолей по-служили результаты анализа отдаленных результатов хирургического лечения неинвазивных опухолей, продемонстрировавших отсутствие рецидивирова-ния и летальности, связанной с прогрессированием опухоли ЩЖ.…”