Improving the diagnostic accuracy of thyroid follicular neoplasms: cytological features in fine-needle aspiration cytology.

Cytological diagnosis in follicular neoplasms of the thyroid has to surmount some difficulties. Capsular/vascular invasions or metastasis are the histological criteria for follicular carcinoma (FC), and, on fine-needle aspiration (FNA) samples, marked cytological atypias are only observed in moderately to poorly differentiated FC, while they may be completely lacking in well differentiated angio- or capsulo-invasive FC. To clarify the cytological features and to improve the accuracy and reliability of aspiration cytology, 892 follicular adenomas and 82 FCs were reviewed. A macrofollicular pattern or large sheet pattern of follicular cells with thin colloid in the background were found to be indicators of follicular adenoma. Crowding or irregular arrangement of follicular cells were found to indicate microfollicular lesions but could not discriminate between benign and malignant conditions. High nucleo-cytoplasmic ratio, nuclear atypia, and coarse granular or dense chromatin were more important criteria for malignancy than nuclear grooves or intranuclear cytoplasmic inclusions. The cytomorphologic features of the follicular neoplasms of the thyroid are described, and the difficulties encountered in the cytodiagnosis of follicular lesions are discussed at length.