Interobserver agreement on microfollicles in thyroid fine-needle aspirates.

Abstract

CONTEXT: Although microfollicles are a well-known feature of follicular neoplasms in fine-needle aspirates of the thyroid, the level of agreement about classifying groups as microfollicles is not known. OBJECTIVE: To determine what features are present in groups that are consistently classified as microfollicles. DESIGN: We showed 45 small groups of follicular cells each composed of fewer than 50 cells from fine-needle aspirates of the thyroid to 12 members of the College of American Pathologists Cytopathology Committee, who then classified the groups into microfollicles, macrofollicles, and indeterminate groups. RESULTS: Twenty groups were consistently classified as microfollicles, 7 groups as macrofollicles, and 18 groups as indeterminate. Cases that were consistently classified as microfollicles were composed of fewer than 15 cells (19/20 cases), were arranged in a circle with a lumen (13/20 cases) that was at least two-thirds complete (12/20 cases), and were flat (18/20 cases). Cases that were classified as macrofollicular had between 8 and 35 cells, were arranged in sheets composed of at least 15 cells (5/7 cases) or rows of 8 cells (2/7 cases), and were flat (7/7 cases). In contrast, cases that were indeterminate were composed of either 3-dimensional groups (5/18 cases), flat groups of fewer than 10 cells (11/18 cases), or single cells (1/18 cases). CONCLUSION: Not all small groups of follicular cells are consistently classified as microfollicles, and some are more often classified as macrofollicles. The criteria described here for reproducible microfollicles (<15 cells, arranged in a circle that is at least two-thirds complete, and flat) may help improve the agreement in classification of microfollicles and lead to more consistent classification of thyroid fine-needle aspirates.