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.
To determine what features are present in groups that are consistently classified as microfollicles.
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.
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).
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.