In thyroid fine needle aspirates, cytologists should be careful about classifying all small groups of follicular cells as microfollicular, and should restrict this term to groups with reproducible features, such as those containing a cluster of fewer than 15 cells, US researchers advise.
The team makes these recommendations based on results of an assessment of the level of agreement among 12 members of the College of American Pathologists and Cytology Committee about classification of groups of cells as microfollicles.
Andrew Renshaw, from the Baptist Hospital of Miami in Florida, and co-workers, showed the committee members 45 small groups of follicular cells, each composed of fewer than 50 cells from fine-needle aspirates of the thyroid.
Of these, the pathologists consistently classified 20 groups as microfollicles, seven groups as macrofollicles, and 18 groups as "indeterminate."
Cases that were repeatedly classified as microfollicles were made up of fewer than 15 cells (95%), were arranged in a circle with a lumen that was at least two-thirds complete (65%), and were flat (90%).
Conversely, cases that were classified as macrofollicular had between eight and 35 cells, were arranged in sheets of at least 15 cells (71%), or rows of eight cells (29%), and were flat (100%).
Cell groups that were defined as being of indeterminate status were characteristically organized as either a three-dimensional group (28%), in flat groups of fewer than 10 cells (61%), or presented as single cells (6%).
"Not all small groups of follicular cells are consistently classified as microfollicles, and some are more often classified as macrofollicles," Renshaw et al write in the Archives of Pathology and Laboratory Medicine.
"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," they suggest.