The group of aspirates diagnosed as "suspicious for a follicular neoplasm" includes both follicular adenomas and FC. Aspirates are cellular and are characterized by follicular cells arranged in any of three patterns: microfollicles, trabeculae, or crowded three-dimensional groups (Figures 6.14, 6.15,
6.16). Aspirates with a combination of these patterns can also be seen. This approach to diagnosing follicular lesions works because FCs are virtually never predominantly composed of normal-sized follicles or macrofollicles. A key point in evaluating the follicular architecture of the groups is to always focus on the predominant pattern. In cases diagnosed as a fol-licular neoplasm, there is a predominant nonmacrofollicular architectural pattern that will dictate the FNA diagnosis.
Aspirates of follicular neoplasms are hypercellular and contain scant background watery colloid, reflecting the paucity of macrofollicles. When colloid is present, it is most often as small clumps or droplets of dense colloid. Confusion sometimes exists over the definition of a microfollicle. Microfollic-les are easily recognized in aspirates because they maintain
Figure 6.16. Follicular neoplasm. Note the prominent microfollicular pattern. (Smear, Diff-Quik.)
their architectural arrangement rather than fragmenting as macrofollicles do. Microscopically, microfollicles are small follicular groups of approximately 6 to 12 follicular cells in a ringlike or wreathlike arrangement, sometimes with a small droplet of central dense colloid (see Figures 6.5, 6.14, 6.16). Occasionally, follicular neoplasms can be cystic, which can create a diagnostic pitfall if the epithelial component of the specimen is inadequate due to sampling.
When a trabecular cytoarchitectural pattern predominates, it is characterized by crowded follicular cells forming ribbons or trabeculae (see Figure 6.15). Sometimes aspirates of fol-licular neoplasms contain crowded three-dimensional groups of overlapping follicular cells (Figure 6.15). These crowded, often irregularly shaped groups are of various sizes that can be distinguished from macrofollicles by their lack of associated colloid and absence of an orderly honeycomb pattern of cells. As with adenomatous nodules, variable degrees of nuclear atypia, including chromatin clumping, nuclear grooves, and irregular nuclear contours, may be seen, but this atypia is generally not predictive of malignancy.
Three Cytoarchitectural Patterns of Follicular Neoplasms
• Crowded, three-dimensional groups
Other Cytologic Features of Follicular Neoplasms
• Scant background colloid in small dense droplets
• Absent nuclear features of PTC
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