Figure 3.

Local invasion by thyroid neoplasms. (A) Local invasion by an unencapsulated lesion is usually recognized as nests of tumor cells beyond the edge of the thyroid neoplasm, infiltrating into and around adjacent nontumorous follicles (arrows); this invasive behavior is not currently defined, as it does not represent ‘capsular invasion.’ Scale bar, 0.5 mm. (B) In some instances, tumor cells with marked cytologic atypia forming microfollicles (arrows) can be seen infiltrating around cytologically and architecturally benign components of a follicular neoplasm (*), suggesting that this represents malignancy arising in a follicular adenoma; does this invasive behavior qualify as ‘capsular invasion’? Scale bar, 0.25 mm.