What distinguishes follicular adenoma from follicular carcinoma?

The distinguishing feature between follicular adenoma and follicular carcinoma is the presence of capsular invasion. Follicular adenomas are benign tumors characterized by a well-defined capsule that separates them from the surrounding thyroid tissue. In contrast, follicular carcinoma presents with evidence of capsular invasion, meaning that the tumor cells infiltrate the capsule and potentially invade surrounding tissues, indicating malignant behavior.

Capsular invasion is critically important in the diagnosis of these conditions because it demonstrates aggressiveness and the potential for metastasis, which are hallmarks of malignant tumors. While other factors such as follicular size, cellular atypia, and amyloid stroma may be associated with different thyroid pathologies, they do not play a definitive role in delineating between adenoma and carcinoma in the way that capsular invasion does. Therefore, the presence of capsular invasion is the crucial factor that differentiates follicular carcinoma from follicular adenoma.