Follicular carcinoma.

The workshop participants agreed on the following points regarding follicular carcinoma: Follicular carcinoma should be divided into two groups according to its degree of invasion: encapsulated and widely invasive. Patients in the first group only occasionally develop distant metastases, whereas in the second group the prognosis is much poorer. In encapsulated follicular tumors, high cellularity and nuclear atypia should not be used as criteria of malignancy; this diagnosis should be based on the presence of vascular or capsular invasion. Only tumor thrombi occurring in vessels in or outside the capsule should be regarded as indicative of vascular invasion. Capsular invasion should be diagnosed only if penetration of the whole capsule is seen. We agree that some tumor islands within the capsule may represent true tumor invasion, but we believe that some others may be due to capsular infoldings or tangential sectioning. Whenever tumor tissue within the capsule is seen, additional tissue blocks from the capsular area should be processed in a search for capsular penetration or vascular invasion. The degree of differentiation in follicular carcinoma does not correlate to the course of disease as clearly as the degree of invasion, although the so-called insular or poorly differentiated, subtype seems to have a poorer prognosis. Thyroid carcinomas composed of large eosinophilic cells (Hürthle cell carcinomas) usually show follicular differentiation and are therefore included in the category of follicular carcinoma. The same diagnostic criteria of malignancy that apply for other follicular tumors should be used when evaluating these tumors. Although follicular carcinomas often show foci of clear cells, tumors composed solely of clear cells are rare. Most pure clear-cell tumors in the thyroid represent metastatic tumors, usually from the kidney. In the distinction of follicular carcinoma from papillary carcinoma, all differential diagnostic criteria should be used. However, in some cases, the diagnosis can be based on one criterion only, mainly the presence of widespread ground-glass nuclei or abundant neoplastic papillae in papillary carcinoma. The presence of occasional papillae in encapsulated tumors composed of large eosinophilic cells is not sufficient for the diagnosis of papillary carcinoma if the other microscopic features of this tumor are lacking.