Ultrastructural features of solid/trabecular areas in differentiated thyroid carcinoma.

The presence of areas exhibiting a solid/trabecular pattern of growth within an otherwise differentiated thyroid carcinoma represents a source of controversy as regards its proper classification and biologic and prognostic significance. The aim of the current study was to investigate the ultrastructural features of solid/trabecular areas in differentiated thyroid carcinoma and to compare those features with the submicroscopic profile of differentiated, poorly differentiated (insular), and undifferentiated (anaplastic) variants of thyroid cancer. The study series included differentiated carcinoma with solid/trabecular areas (3 cases), conventional papillary carcinoma (4 cases), follicular variant of papillary carcinoma (4 cases), poorly differentiated (insular) carcinoma (3 cases), and undifferentiated (anaplastic) carcinoma (3 cases). It was found that the solid/trabecular areas in differentiated carcinoma and poorly differentiated (insular) carcinoma share similar ultrastructural features and overall retain, even if attenuated, many of the submicroscopic attributes of differentiated carcinomas. In particular, nests of neoplastic cells were observed showing a highly developed cytosecretory apparatus and the presence of numerous abortive/rudimentary follicles, and intercellular and intracellular (intracytoplasmic) lumina/canaliculi of variable morphology. The study supports the hypothesis that the solid/trabecular areas do not merely represent an architectural pattern but rather should be regarded as the expression of a process of reduced differentiation similar to that of poorly differentiated (insular) carcinoma.