Ultrastructural features of solid/trabecular areas in differentiated thyroid carcinoma

The presence of areas exhibiting a solid/trabecular pattern of growth withi n an otherwise differentiated thyroid carcinoma represents a source of cont roversy as regards its proper classification and biologic and prognostic si gnificance. The aim of the current study was to investigate the ultrastruct ural features of solid/trabecular areas in differentiated thyroid carcinoma and to compare those features with the submicroscopic profile of different iated, poorly differentiated (insular), and undifferentiated (anaplastic) v ariants of thyroid cancer. The study series included differentiated carcino ma with solid/trabecular areas (3 cases), conventional papillary carcinoma (4 cases), follicular variant of papillary carcinoma (4 cases), poorly diff erentiated (insular) carcinoma (3 cases), and undifferentiated (anaplastic) carcinoma (3 cases). It was found that the solid/trabecular areas in diffe rentiated carcinoma and poorly differentiated (insular) carcinoma share sim ilar ultrastructural features and overall retain, even if attenuated, many of the submicroscopic attributes of differentiated carcinomas. In particula r, nests of neoplastic cells were observed showing a highly developed cytos ecretory apparatus and the presence of numerous abortive/rudimentary follic les, and intercellular and intracellular (intracytoplasmic) lumina/canalicu li of variable morphology. The study supports the hypothesis that the solid /trabecular areas do not merely represent an architectural pattern but rath er should be regarded as the expression of a process of reduced differentia tion similar to that of poorly differentiated (insular) carcinoma.