Insular (poorly differentiated) carcinoma of the thyroid: an ultrastructural and immunocytochemical study of two cases.

Two cases of pure insular (poorly differentiated) carcinoma of the thyroid are reported, with emphasis on their ultrastructure. Notwithstanding their solid/primitive histologic character, a high degree of structural organization and differentiation was present at the submicroscopic level, namely a highly developed cytosecretory apparatus and numerous abortive/rudimentary follicles. A prominent inclusion/dot-like pattern (often paranuclear) of thyroglobulin immunoreactivity was observed, ultrastructurally corresponding to the following features: a) abortive/rudimentary follicular lumina with abundant periluminal dense bodies, and with varying amounts of colloid and microvilli, both at the intercellular and intracellular level; b) large inclusions of fibrillar/lamellar or finely granular material within dilated cisternae of endoplasmic reticulum and possibly of lysosomal nature as well, likely reflecting altered production and/or storage of thyroglobulin substrate. One tumor had many cytophenotypical attributes ascribed to differentiated papillary carcinoma (PC), whereas the other tumor was more analogous to differentiated follicular carcinoma (FC). The present study supports the concept that the insular pattern is a common pathway of dedifferentiation for follicular cell neoplasia of both PC and FC types, yet with maintenance of most of their respective cytophenotypical features, and with evidence of a high level of differentiation/maturation at the submicroscopic level.