Medullary carcinoma of the thyroid with atypical patterns.

A case of a highly invasive thyroid carcinoma, which occurred in a 68-year-old woman, was studied by light and electron microscopy, and histochemical and biochemical analysis. Light microscopical, histochemical, and biochemical features were consistent with a diagnosis of a calcitonin-producing, amyloid-rich medullary carcinoma; electron microscopical patterns, mainly the presence of lumina, microvilli, and extensively dilated cytoplasmic E.R., were reminiscent of the ultrastructural features of the follicular carcinoma. Electron-dense bodies interpreted as "secretory granules" were very scarce. This case appears very similar to the case recently presented by Valenta et al. and interpreted as a microfollicular carcinoma. The atypical features of our case of MCT seem to indicate that this tumor should be included in a group of atypical MCT; these should be kept separate from the typical (differentiated) ones on morphological, functional, and prognostic grounds.