Poorly differentiated (PD) thyroid carcinomas lie both morphologically and behaviorally between well-differentiated and undifferentiated (anaplastic) carcinomas. Following the original description of this entity, different diagnostic criteria have been employed, resulting in wide discrepancies and confusion among pathologists and clinicians worldwide. To compare lesions occurring in different geographic areas and the diagnostic criteria applied in those countries, we designed a study with a panel of internationally recognized thyroid pathologists to develop consensus diagnostic criteria for PD carcinomas. Eighty-three cases were collected from Europe, Japan, and the United States, and circulated among 12 thyroid pathologists. Diagnoses were made without any knowledge of the clinical parameters, which were subsequently used for survival analysis. A consensus meeting was then held in Turin, Italy, where an agreement was reached concerning the diagnostic criteria for PD carcinoma. These include (1) presence of a solid/trabecular/insular pattern of growth, (2) absence of the conventional nuclear features of papillary carcinoma, and (3) presence of at least one of the following features: convoluted nuclei; mitotic activity >or=3 x 10 HPF; and tumor necrosis. An algorithmic approach was devised for practical use in the diagnosis of this tumor.

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a-Smooth muscle (a-sm) actin, an isoform typical of smooth muscle cells (SMC) and present in high amounts in vascular SMC, was demonstrated in the cytoplasm ofpericytes of various rat and human organs by means of immunocytochemistry at the electron microscopic level. In SMC and pericytes, a-sm actin was localized in microfilament bundles, strength-I Supported by the Swiss National Science Foundation, Grants 3.107-0.85 and 3.404.86, and by the Fondation Centre de Recherches M#{233}dicales Carbos et Elsie de Reuter.

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