[Problem areas of tumour classifications--thyroid carcinomas].

Thyroid carcinoma has been traditionally subdivided into the four major groups papillary, follicular, medullary, and anaplastic carcinoma. The WHO classification of thyroid tumours, published in 2004, has added to these four tumour groups the entity of poorly differentiated carcinoma as well as a broad variety of rare thyroid malignancies. Another important change concerns the histological hallmarks of papillary carcinoma, the diagnosis of which is now exclusively dependent on characteristic nuclear features. The aim of the present paper is to highlight diagnostic problems particularly caused by the alteration introduced onto the WHO classification, This includes a proposal of a more systematic thyroid carcinoma classification based on the histogenetic differentiation (follicular cell differentiation. C cell differentiation, rare carcinomas) and tumour grading of carcinomas with follicular cell differentiation (differentiated, poorly differentiated and anaplastic carcinomas) as well as commentaries on the diagnosis of papillary carcinoma, poorly differentiated carcinoma, and rare types of thyroid carcinoma (squamous cell carcinoma, mucoepidermoid carcinoma, sclerosing mucoepidermoid carcinoma with eosinophilia, mucinous carcinoma, SETTLE, and CASTLE).