Changes in the diagnosis of thyroid tumours in the 5th edition of the WHO classification of endocrine neoplasms.

The WHO classification of thyroid tumours enters its second half-century of development with the 5th edition. Compared to the previous 4th edition of the clas- sification, the permanent increase in information is mainly at the molecular biological level. This has changed the view of very traditional entities - the preferred name for polynodous goiter is (given the monoclonal nature of some nodules) follicular nodular thyroid disease. Some terminological relics have also been re- moved - Hürthle cells are definitively referred to as oncocytes. Follicular adenoma has a new subtype with papillary arrangement (and missing nuclear features of papillary carcinoma). In the already used NIFTP unit, subtypes smaller than 10 mm and oncocytic are newly defined. All oncocytic tumours have an arbitrarily set minimum proportion of oncocytes at 75 %. A multidisciplinary approach to the treatment of thyropathies and the stratification of therapeutic procedures according to risk brought about the introduction of grading into several nosological units of papillary, follicular, and medullary carcinomas. Grading using the number of mitoses determines their quantification at 2 mm² instead of the previously used non-uniform HPFs (high power fields of view). Clarification was made on the basis of genetic findings in a number of other, less frequent diagnoses (e.g. classification of squamous cell carcinoma among anaplastic). Among rare tumors a new category of salivary gland - type carcinomas is formulated with two representatives: mucoepidermoid and secretory carcinoma. Cribriform morular carcinoma previously classified as a variant of papillary carcinoma is newly separated on the basis of the immunological and genetic profile into the newly created category of tumors of uncertain histogenesis. This category also includes sclerosing mucoepidermoid carcinoma with eosinophilia. Microcarcino- ma as a separate entity is not included in the 5th edition. A tumor smaller than 10 mm must be characterized by the appropriate features of the corresponding category. Thyroblastoma replaces terminologically malignant teratoma from the previous classification. Part of the newly established diagnostic criteria is also applicable in FNAB diagnosis. The newly introduced grading in some nosological units can exceptionally change the diagnosis (NIFTP/EFVPTC/non-invasive HG FVPTC), but above all it will affect the choice of therapeutic procedures.