Evolving concepts and new entities in the 2017 WHO classification of salivary gland tumors

For the past decades, many new salivary gland entities have been described which are somewhat related to the discovery of unique molecular alterations in these tumors. The 4th edition of World Health Organization (WHO) classification of head and neck tumors has included several new entities, e.g. secretory carcinoma, sclerosing polycystic adenosis and intercalated duct lesions and modified several carcinomas, e.g. clear cell carcinoma, intraductal carcinoma and polymorphous adenocarcinoma. In addition, in the 4th edition, the concept of high grade transformation has been introduced. In this review, we aimed to illustrate the major changes in the WHO classification, focusing on the rationale behind these changes, the morphologic features of the new described entities and the ancillary diagnostic tools that may help with the differential diagnoses of salivary gland neoplasms.