Update from the 4th Edition of the World Health Organization of Head and Neck Tumours: Tumours of the Oral Cavity and Mobile Tongue.

There have been several additions and deletions in Chapter 4 on Tumours of the oral cavity and mobile tongue in the 2017 fourth edition of the World Health Organization Classification of Tumours of the Head and Neck. This chapter excludes the oropharynx, which now is a stand-alone chapter acknowledging the uniqueness of the oropharynx from the oral cavity. New entries in Chapter 4 include rhabdomyoma, haemangioma, schwannoma, neurofibroma and myofibroblastic sarcoma in the section titled Soft tissue and neural tumours. Discussion of salivary gland entities have been reduced and includes mucoepidermoid carcinoma and pleomorphic adenoma as the other salivary gland types are discussed elsewhere. In the Haematolymphoid tumours section, like the salivary gland section, only tumors that commonly present in the oral cavity are discussed in Chapter 4. Excluded entities in the updated classification include papillary hyperplasia, median rhomboid glossitis, keratoacanthoma, focal oral mucinosis, and secondary tumors. This article will summarize the changes in the new classification since the 2005 edition focusing on selected entities that have had significant changes along with new entries.