Update from the 5th Edition of the World Health Organization Classification of Head and Neck Tumors: Odontogenic and Maxillofacial Bone Tumours

The 5th edition of the World Health Organization (WHO) Classification of Head and Neck Tumours (2022) comes out only five years after the previous edition, however it presents important updates that run in parallel with the rapid progression involving the increasingly sophisticated molecular investigation and its interpretation, some of which already have therapy-related impact. This manuscript provides an overview of the leading changes introduced in the classification of Odontogenic and Maxillofacial Bone Tumours that encompasses cysts of the jaws, odontogenic tumours, giant cell lesions and bone cysts, and bone and cartilage tumours. This is the first edition that Essential and Desirable Diagnostic Features were added for each entity, so that the most important clinical, microscopic and/or radiologic features were encapsulated and briefly highlighted. Surgical ciliated cyst was added to the group of odontogenic cysts, adenoid ameloblastoma was a newly recognized benign epithelial odontogenic tumour, and segmental odontomaxillary dysplasia was introduced in the group of fibro-osseous tumours and dysplasia. In addition, rhabdomyosarcoma with TFCP2 rearrangement, was introduced into the group of malignant jawbone tumours. The unique genetic aberrations distinguish it from other types of rhabdomyosarcomas. On the other hand, melanotic neuroectodermal tumour of infancy and osteoid osteoma were deleted from the benign bone and cartilageneous tumours, as was the hematolymphoid tumour of solitary plasmacytoma of bone. We systematically reviewed each entity in this chapter and provided important updated findings for selected topics that can further aid in the diagnostic process for challenging cases, broaden insights on the logic of the present classification, and finally, emphasize the potential that some of the molecular results may have in the near future to set new treatment approaches.