The World Health Organisation classifications of tumours (often referred to as the 'Blue Book' series) provides the internationally recognised standard for diagnosis and research in neoplastic lesions. The 5th edition of the Classification of head and neck tumours,1 which will be published in 2022, updates current knowledge and integrates developing understanding of these conditions, including insights from molecular pathology techniques. There are several changes from the 4th edition (2017) which bring a more logical hierarchical classification, consensus definitions and provide greater insight into advances in our understanding of the pathogenesis of these varied lesions.2,3 Whilst there are 17 chapters in the 5th edition, covering different tumours and sites within the head and neck, this update will focus on the main changes of relevance to dental professionals working in primary and secondary care.

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For oral potentially malignant disorders (OPMDs), chronic candidiasis, syphilitic glossitis and actinic keratosis have been removed from the table of OPMDs, whilst oral lichenoid lesions (resemble oral lichen planus but lack the classical clinical or histopathological features; please note - lichen planus remains an OPMD), oral graft versus host disease and familial cancer syndromes have been added. Smokeless tobacco keratosis has been updated to note that the risk varies with tobacco type.

There is greater clarity in the new edition for grading of oral epithelial dysplasia (OED), taking into account numerous studies highlighting challenges in reaching diagnostic agreement. The grading system is still three-tiered (mild, moderate and severe epithelial dysplasia) and there is increased recognition of a lichenoid response against dysplastic epithelium. HPV-associated dysplasia has been described under a separate heading with defined diagnostic criteria, and which is not graded like classical OED. Molecular techniques may be useful: demonstration of p16 overexpression is desirable, but the positivity threshold has not been validated for diagnosis.

Greater detail has been provided for rare variants of oral squamous cell carcinoma with separate sections for verrucous carcinoma

Greater detail has been provided for rare variants of oral squamous cell carcinoma with separate sections for verrucous carcinoma (pushing border interface with a proliferative verrucous exophytic architecture) and carcinoma cuniculatum (which has a burrowing invasive pattern).4

In terms of lesions of the maxillofacial bones, the surgical ciliated cyst is a new entity in the cysts of the jaws section. This is caused by traumatic implantation (usually surgical) of respiratory (sinus lining) epithelium in the gnathic bones. Risk factors include Caldwell-Luc and Le Fort I procedures amongst other sinus and facial surgeries. Another new entity is the adenoid ameloblastoma which is an epithelial odontogenic neoplasm characterised by cribriform architecture, duct-like structures and an ameloblastoma-like component. Dentinoid, clear cells and ghost cells can also be present. These tumours have a recurrence rate of up to 70%. Metastasising ameloblastoma is also discussed in a separate section.

The beta version of the 5th edition of the WHO classification1 is currently live to allow early access; however, the content may change during further editing. The book is anticipated to be published in 2022. We hope this brief update and link to references signposts readers to further reading.