Current classification of precursor lesions of oral squamous cell carcinoma. Principles of the WHO classification 2005

The WHO classification of oral tumours summarizes the precancerous squamous cell lesions under the term epithelial precursor lesions. For the first time three classification schemas that histologically categorize oral epithelial precursor lesions are used analogously. According to the WHO suggestion of 2005 the traditional schema of grading dysplasia as mild dysplasia, moderate dysplasia, severe dysplasia and carcinoma in situ continues to be used. In addition the concept of intraepithelial neoplasia is introduced as squamous intraepithelial neoplasia I-III. Squamous intraepithelial neoplasia III (SIN III) combines severe dysplasia and carcinoma in situ. The Ljubljana classification of squamous intraepithelial lesions was originally established to grade laryngeal epithelial precancerous lesions. The clear and succinct nomenclature and the simple clinical utility of the Ljubljana classification have also proven to be useful for oral epithelial precursor lesions: squamous cell (simple) hyperplasia; basal/parabasal cell hyperplasia (analogous to mild dysplasia and to SIN I); atypical hyperplasia (analogous to moderate-severe dysplasia and to SIN I-III and is also called risky epithelium); carcinoma in situ (analogous to WHO carcinoma in situ and to SIN III). Atypical hyperplasia (risky epithelium) and carcinoma in situ are defined as lesions requiring either total excision or close clinical monitoring.