Objectives: Oral squamous cell carcinomas (OSCCs) are often diagnosed late. This study aimed to determine how frequently oral epithelial dysplasia (OED) transforms to OSCC and to identify histological features that could influence the rate of malignant transformation.

Materials and methods: The study was a retrospective analysis of OED over 29 years at the Institute of Dentistry, University of Turku, Finland. OEDs with co-existing carcinomas were excluded from the data (5.8%). OED patients who developed carcinoma were identified from the Finnish Cancer Registry database.

Results: Altogether 681 OED patients had a mean age of 59.0 years, and the male:female ratio was 0.67. Of all OED samples, 21.8% were on the tongue, followed by lining mucosa (21.3%), lip (5.3%), and masticatory mucosa (4.85%). In addition, 46.7% had no location cited. The prevalence of mild dysplasia was 62.4%, moderate dysplasia 29.1%, and severe dysplasia 3.2%. Of the patients, 94.7% had an additional histological diagnosis alongside OED. Candidiasis, lichenoid inflammation, and ulcer were found in 18.2%, 0.0%, and 22.7% of severe dysplasias, in 12.1%, 12.2%, and 22.7% of moderate dysplasias, and in 6.6%, 12.2%, and 15.8% of mild dysplasias, respectively. An additional histopathological diagnosis did not increase the risk for OED to transform to OSCC. In a mean time of 5.2 (range 0.7-29.0) years, 7.5% of OED patients developed OSCC.

Conclusions: Location on the tongue and the more severe OED grades increased the risk of malignant transformation of OED. These patients may benefit from an intensified follow-up schedule to ensure early diagnosis of OSCC.