The lining epithelium of odontogenic keratocysts is affected by various types of inflammatory stimuli. However, malignant transformation of the cyst is extremely rare. We report a newly diagnosed case of squamous cell carcinoma arising in an odontogenic keratocyst. A 52-year-old man was referred to our outpatient clinic because of a swelling in the labial-side gingiva of the maxilla. X-ray examination showed cyst-like radiolucent bone defects in the middle of the maxilla, with an impact tooth overlaying the lesion. The patient received radiotherapy to the oral region because of a malignant lymphoma.
The cystic lesion was enucleated. Histopathological examination of the surgical material revealed an odontogenic cyst with malignant transformation. The thick squamoid epithelium showed irregular epithelial ridges with various types of atypism. The lesion was attributed to malignant transformation of the odontogenic keratocyst. The patient has been followed up for 13 months without recurrence.