Soft tissue keratocyst of the cheek mucosa associated with a cluster of epithelial nests: a case report

The vast majority of odontogenic keratocysts (keratocystic odontogenic tumour) (OKC) are intra-osseous, but a rare variant has been documented in the mucosa of the cheek. Despite its extra-osseous location, the source of this variant is assumed to be remnants of the dental lamina. We describe a keratocyst which was situated in the buccal space of a 76 year-old man. Microscopically, a cluster of epithelial nests was present in the cyst wall. Basal palisading and reversed nuclear polarity were features of both the nests and the cyst lining. Some nests had lumina. The epithelial cluster was adjacent to small nerves. The nests and the cyst lining were strongly positive for cytokeratin (CK) 5 and p63, and negative for CK18 and 20. CK7-positive cells were present in the centre of the nests, but the cyst lining was entirely negative. CK8 and CK19 were strongly expressed by the epithelial nests, but only focally by the cyst lining. Whereas the nests were negative for Ki67, the cyst lining showed the pattern of basal and parabasal positive nuclei seen in OKC. The origin of these nests is uncertain; the most likely candidates are ectopic remnants of dental lamina, salivary glands, isthmus of the cutaneous hair follicle and the juxta-oral organ of Chievitz. On the basis of immunophenotype, we were unable to determine whether their presence was coincidental, or whether the epithelial nests were the source of the cyst. The cyst has not recurred in 34 years.