Primordial Odontogenic Cyst with Induction Phenomenon (Zonal Fibroblastic Hypercellularity) and Dentinoid Material Versus Archegonous Cystic Odontoma: You Choose!

The most recent A.F.I.P. fascicle defines primordial odontogenic cyst (POC) as a distinct, nonkeratinized, odontogenic cyst of “undetermined origin” forming in the place of a developing normal or supernumerary tooth. However, the majority of examples reported in the literature under this term represent odontogenic keratocysts (keratocystic odontogenic tumors). In addition, there are rare reported cases of cystic odontomas. An 18-year-old Caucasian male presented with a unilocular mandibular radiolucent lesion in the place of a congenitally missing molar. Histologically, it featured nonkeratinizing, thin stratified squamous epithelial lining with areas of spongiosis and foci of vacuolization of individual basal cells without significant nuclear palisading. Focally, budding of the basal cell layer was identified. A zone of increased cellularity featuring induction-type fibroblasts was present subepithelially as well as dentinoid deposits with odontogenic epithelial nests. Immunohistochemically, the epithelial lining was negative for calretinin and the induction-like zone negative for S100 protein, smooth muscle actin, and CD34. The case was externally reviewed by five oral pathologists who provided various diagnostic interpretations including primordial cyst, odontogenic cyst not otherwise specified (NOS), cyst with ameloblastic changes, and unicystic ameloblastoma. At that time, a final diagnosis of odontogenic cyst NOS was rendered with a comment that it may represent a true example of POC or a cystic odontoma. The lesion has not recurred within a 13 year follow-up period after initial excision. An unusual cystic lesion is presented that may represent a true example of POC with dentinoid formation or an archegonous cystic odontoma.