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Odontogenic tumors composed of two or more distinct types of lesions are unusual. In this paper, a case of an odontogenic lesion characterized by simultaneous occurrence of areas of calcifying odontogenic cyst (COC) and orthokeratinized odontogenic cyst (OOC) is described. The lesion was asymptomatic and presented at the radiographic examination as a unilocular well-delimited radiolucency extending from left incisor to right premolar area in the mandible. To date, this is the first report of COC associated with an OOC.
Odontogenic tumors composed of two or more distinct types of lesions are unusual and the biologic mechanism causing such a unique combination is not readily apparent .
The calcifying odontogenic cyst (COC) was first defined as a separate entity in 1962 . According to the new WHO classification in 2005 COC has now been reclassified as calcifying cystic odontogenic tumor (CCOT) . Although there is no consensus regarding the classification and terminology of COC, this benign lesion is categorized as either a cyst or neoplasm (solid) [3–5]. In the cyst variant, three different types may be found: simple unicystic type, unicystic odontoma-associated type, and unicystic ameloblastomatous proliferating type. COC is usually asymptomatic and may be an incidental radiographic finding . Radiographically, the lesion appears as a unilocular or multilocular well-defined radiolucency that may contain small irregular calcified bodies of varying sizes, and it may be associated with an odontoma or an unerupted tooth [2, 4].
The orthokeratinized odontogenic cyst (OOC) was first described in 1927 as a dermoid cyst  and in 1945 it was considered as an eventual type of odontogenic keratocyst (OKC) . In 1981, Wright  specified its clinicopathologic aspects, assuring that OOC was an individual entity, distinct from other odontogenic cysts, including the OKC. OOC has been described as a solitary lesion, usually small and nonexpanding, radiolucent, and asymptomatic .
In this article, a COC associated with an OOC is described. As far as we are concerned, this is the first case reported in the English-language literature.
A 24-year-old woman was referred to the Oral Diagnosis Service of Odilon Behrens Hospital for consultation regarding a radiolucent lesion in mandible. The lesion was asymptomatic and a discrete swelling was noted in the region. The overlying mucosa was normal, and radiological findings revealed a unilocular well-delimited radiolucency with delicate septae extending from first lower left premolar to lower right lateral incisor (Fig. 1a). The vitality test revealed that all teeth were vital. Aspiration of the lesion provided a viscous content presenting creamy material composed of keratin intermingled by yellowish granules. Considering a provisional diagnosis of an odontogenic benign lesion an excisional biopsy was performed. Grossly, the material presented an oval format with a cystic cavity containing yellowish granules. Histopathological examination revealed a cystic lesion presenting a fibrous capsule lined with odontogenic epithelium of variable thickness. Cuboidal and columnar cells resembling ameloblasts were observed at the basal layer. Above the basal layer, the epithelial cells showed a loose arrangement together with groups of ghost cells (Fig. 1b, c). It was observed a transition from COC to OOC in which the lining was represented by orthokeratinized stratified epithelium with uniform thickness (Fig. 1d, e). In addition, ameloblastomatous proliferation was seen in the connective tissue (Fig. 1f). The COC represented about 70% of the lesion. The final diagnosis was COC associated with an OOC. There was no evidence of recurrence 5 months after the enucleation of the lesion.
Hybrid odontogenic tumors have been reported occasionally; most of these tumors have been diagnosed as an odontogenic tumor (most commonly odontomas, ameloblastomas, ameloblastic fibroma) combined with COC [1, 10, 11]. However, an association of COC with an OOC has not been described yet.
We suggest that the presence of two distinctive epithelial odontogenic patterns, present in a single lesion, is probably not a collision of a COC and an OOC, but it is instead an expression of the histomorphodifferentiation potential of the epithelium from which these lesions are derived. This assumption is supported by the finding of both histological types lining the same cystic cavity. As COC is more undifferentiated than the OOC, we suggest that the former arose in the epithelium lining of the later, but the possibility of COC mimicking OOC should also be considered.
OOC has been specified as an individual entity, distinct from other odontogenic cysts, including OKC . OOC is characterized by the presence of an epithelium-lined cavity in which a significant portion of the epithelium exhibited orthokeratinization and in which the basal cells showed little tendency to palisade or polarize. Frequently, there is a granular cell layer associated with the orthokeratin. The intraosseous occurrence within the jaws, predilection for the posterior mandible areas and the ability to keratinize in their linings suggested initially that OOCs may share the same histogenic origin as OKCs (from dental lamina and its remnants) . However, many investigations have demonstrated significant differences in histological features and clinical behavior between OKC and OOC [13, 14]. In addition, PTCH gene alterations commonly described in OKC  have not been demonstrated in OOC. Mutation on the β-catenin gene was previously reported in COC , but has not been investigated in OOC. Molecular studies are necessary to understand the histopathological changes observed in the current case.
The present paper shows an example of the importance of microscopic analysis of the entire specimen when studying odontogenic cysts or tumors in general. The treatment of COC involves simple enucleation of the lesion and long-term follow-up. In most cases, the prognosis is good, but in cases of solid neoplastic COC, recurrence is more prone to occur. When it is present in association with other odontogenic tumors, the treatment is based on the associated tumor . Referring to behavior of OOC, although it can become relatively large, this lesion seems to be less aggressive than OKC .
Dr. RS Gomez and MCF Aguiar are research fellows of CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico)—Brazil.
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