Discussion
Unlike solitary tumors, hybrid tumors are rare. However, 25% of cases of calcifying odontogenic cyst (COC) occurs concurrently with other odontogenic lesions and the most common concurrent lesion seen in this association is odontoma.6 There are very few cases of ameloblastic fibro-odontoma (AFO) associated with COC and the first reported case was published in 1987.8
Developing odontoma was formerly called AFO until 2017 when WHO changed its name.1 This report is the first case of developing odontoma as a new entity arising from calcifying odontogenic cyst. Although the last WHO classification of odontogenic tumors considered the presence of dental hard tissue structures within an ameloblastic fibroma-like tissue as early stage of a developing odontoma, there are still some argue regarding whether all AFO represent early stage of odontoma. Soluk Tekkesin and Verde suggested that a combination of a cut-off age over 13.5 and size bellow 2.1cm can suggest a developing odontoma. Also the presence of ameloblastic fibroma-like tissue in the periphery of the lesion, lobular arrangement of stromal component in a way that hypercellular areas being located mostly around the epithelial component, a well-developed ameloblastic epithelium and also the presence of ghost cells and cystic structures can serve as histological clues that helps to differentiate developing odontoma from AFO.9 Except to histopathologic findings, this concept also revealed that our case represents a developing odontoma.
Developing odontoma is a rare odontogenic tumor. It shows proliferation of odontogenic epithelium and primitive ectomesenchymal tissue in association with tooth structure.2 It accounts for about 3% of odontogenic tumors and usually occurs in patients younger than 20 years old.3 75% of developing odontomas are located in the mandible and 67% of these are located in the posterior mandible. Over 95% of developing odontoma are associated with impacted permanent teeth and present clinically as painless slow-growing mass. Radiographically, developing odontoma depicts a clearly defined mixed unilocular or multilocular lesion with various amounts of radiopaque calcifications.10
COC is a rare odontogenic lesion presented as a painless slow-growing lesion with predilection for the anterior region of the jaws. It affects both maxilla and mandible equally.11 It also occurs equally in males and females and shows no race predilection. Intraosseous COC lesions are more common than the peripheral forms.2 COC affects patients between 5 to 92 years and the age of peak incidence is between the second and sixth decade of life.12 Radiographically, it shows either well-defined unilocular or multilocular radiolucencies and sometimes diffuse radiopacities.11
COC may occur in association with an impacted tooth. The distinguishing histopathologic feature of COC is the presence of ghost cells, which may calcify, in an ameloblast-like epithelium.11 It can arise in association with other odontogenic lesions like odontoma, ameloblastic fibroma, and ameloblastic fibro-odontoma (AFO). The most frequent concurrent odontogenic lesion with COC is odontoma.6 Association of COC with ameloblastic fibro-odontoma is extremely rare and so far, only 3 cases have been reported.8,13,14
The first case of Calcifying odontogenic cyst with ameloblastic fibro-odontoma was reported by Farman, et al in 1978.8
Matsuzaka, et al reported a case of ameloblastic fibro-odontoma arising from a calcifying odontogenic cyst in 2001. The patient was a 23 year-old male with the chief complaint of painful swelling on the left mandibular molar region. It was a multilocular mixed lesion in the panoramic radiograph. Tooth impaction was also evident.13
Lee et al., in 2014 reported calcifying odontogenic cyst associated with ameloblastic fibro-odontoma of the anterior mandible in a 4 old-year girl. The chief complaint of the lesion was swelling. It had caused tooth displacement. It was a unilocular mixed lesion that caused root resorption and cortical perforation. The lesion was also around an impacted tooth.14
Except for these two combinations of COC with ameloblastic fibro-odontoma, Imani, et al in 2017 also reported a hybrid odontogenic tumor in a 14 old year boy with a painless lesion in the left maxillary canine which was without expansion. The lesion had a mixed radiolucent-radiopaque appearance with a well-defined border in the panoramic radiograph. In histopathologic examination combination of three odontogenic lesions including calcifying odontogenic cyst, complex odontoma and ameloblastic fibro-odontoma were evident.15
The mechanism that causes to arise two odontogenic lesions together is not well known. Nevertheless, various theories have been proposed to explain the phenomenon including a transformation of one lesion into another, a collision of two separate lesions, and an inductive effect of one lesion on the another one.16 According to histopathologic findings, the first and third theories seem to be more plausible in our case.