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.