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Unicystic ameloblastoma of mandible: imaging features and literature review

Ameloblastoma is the most common benign odontogenic tumor accounting for approximately 1% of tumors and cysts of the jaw and 10% of all the odontogenic tumors.1 It is a slow-growing, persistent and locally aggressive neoplasm that may originate from the epithelium involved with the formation of teeth such as enamel organ, odontogenic rests of Malassez, reduced enamel epithelium and odontogenic cyst lining.2 Ameloblastoma may occur centrally within the bone or peripherally, without an intraosseous component in the soft tissues overlying the alveolar ridge. Intraosseous lesions are of two types solid/conventional/multicystic and unicystic.3 Unicystic Ameloblastoma (UA), a variant of ameloblastoma first described by Robinson & Martinez4 in 1977, refers to those cystic lesions that show clinical and radiologic characteristics of an odontogenic cyst, but in histologic examination, show a typical ameloblastomatous epithelium lining part of the cyst cavity with or without luminal and/or mural tumor proliferation. Prior to the report by Robinson and Martinez, this variant had been referred to as a mural or intraluminal ameloblastoma. Recognition of this growth pattern is very important because of its Unicystic radiographic appearance, histologic findings, association with an unerupted tooth, occurrence in the mandible of younger patients, and a recurrence rate after conservative surgical treatment lower than that of its conventional counterpart.5 We present a case of a Unicystic ameloblastoma in a 10-year-old child patient who reported with a complaint of swelling of her right lower jaw.


Introduction
Ameloblastoma is the most common benign odontogenic tumor accounting for approximately 1% of tumors and cysts of the jaw and 10% of all the odontogenic tumors. 1 It is a slow-growing, persistent and locally aggressive neoplasm that may originate from the epithelium involved with the formation of teeth such as enamel organ, odontogenic rests of Malassez, reduced enamel epithelium and odontogenic cyst lining. 2 Ameloblastoma may occur centrally within the bone or peripherally, without an intraosseous component in the soft tissues overlying the alveolar ridge. Intraosseous lesions are of two types solid/conventional/multicystic and unicystic. 3 Unicystic Ameloblastoma (UA), a variant of ameloblastoma first described by Robinson & Martinez 4 in 1977, refers to those cystic lesions that show clinical and radiologic characteristics of an odontogenic cyst, but in histologic examination, show a typical ameloblastomatous epithelium lining part of the cyst cavity with or without luminal and/or mural tumor proliferation. Prior to the report by Robinson and Martinez, this variant had been referred to as a mural or intraluminal ameloblastoma. Recognition of this growth pattern is very important because of its Unicystic radiographic appearance, histologic findings, association with an unerupted tooth, occurrence in the mandible of younger patients, and a recurrence rate after conservative surgical treatment lower than that of its conventional counterpart. 5 We present a case of a Unicystic ameloblastoma in a 10-year-old child patient who reported with a complaint of swelling of her right lower jaw.

Case report
A ten year old female child came with a complaint of swelling on right lower side of her jaw since one month which was insidious in onset and got gradually progressed to present size. It was associated with severe, intermittent and dragging type of pain which radiates to right ear. Pain relieved with medication. It was also associated with extra oral swelling since15 days. On Extraoral examination facial asymmetry was seen in the right lower third of the face. On Intra oral examination, a solitary diffuse swelling was seen in the buccal vestibular region of 85 and 46, extending anteroposteriorly from middle third of 85 to distal surface of 46. Superoinferiorly 0.5cms below the marginal gingiva of 85 and 46 extending into the buccal vestibule with vestibular obliteration. Sinus opening with pus discharge was seen on the attached gingiva of 85 (buccal or lingual). On palpation, swelling was tender, hard in consistency, non-compressible, non-reducible. Expansion of buccal cortical plate was felt irt 85 and 46. Based on the history and clinical examination a provisional diagnosis of ameloblastoma was made. Under differential diagnosis ameloblastic fibroma, odontogenic keratocyst was considered. IOPA, Occlusal view, Orthopantanogram and CT scan was taken. IOPA taken irt 85 and 46 ( Figure 1) showed diffuse radiolucency distal to 46 with absence of 47 tooth bud. OPG (Figure 2) view showed a solitary, well defined radiolucency of size 3x2cms extending anteroposteriorly from mesial root of 46 to 1cm from the posterior border of ramus of the mandible, superoinferiorly 1cm below the sigmoid notch to inferior border of mandible thinning the inferior cortical margin. Internal structure was radiolucent with the displaced tooth bud of 47 in the ramus region. Panoramic CT section (

Discussion
UA accounts for 6% to 15% of all intraosseous ameloblastomas. 1 It is less aggressive, usually occurs in an earlier age group than the solid or multicystic with about 50% of the cases occurring in the second decade of life. As in the present case more than 90% are located in the mandible. 6 In most cases UA are associated with impacted tooth, mandibular third molar being the most common. 7 The term unicystic is derived from the macroscopic and microscopic appearance, the lesion being essentially a well-defined, often large monocystic cavity with a lining, focally but rarely entirely composed of odontogenic (ameloblastomatous) epithelium. 8 The pathogenesis of cystic ameloblastomas remains obscure. Some investigators believe that UCA arises from pre-existing odontogenic cysts, in particular a dentigerous cyst, while others maintain that it arises de novo. The reason why some ameloblastomas become completely cystic may be related to epithelial dysadhesion (e.g., defective demosomes) or, more likely, to the intrinsic production of proteinases (e.g, metalloproteinases, and serine proteinases); enzymes that normally degrade the central zone of the enamel organ after tooth development. 9 Radio-graphically, the unilocular: multilocular ratio is 13:3 when the lesion is associated with an impacted tooth. For the `non-dentigerous' variant this ratio changes to 8:7. Further, the `dentigerous' type occurs on average 8 years earlier than the `non-dentigerous' variant. Lastly, the mean age for unilocular, impaction-associated UAs is 22 years, whereas the mean age for the multilocular lesion unrelated to an impacted tooth is 33 years. 7 Ackerman et al., 10 classified unicystic ameloblastoma into three types with prognostic and therapeutic implications.       Group III: Mural (invasive islands of ameloblastomatous epithelium in the connective tissue wall not involving the entire epithelium).
The microscopic pattern that exhibits mural invasion in UA suggests a more aggressive potential. 11   Another histologic subgrouping by Philipsen & Reichart 12 has also been described as follows:  13 is also advocated for subgroups 1 and 1.2. Subgroups 1.2.3 and 1.3 have a high risk for recurrence, requiring more aggressive surgical procedures. 14 Recurrence rates for Unicystic Ameloblastoma after conservative surgical treatment (curettage or enucleation) are generally reported to be less than 25%. For intraluminal and plexiform type of Unicystic Ameloblastoma recurrence rate was found to be as low as 10.7%. 15 Recurrence rates for Solid Multicystic Ameloblastoma was found to be about 50 to 90%. The present analysis included only publications in English. All well-documented publications during the last 20 years were collected, and several clinicopathological features of each case were studied. The following data were recorded: age (less than or equal to 10 years), sex, location, clinical features/symptoms, histological type, radiographic appearance, treatment. Included only reports of Unicystic ameloblastoma in children less than 10 years confirmed by histological analysis with all the data required for tabulation were included and the Articles not having enough information were excluded (Table 1).

S. no
Year

Conclusion
Ameloblastomas in children differ from adults with a higher percentage of unicystic tumors. Unicystic ameloblastoma is a tumor with a strong propensity for recurrence, especially when the ameloblastic focus penetrates the adjacent tissue from the wall of the cyst. Although enucleation has been claimed to give acceptable recurrence rates in unicystic ameloblastoma, there are no large series with long follow-up in children. The histologic pattern that exhibits mural invasion in unicystic ameloblastoma suggests that more aggressive surgery is necessary. Present case was treated with Carnoy's solution along with the enucleation, which suggests a possible benefit against recurrence.