Volume 3, Issue 4 , Pages 192-196, December 2008
Unicystic ameloblastoma in a child: A case study and review of the literature
Article Outline
Summary
We report a case of a 15-year-old female patient referred for diagnosis and treatment of an asymptomatic unilocular rather ill-defined radiolucent lesion localized distal to the lower second right molar. A previous panoramic radiograph taken two years earlier by his practitioner demonstrated the presence of a well demarcated unilocular radiolucency surrounding the crown of the partially erupted mandibular right second molar (#47) but no radiologic evidence of the present lesion. The lesion was removed by enucleation, curettage and extraction of the #48** tooth germ. Differential diagnosis included a residual dentigerous cyst, a keratocyst or an odontogenic tumor. Histopathological examination of the enucleated material provided a diagnosis of plexiform unicystic ameloblastoma (UA) with mural invasion. The patient underwent regular clinical and radiological controls and a last follow-up examination at 30 months showed no recurrence of the tumor. This case illustrates the importance and difficulty of the differential diagnosis of unicystic ameloblastoma presenting in the posterior region of the mandible.
Keywords: Adolescent, Dentigerous cyst, Differential diagnosis, Unicystic amelobastoma
1. Introduction
Ameloblastoma is a benign but locally aggressive odontogenic epithelial neoplasm. It is classified as solid or multicystic, unicystic or peripheral showing important differences with respect to the clinical behavior [1]. It usually occurs in adulthood and is well known for the propensity to recur. The distinct variant of ameloblastoma referred to as unicystic ameloblastoma (UA), described by Robinson and Martinez in 1977 [2], accounts for 10–15% of all ameloblastomas. On the basis of previous reports, UA tends to occur at an earlier age than the solid or multicystic forms and frequently presents as a unilocular well-defined radiolucency surrounding the crown of an unerupted mandibular third molar, such as a dentigerous cyst. This type is considered by many to be less aggressive than the other types of ameloblastomas.
The aim of this paper is to report a case of a mandibular unicystic ameloblastoma in a child.
2. Case report
A 15-year-old female patient was referred to our department for a rather ill-defined unilocular radiolucencent lesion, discovered upon examination of a routine panoramic radiography. The orthopanthomography showed an osteolytic lesion measuring approximately 2.0
cm×1.5cm, localized at the right retromolar area distal to the lower right second molar, and displacing the third molar germ towards the inferior alveolar canal (Fig. 1). Clinical examination revealed no vestibular swelling in the right posterior region of mandible, no erythematous tissue above the osteolytic lesion and no pain upon palpation.
Fig. 1.
Panoramic radiograph at patient presentation showing a radiolucent lesion distal to the mandibular second right molar and displacing the third molar germ towards the inferior alveolar canal.
A previous panoramic radiography taken two years earlier by her orthodontist showed an unilocular radiolucency surrounding the crown of a partially erupted second mandibular molar (Fig. 2).
Fig. 2.
Panoramic radiograph of the patient taken two years earlier showing a well-limited radiolucent osteolytic lesion surrounding the crown of a lower right second molar.
The present lesion was considered to be compatible with a residual dentigerous cyst persisting after the eruption of tooth #47, an odontogenic keratocyst or an odontogenic tumor such as an ameloblastic fibroma or an ameloblastoma.
A CT scan was carried out and it showed a lesion with a well-defined homogeneous unilocular osteolytic lesion, without resorption of adjacent dental roots of the second mandibular molar. There was a mild expansion of the cortical bone. The computer 2D reconstruction demonstrated that the osteolytic lesion was not related but in some sections extended to the third molar germ (Fig. 3).
Fig. 3.
Transversal CT scan reconstruction showing the lesion.
The young patient was surgically treated by enucleation of the lesion, thorough curettage of the bone cavity and third molar germ extraction. Postoperative course was uneventful.
Histological analysis of the surgical specimen revealed a cystic lesion represented by a fragment of 3cm with an irregular external surface, showing on the internal side a nodule of 0.9cm. The cyst was lined by columnar epithelial basal cells containing a hyperchromatic nucleus, showing reverse polarity, with the overlying cells resembling to the stellate reticulum (Fig. 4). The nodule showed a proliferation of ameloblastomatous tissue of plexiform type within the lumen of the cyst. The connective tissue wall contained in some areas intramural ameloblastic tissue islands extending near the inferior surgical margin (Fig. 5). The diagnosis was then of a plexiform UA with mural invasion.
Fig. 4.
Ameloblastoma of luminal type showing columnar basal cells with prominent nuclear palisading and loosely arranged stellate, reticulum-like upper cellular layers. Hematoxylin–eosin–safran stain; original magnification 6.4×.
Fig. 5.
Ameloblastoma islands and strands within the connective tissue wall (mural invasion) with some follicles showing squamous metaplasia. Hematoxylin–eosin–safran stain; original magnification 20×.
It was discussed if the patient should receive a further treatment in view of the histological subtype. In agreement with the patient's parent it was decided to schedule clinical and radiological controls at regular intervals. A CT scan (Fig. 6a and b) taken at 24 months after initial treatment showed no recurrence of the tumor with satisfactory bone healing of the right retromolar region of the mandible.
Fig. 6.
Axial CT scan taken at 24 months follow-up demonstrating bone healing distal to the right second mandibular roots (a). Transversal CT scan reconstruction showing normal bone thickness of the right mandibular retromolar area (b).
3. Discussion
Ameloblastomas are benign odontogenic tumors of the jaws of epithelial origin. They account for about 20% of the all odontogenic tumor but their prevalence and incidence are not yet established because of significant deficiencies in the published literature [3].
The mandible is involved in 80% of cases. Men and women are affected equally, generally in the third or fourth decade of life. The tumor is thought to develop from residual epithelium of the tooth-forming apparatus, such as the rests of dental lamina, epithelium of odontogenic cysts, basal cells of the surface epithelium and epithelium of the enamel organ. Ameloblastomas are currently classified into three categories [1]: (a) solid or multicystic, the most frequent form and radiologically characterized by multilocular aspect; (b) unicystic, radiologically characterized by unilocular aspect and (c) extraosseous or peripheral which affects the oral soft tissues and especially the gingivae, showing diverse histological subtypes. Our case belongs to the second type of ameloblastomas and the clinicopathological features of the patient are in agreement with recent studies showing that the UA is the most commonly form in children [4], [5]. The lack of signs and symptoms of this lesion in the childhood, resulting from its slow growth, may lead in some cases to a late diagnosis [6].
Various contradictory theories on the development of UA have been proposed by many authors. Some authors suggested that UA arises de novo [1], [7], others suggested that UA may develop by mural and/or luminal ameloblastomatous changes in a pre-existing dentigerous cyst because of the coexistence of non-specific epithelial lining similar to that of a dentigerous cyst and ameloblastic epithelium [8], [9], [10]. This eventuality would be further supported by the frequent involvement of an impacted tooth, mainly the third molar, and by the fact that many UAs are preoperatively diagnosed as a cyst. Both theories are likely although difficult to be proved.
In the present case, it is not possible to establish if the UA arose in a dentigerous cyst or developed from odontogenic remnants present in the region. This definitely demonstrates the diagnostic difficulty before surgery.
It is often difficult to distinguish UAs upon panoramic radiograph or CT scan images because they appear as round unilocular radiolucencies. In a clinicopathologic study, Li et al. [4] showed that 75% of UA are in fact temporarily diagnosed as an odontogenic cyst such as a dentigerous cyst or a keratocyst. Some authors attempted to differentiate UAs from odontogenic cysts using MRI [11], [12], [13], [14]. By using contrast enhanced-MRI, Konouchi et al. [14] reported distinctive features: a low signal intensity observed on T1-weighted images (WIs), a markedly high signal intensity on T2-WIs and a relatively thick rim-enhancement with/without small intraluminal nodules upon contrast enhanced-T1WIs.
In our case, the radiologic appearance of the lesion on the first panoramic X-ray was consistent with a cyst (Fig. 2), like a dentigerous cyst, associated with the second mandibular molar being still in eruption. On the other hand, the second panoramic radiography taken two years later, showed a radiolucent image being unrelated to a normally positioned second mandibular molar as well as from the third molar germ which was pushed towards the inferior alveolar canal (Fig. 1). The CT scan confirmed this finding allowing in addition to appreciate the regular contours of the lesion, an empty cavity and cortical bone lysis.
Differential diagnosis in this case might also have included a small ameloblastic fibroma, a calcifying odontogenic cyst and an adenomatoid odontogenic tumor. Among the extremely rare malignant lesions occurring in children a primary intraosseous carcinoma, and a mucoepidermoid carcinoma would have been considered [15], [16].
The histological examination of the patient's lesion revealed a plexiform UA with mural infiltration. UAs represent 5–63% of all ameloblastomas [7], [17], [18]. UAs have been classified by some authors in four histologic subtypes [7]: (1) a single cystic sac lined by ameloblastomatous epithelium, which may often be seen in focal areas (minimum criterion for diagnosis a lesion as UAs); (2) features of subtype 1 plus intraluminal proliferations; (3) features of subtype 1 plus both intraluminal and intramural proliferations; (4) features of subtype 1 plus intramural proliferations. This case showing luminal, intraluminal as well as mural ameloblastomatous proliferation belongs then to the third subtype.
Various treatment modalities for the UA have been proposed in the literature [19] with recurrence rate ranging from 10% to 40%. The surgical enucleation is considered sufficient for subtypes 1 and 2, whereas subtypes 3 and 4, which show mural invasion, should be treated more radically as a solid or multicystic form of ameloblastoma [20], [21]. However a recent study reported a recurrence rates ranging from 50% to 80% for UA treated with a conservative technique [22].
The surgical treatment of the present case consisted of careful enucleation of the whole lesion, curettage of the surgical site and extraction of third molar germ.
Clinical and radiological exams were performed at regular intervals and 30 months after the initial surgery there is no recurrence with good bone healing of the surgical site.
In conclusion, preoperative diagnosis of UA can be difficult or sometimes impossible since this variant of ameloblastoma shares significant clinical and radiographic similarities with odontogenic cysts and tumors, and because incisional biopsy may not be able to reflect the true nature of the lesion. This highlights the importance to routinely submit the removed surgical specimen for histopathological examination. Long term follow-up is mandatory because recurrence may appear years after tumor removal.
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PII: S1871-4048(08)00048-8
doi:10.1016/j.pedex.2008.06.004
© 2008 Elsevier Ireland Ltd. All rights reserved.
Volume 3, Issue 4 , Pages 192-196, December 2008
