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Case of the Month - June 2009

Peripheral Ameloblastoma - A Rare Case Report

Abstract:
Peripheral ameloblastoma is a rare benign odontogenic tumor defined as an ameloblastoma that is confined to the gingival or alveolar mucosa. It infiltrates the surrounding tissues; basically the gingival connective tissue like any other ameloblastoma but it does not invade the underlying bone. The peripheral ameloblastoma is thought to arise from rests of the dental lamina or from basal cells of the surface epithelium. Histologically, the peripheral ameloblastoma resembles the intraosseous form, consisting of ameloblastic epithelium against a background stroma of fibrous connective tissue and in proximity to surface epithelium. A rare case of peripheral ameloblastoma has been reported based on the clinical, radiographic and histological findings.

Introduction:
The peripheral or extraosseous ameloblastoma is a rare benign odontogenic tumor1 which histologically resembles the typical central or intraosseous ameloblastoma, but occurs in the soft tissue outside and overlying the alveolar bone. The peripheral lesion is relatively innocuous and lacks the persistent invasiveness of the intraosseous lesion. It was first described in the literature by KURU in 19112 .The reported incidence of peripheral ameloblastoma varies between 1 and 5% of all ameloblastomas .This lesion generally occurs in the 4th and 5th decades of life, predominantly in the mandible especially in the premolar region.

The peripheral ameloblastoma is thought to arise from the cell rests of the dental lamina or from the basal cells of the surface epithelium .Clinically it is usually a painless , non-ulcerated, sessile or pedunculated gingival or alveolar mucosal lesion ,measuring less than 1.5 cm .Radiographically a few cases may exhibit a type of bony involvement referred to as cupping or saucerization representing pressure resorption of underlying bone. Histologically, the peripheral ameloblastoma resembles the intraosseous form, consisting of ameloblastic epithelium against a background stroma of fibrous connective tissue and in proximity to surface epithelium .Criteria for diagnosis of peripheral ameloblastoma have included features such as an origin from overlying epithelium, the presence of odontogenic epithelial islands and lack of potential to bone infiltration. This article describes the clinical and microscopic features of peripheral ameloblastoma.

Case report:
A 25-year-old male patient reported to the out patient department of Sibar Institute of Dental Sciences for the complaint of painless swelling in the lower right canine-premolar region of 3 months duration. History revealed that, since the onset of the swelling, there was no alteration in size. The medical and dental history of the patient was non-contributory. The patient did not give any history of trauma to the involved area.

Extra-oral examination of the head and neck did not show any abnormalities. On intraoral examination a non tender, firm, pedunculated swelling measuring 1x1cm was present on the gingiva of 43, 44, & 45 region covered by normal mucosa. Bleeding on probing also seen. The teeth in relation to the mass revealed no pathology. OPG revealed no abnormalities (fig-1) . The patient was not diabetic, not hypertensive and the routine hematological findings were within normal limits. A provisional clinical diagnosis of pyogenic granuloma was made.

Excisional biopsy of the lesion was done under local anesthesia. Microscopic examination of the biopsy specimen revealed an overlying stratified squamous epithelium with hyperplasia in some areas (fig-2,3) .Beneath the epithelium within the connective tissue region numerous odontogenic epithelial islands are evident resembling ameloblastoma like tissue ( fig-4) . The tumor cells show follicular pattern mostly and plexiform arrangement in some areas. The peripheral cells are short columnar and cuboidal and the central cells resemble stellate reticulum cells (fig-5). Within some follicles, the central cells reveal squamous metaplasia and also keratin pearl formation is evident in one or two areas (fig-6) .A band of collagen fibres separate the overlying surface epithelium from the underlying tumor cells. The supporting connective tissue stroma seems to be delicate with few blood vessels. A diagnosis of peripheral ameloblastoma was made based on the histopathological features.

The patient was advised to come for a regular follow-up. Six months after the excisional biopsy there was no recurrence.

Discussion:
The peripheral ameloblastoma is a rare, benign, extraosseous odontogenic soft tissue tumor that was first reported in the literature by KURU in 19112. According to Buchnor and Scuibba 8 peripheral ameloblastoma is defined as a tumor with the histological characteristics of an intraosseous ameloblastoma but occurring in the soft tissue overlying the tooth bearing regions of the maxilla and mandible. In our case the lesion was seen in canine-premolar region of lower right mandible.

In resemblance to the present case, clinically the peripheral ameloblastoma usually presents as a painless, nonulcerated, sessile or pedunculated , gingival or alveolar mucosal lesion with normal coloured smooth surface.

The occurrence of peripheral ameloblastoma is seen in a significantly older age group, average age of 51yrs8. The case reported here was found in a 25-year- old patient, which is uncommon.

In resemblance to the present case most tumors are smaller than 1.5 cm in diameter and the most common sites of occurrence are premolar or anterior regions of the mandible9. Bone involvement is rare, when it occurs, in the form of superficial erosion than neoplastic invasion.

Several histopathological patterns of peripheral ameloblastoma include follicular, plexiform, acanthomatous, granular cell, and basal cell pattern. Among these, plexiform and follicular patterns are very common. In the present case, the biopsy specimen revealed an overlying stratified squamous epithelium with hyperplasia in some areas .Beneath the epithelium within the connective tissue region numerous odontogenic epithelial islands are evident resembling ameloblastoma like tissue. The tumor cells show follicular pattern mostly and plexiform arrangement in some areas. The peripheral cells are short columnar and cuboidal and the central cells resemble stellate reticulum cells. Within some follicles, the central cells reveal squamous metaplasia and also keratin pearl formation is evident in one or two areas. A band of collagen fibres separate the overlying surface epithelium from the underlying tumor cells. The supporting connective tissue stroma seems to be delicate with few blood vessels. The above said features suggest that it is a case of peripheral ameloblastoma.

The accepted surgical treatment of peripheral ameloblastoma involves excision of the lesion down to periosteum, including a small amount of normal tissue without removal of teeth6. Recurrences are rare.

References:
1. Gurol M,Burks EJ Jr. Peripheral ameloblastoma. J.Periodontal 1995 ; 66 : 1065-8
2. Zhu Ex, Okada N, Takagi M. Peripheral ameloblastoma : case report and review of literature : J Oral Maxillofacial Surgery 1995 ; 53 : 590-4
3. Lin SC, Lieu CM, Hahn LJ, Kwan HW. Peripheral ameloblastoma with metastasis. Int .J.Oral Maxillofacial Surgery 1987 ; 16 :202-6
4. Susan F.Connolly, Stephen Sonic, Peter B.Lockhart. An unusually located early peripheral ameloblastoma. J Oral Surgery 1984, 50 : 180
5. Hescellio Mastelli-Junior, Leandro N. Souza, Luis Antonia Nogueria Santos, Mario R .Melo.Filho, Alfredo M.B.De Paula. Peripheral ameloblastoma : A case report. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005; 99: E 31 -3
6. Alfredo M. B.De Paule. Peripheral ameloblastoma : A Case report. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005; 99: E 31 -3.
7. Nevelli, Damm, Allen, Bouquot. Oral and Maxillofacial Pathology; 2nd edition: Elsevier publications; India; 2004; 611-18.
8. Tajima et al.Peripheral ameloblastoma with potentially malignant features: report of a case with special regard to its keratin profile. J Oral Pathol Med 2001:30:494-8.

Appendix

Fig 1 - Orthopantamograph	Fig 2 - Microphotograph-4x	Fig 3 - Microphotograph-4x

Fig 4 - Microphotograph-10x	Fig 5 - Microphotograph-10x	Fig 6 - Microphotograph-4x

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