Hybrid peripheral ameloblastoma of cheek mucosa

Abstract

An unusual case of large buccal tumour has been described. The lesion was clinically compatible with a squamous cell carcinoma, but it had pathological features of an ameloblastoma arising in the soft tissues. Only few cases of extragingival peripheral ameloblastoma were found in a review of the literature. Possible origins of these tumours are still debatable.

Background

Ameloblastoma is an odontogenic neoplasm that most often occurs in bone, and its origin has been accepted to be one of the following sources: (1) the developing enamel organ, (2) cell rests of the enamel organ, (3) epithelium of odontogenic cysts, (4) basal cells of the surface epithelium or (5) heterotopic epithelium, as in lesions that occur at extraoral sites. It can be divided into central and peripheral variety. Peripheral ameloblastoma (PA) is described as a neoplasm arising in the soft tissue overlying the tooth-bearing region, not including the buccal mucosa, the lips, the craniopharyngeal duct or the tibia. Peripheral variety is mostly considered to be gingival location; however, extragingival ameloblastoma has also been reported in the past. Extraginigval PA term was introduced for the PA occurring in an extragingival area.¹ The histogenesis of the PA is controversial. Various authors have suggested one or two potential sources of odontogenic cells: (1) the gingival remnants, (2) basal cells of gingival epithelium, (3) odontogenic remnants of the vestibular lamina, (4) pluripotent cells in the basal cell layer of the mucosal epithelium and (5) pluripotent cells from minor salivary glands. Incidence of extragingival PA is less. Of the 12 cases of PA reported so far in the literature at the extragingival location, 10 were buccal mucosa,² 1 at the base of the tongue³ and 1 in the floor of the mouth.⁴ We report the 13th case of an extragingival PA and 11th case involving the buccal mucosa, which was clinically resembling squamous cell carcinoma.

Case presentation

A 34-year-old Indian woman was referred to the dental hospital, for painless enlargement and growth in the right side of her face and in the right buccal mucosa (figure 1A,B). The patient had noticed swelling 1 year ago, however, she did not seek any medical attention at that time. Extraorally, the swelling has extended from lower border of mandible to upper zygomatic arch. The lesion had gradually increased and she had bitten intraoral swelling many times. Intraorally, the lesion in right buccal mucosa was firm and non-tender with granular texture resembling squamous cell carcinoma (SCC) initially (figure 2). It was extending superioinferiorly from lower to upper buccal vestibule and anterioposteriorly from commissar up to second molar posteriorly. It was about 4 cm in diameter. With provisional diagnosis of SCC, incisional biopsy was performed. Histopathological examination revealed the ulceroproliferative squamous epithelium resembling odontogenic epithelium with acanthomatous changes. The epithelium showed palisaded, peripheral basal cell layer with hyperchromatic with central stellate reticulum-like cell arranged in strands in plexiform pattern with acanthomatous changes (figure 3A,B). Few areas were showing the granular change within the cytoplasm of stellate reticulum-like cells (figure 4). Histologically, it was diagnosed as hybrid (acanthomatous and granular cell variant) peripheral plexiform ameloblastoma of the cheek mucosa. In our case, we were not able to diagnose PA based only on clinical findings. As the patient required complete excision with reconstructive surgery, she was referred to advanced facility for further management.

Figure 1.

(A and B) Painless enlargement and growth in right side of the face.

Figure 2.

Firm and non-tender with granular texture lesion in right buccal mucosa resembled the squamous cell carcinoma.

Figure 3.

(A and B) The epithelium showed palisaded, peripheral basal cell layer with hyperchromatic with central stellate reticulum-like cell arranged in strands in plexiform pattern with acanthomatous changes.

Figure 4.

Few areas were showing the granular change within the cytoplasm of stellate reticulum-like cells.

Investigations

Incisional biopsy was performed under general anaesthesia and tissue was sent for histopathology examination. However, it was diagnosed as plexiform ameloblastoma.

Differential diagnosis

PA is difficult to diagnose based only on clinical findings, and it can be confused clinically with a fibroma or carcinoma, including intraoral basal cell carcinoma (BCC). Clinically, lesion appearance were suggestive of oral SCC (OSCC) and too certain extent BCC. OSCC was excluded as tissue did not show any malignant features. Intraoral BCC showed a sharp demarcation between the peripheral cells and the central reticular cells, which can be helpful in differentiating the lesion.

Outcome and follow-up

The patient required complete excision with reconstructive surgery. She was referred to the advance surgical care facility for further management. However, patient did not report and could not trace that patient for the follow-up.

Discussion

PA—also known as the extraosseous ameloblastoma, soft tissue ameloblastoma, ameloblastoma of mucosal origin or ameloblastoma of the gingiva—is a very uncommon odontogenic tumour. Zhuet al⁵ stated that the term ‘Peripheral ameloblastoma’ should be used not only to describe a neoplasm arising in the soft tissues overlying a tooth-bearing region, but also to include those found in more remote locations, such as the buccal mucosa, lips, palate and other part of oral mucosa. Similarly, Shiba⁶ emphasised that ameloblastoma arising in buccal mucosa or labial mucosa should come within the same category of PA. Philipsen et al,⁷ however, in their analysis of 160 cases of PAs excluded the extragingival cases as they believed that they most likely represent basal cell adenomas with a histopathological resemblance to an ameloblastoma or the rare ameloblastoid variant of the SCC.

The first completely documented case of a PA must be attributed to Stanley and Krogh,⁸ who defined the clinical and histopathological characteristics in 1959. The aetiology of PA is unclear. Histologically, both extragingival PA and intraosseous ameloblastoma are similar but PA is limited to the soft tissue of the gingiva and does not involve any osseous structure. Based on the information available from the literature, it is reasonable to consider that buccal PA may be linked to the developmentally included enamel organ or its remnants as parts of the buccal mucosa during the embryogenesis of the vestibular lamina.⁹ The misplaced odontogenic epithelium could well explain the pathogenesis of unusual buccal lesions of tooth-forming teratoma¹⁰ and keratocystic odontogenic tumour.¹¹ In buccal PA, the frequent observation of a focal connection between the tumour and the covering epithelium must be kept in mind. Because cases that had the surface epithelial contact invariably showed ulceration, this change appears to be a secondary event resulting from local trauma and not a point of origin. Champion et al¹² stated that the oral mucosa had the potential to differentiate into ameloblastic cells, while Klinar et al believed that the buccal mucosa may have an abortive odontogenic potential.

PA is rarely the initial preoperative diagnosis and the final diagnosis requires histological evaluation.⁷ Histologically, the tissue is comprised of odontogenic epithelial islands and strands, resembling the follicular ameloblastoma. Sometimes, these strands and islands demonstrate the acanthomatous changes with central areas of keratin formation, occasionally the cystic pattern. However, the epithelial islands and strands may be surrounded by fibrous tissue.^{5 13–16} In the present case, histopathologically, the lesion showed features similar to acanthomatous as well as granular cell ameloblastoma. Only one case of extragingival ameloblastoma with two histological types/variants was reported. Majority extragingival PA was of follicular type whereas in case of PA¹⁷ the most common histological pattern is plexiform rather than follicular or acanthomatous. Simpson¹⁸ reported that because of the location and histopathological features it is possible to mistake PA for BCC. Greer and Hammond¹⁹ believed that the PA exhibits central polarisation of the nuclei of the peripheral cells, while the true intraoral BCC shows a sharp demarcation between the peripheral cells and the central reticular cells. On the other hand, Gardner²⁰ considered these two lesions to be indistinguishable. Tjioe et al²¹ were in view that the incipient odontogenic tumours often display intermediate features between two or more lesions leading to diagnosis dilemma. Philipsen et al documented three lesions which may be considered by the pathologist in the differential diagnosis of PA: peripheral odontogenic fibroma; peripheral variant of the squamous odontogenic tumour; and odontogenic gingival epithelial hamartoma. When a PA arises on the edentulous alveolar mucosa in denture-wearing patients, it may be incorrectly diagnosed as denture irritation hyperplasia. The fibrous capsule was part of histological findings but it was also observed that multicystic and solid type of intraosseous ameloblastoma and PAs usually do not possess fibrous capsules. It is unknown why extragingival PA possesses fibrous capsules, but this clinical characteristic would suggest the benign nature of the extragingival PA. From treatment of the tumour,²² its prognosis is considered better that its intraosseous counterpart. Nevertheless, Woo et al stated²³ that recurrences were observed in all cases that were treated by simple local excision of the lesion. Follow-up is very important after surgical treatment as it very difficult to predict the biological behaviour of the PA.²⁴

In conclusion, extragingival PA of cheek mucosa is an uncommon and rare odontogenic neoplasm. As the isolated BCC of the oral cavity has also been reported, a careful histopathological evaluation is necessary to differentiate PA from BCC of the oral cavity and all possible cases should be evaluated by immunohistochemistry. Microscopic examination of the specimens is needed throughout to ensure that the margins are clear of tumour. Long-term follow-up is needed to ensure no late recurrence in treated cases of PA.

Learning points.

• Extragingival peripheral ameloblastoma (PA) of cheek mucosa is an uncommon and rare odontogenic neoplasm.

• A careful histopathological evaluation is necessary to differentiate PA from squamous cell carcinoma and basal cell carcinoma of the oral cavity and all possible cases should be evaluated by immunohistochemistry.

• Long-term follow-up is needed to ensure no late recurrence in treated cases of PA.