‘A harmless but confusing tumour on the anterior maxilla’

Abstract

Squamous odontogenic tumour (SOT) is a very rare benign neoplasm probably arising from rests of Malassez. Patients may present with an increase in the volume of the maxilla or mandible, tooth mobility, ulceration of the oral soft tissue, painful symptoms and tooth displacement. Radiographic features of SOT consist of a triangular-shaped radiolucent lesion adjacent to the roots of teeth. Histologically, care should be taken not to misdiagnose this condition as acanthomatous ameloblastoma or well-differentiated squamous cell carcinoma. The authors are presenting a case of a 65-year-old male patient who presented with a painless swelling and diagnosed to be having SOT.

Background

Squamous odontogenic tumour (SOT) is a rare lesion with few cases reported to date. SOT may be histopathologically misdiagnosed as ameloblastoma, particularly the acanthomatous and the desmoplastic variants, well-differentiated squamous cell carcinoma, or pseudo-epitheliomatous hyperplasia similar to a keratoacanthoma. The histopathological characteristics of SOT may thus lead to errors in the microscopical diagnosis, resulting in unnecessary, mutilating surgery. In fact, in several of the published cases, the initial histological diagnosis was squamous cell carcinoma or acanthomatous ameloblastoma. It is also likely that on rare occasions, a case of acanthomatous or desmoplastic ameloblastoma may be misdiagnosed as a SOT with the unfortunate result of inadequate treatment. Radiographically, SOT presents as a triangular well-defined radiolucency between the roots of teeth, which would contrast with the multilocular appearance of an ameloblastoma. An oral radiologist would be instrumental in taking into consideration SOT in the differential diagnosis when coming across triangular radiolucencies between teeth and be of assistance in guiding the pathologist to the right diagnosis.

Case presentation

A 65-year-old man had consulted an ear, nose, throat surgeon for head ache, nasal discharge with occasional nasal bleeding without anosmia. He complained of altered taste in the mouth when he had nasal discharge. They arrived at a working diagnosis of inverted papilloma and asked for investigations. Having noticed a painless swelling of the right maxilla, he was referred to the oral physician. The patient was moderately nourished with an unremarkable medical history. He was unaware of the swelling and his dental history was not significant.

Extraoral examination revealed a diffuse swelling over the right anterior maxilla extending from the right to the philtrum. The skin over the swelling appeared normal and the temperature was not raised. The swelling was hard and non-tender on palpation (figure 1).

Figure 1.

Photograph of the patient with swelling over the anterior maxilla.

Intraorally, a diffuse, hard, non-tender swelling was noted in the attached gingiva extending from the region of the right maxillary lateral incisor to the left maxillary central incisor (figure 2). The mucosa over the swelling appeared normal with no secondary changes. A diffuse swelling measuring 2×2 cm was also noted on palatal aspect which was firm in consistency and non-tender and yielding in some areas (figure 3). There was displacement of the central incisor to the right and the oral hygiene was fair. A provisional diagnosis of extra follicular adenomatoid odontogenic tumour was arrived at.

Figure 2.

Intraoral photograph of the swelling extending from the attached gingival of right lateral incisor to the left central incisor.

Figure 3.

Photograph of the well-defined circumscribed swelling on the palate.

Investigations

A set of radiographs were made which included periapical, occlusal, peripheral nervous system (PNS) panoramic views and CT. Intraoral periapical radiogragh showed a unilocular triangular radiolucency between the central and the lateral incisor of the right side approximately 4×3 cm causing displacement of both the teeth (figure 4). Panoramic radiograph revealed a well-circumscribed, unilocular triangular radiolucency between maxillary lateral and central incisors (figure 5). Maxillary cross sectional occlusal radiograph showed a triangular radiolucency extending to the palate (figure 6). PNS showed opacification of the left maxillary sinus (figure 7). CT showed a massive lesion in the maxillary sinus but did not show the oral lesion (figure 8). Aspiration was negative.

Figure 4.

Intraoral periapical radiogragh showing a unilocular triangular radiolucency.

Figure 5.

Panoramic radiograph showing a well-circumscribed, unilocular triangular radiolucency between maxillary lateral and central incisors.

Figure 6.

Occlusal radiograph showing triangular radiolucency extending to the palate.

Figure 7.

Peripheral nervous system showing opacification of the left maxillary sinus.

Figure 8.

CT showing a massive lesion in the maxillary sinus but did not show the oral lesion.

Differential diagnosis

1. Fibrous dysplasia

2. Ossifying fibroma

3. Ameloblastoma

4. SOT.

Treatment

Incisional biopsy was done and was sent to oral pathology and microbiology department for final evaluation.

Outcome and follow-up

Microscopic examination of the lesion showed irregularly shaped islands of odontogenic epithelial cells in mature fibrous connective tissue stroma. The odontogenic epithelial islands were round to oval in shape and were lined by flat to cuboidal odontogenic epithelial cells and centrally placed squamous cell (figure 9). A biopsy report of SOT was obtained. Excisional of the tumour was done and the patient has been regularly followed up and does not show any signs of recurrence.

Figure 9.

Photomicrograph of squamous odontogenic tumour.

Discussion

SOT is a very rare benign neoplasm with few cases reported to date. In the recent WHO classification of odontogenic tumours¹ SOT has been defined as: a benign but locally infiltrative neoplasm consisting of islands of well-differentiated squamous epithelium in a fibrous stroma with epithelial islands occasionally showing foci of central cystic degeneration. Pullon et al² were the first to describe it. They reported six cases and established the diagnostic criteria and surgical approaches that are still followed today. Typically, lesions are often asymptomatic but may present with mobility of involved teeth, pain and tenderness to percussion and occasionally abnormal sensation.^3–5 Our patient did not have pain or mobility of teeth but there was displacement of teeth.

The SOT is a hamartomatous proliferation of mature epithelial cells that probably arise from the rests of Malassez.⁴ Epithelial cell rests of Malassez (ERM) are quiescent epithelial remnants of Hertwig’s epithelial root sheath (HERS) that are involved in the formation of tooth roots. After completion of crown formation, HERS is converted from cervical loop cells, which have the potential to generate enamel for tooth crown formation. Cervical loop cells have the potential to differentiate into ameloblasts. Generally, no new ameloblasts can be generated from HERS, however studies demonstrated that subcultured ERM can differentiate into ameloblast-like cells and generate enamel-like tissues in combination with dental pulp cells at the crown formation stage.⁶

SOT presents as a painless expansion of the alveolar process that may displace teeth, cause tooth mobility, or resorb roots. It generally occurs in adult life around the age of 40 years, but it has a wide age range. It is a rare tumour for which statistics may not be accurate, but it has been suggested to favour the premolar canine region of the maxilla and the molar region of the mandible. It may occur in a familial pattern in which several members of the same family have one or more lesions, or it may occur without a familial pattern as multiple separate lesions in individuals.⁷

Radiographically, most tumours are well-demarcated, unilocular triangular radiolucencies limited in size to 3 cm or less. In rare instances, they have grown larger. Nearly all are confined to the alveolar bone.⁴

The location and cellularity of the SOT suggests its origin from the epithelial rests of Malassez. The fact that these stimulated rests result in a solid proliferation of squamous cells rather than a lateral periodontal cyst suggests that the lateral periodontal cyst arises from the dental lamina rests rather than the rests of Malassez. It may also be that the stimulus or the genetic alteration that causes each of these lesions is different yet may affect either the rests of Serres or the rests of Malassez.^{4 7}

The SOT is sufficiently rare as to be an unexpected diagnosis when it is discovered. Its alveolar location in the adult suggests cysts and tumours that are more commonly seen in this area, in particular an odontogenic keratocyst and a lateral periodontal cyst. The botryoid odontogenic cyst variant of the lateral periodontal cyst may also be considered. In addition, early true odontogenic neoplasms such as the ameloblastoma and odontogenic myxoma are important considerations.⁴

Most such presentations require direct exploration and removal by enucleation and curettage. Periapical radiograph or a panoramic radiograph is sufficient. The tumour is usually firm and encapsulated within its bony crypt. The resultant defect may create a periodontal pocket or become a residual periodontal defect. To reduce this possibility, maintenance of the crestal alveolar bone is important. If the crestal ridge of bone is lost, a membrane technique to inhibit fibrous tissue replacement may be used.^{4 7}

The SOT consists of islands of well-differentiated squamous epithelium that lack peripheral columnar cells. The cells are uniform and benign in appearance. There may be vacuolation and formation of microcysts. Individually keratinised cells may be present. Intraepithelial calcifications, probably dystrophic in nature, and hyalin globules that are negative histochemically for amyloid may be seen. The stroma is a mature collagenised fibrous tissue that may contain some inflammatory cells.⁵

Recurrences are extremely rare and are related to the encumbered surgical access between and behind roots. If the entire visible portion of the lesion is removed, recurrence is not seen. However, new lesions arise in a different location in as many as 20% of patients.⁷

Leider et al⁸ reported three cases of SOT in siblings, which suggested a possible familial pattern in the occurrence of this lesion. Radiographs of the central variant show a well-defined unilocular and triangular radiolucency in the alveolar process localised between the roots of teeth. Some of the rare extensive lesions may have a multi-locular appearance involving the body of the mandible or pushing aside the maxillary sinus. A few cases have been found in association with an embedded tooth.

According to Hopper et al,⁹ the type of radiographic border might be helpful in defining the type of treatment to be adopted because a more aggressive lesion has poorly defined radiographic borders.

Cases involving an entire jaw quadrant as well as muitiple lesions have been reported.^{2 8 10–14} The majority of cases seemed to arise and develop in the periodontium of the permanent dentition. In one case reported by Schwarz-Arad et al¹⁵ in 1990 the tumour was associated with the deciduous dentition and in four cases,^{12 14 16 17} the lesions were detected in edentulous areas.

The tumour must be differentiated from so-called ‘SOT-like islands’ arising in the walls of odontogenic cysts.^{9 16–22} The latter lesion should not be considered a neoplastic growth but rather a reactive, inflammatory hyperplasia of the epithelial cyst lining.

In 1979, Wright²⁰ reported five cases of SOT-like proliferation occurring in the walls of odontogenic cyst. The question of whether these lesions arise by ‘budding off’ of the cystic lining or from independent proliferation of mural odontogenic rests remains to be answered. Although it has an infiltrative pattern of growth, the reported follow-up of SOT, which is substantial, indicates that they are no more aggressive than the odontogenic cyst with which they are associated.

However in 1985, Hietanen et al²³ reported a recurrent case of peripheral SOT in a 24-year-old woman.

SOT is characterised histologically by the formation of islands of different sizes and shapes made up of of well-differentiated squamous epithelium. The islands are mostly of a rounded shape but in several reported cases,^{2 8 24 14 25} the islands appear to be very irregular and cord-like. This pattern greatly resembles the islands seen in the desmoplastic ameloblastoma. However, the individual islands have an outer layer of low cuboidal or more often quite flat cells with little or no similarity to those of ameloblastoma islands. The centres of these islands may undergo microcystic degeneration of the spinous cells often following single-cell keratinisation. However, lack of polarisation of peripheral cells in the epithelial islands, which is typical of ameloblastomas, is a differential criterion in favour of SOT.

Immunohistochemical studies^{26 27} of the SOT have confirmed the proliferative activity of the odontogenic epithelium indicated by heavy staining for keratin 13/16, and the squamous differentiating cells in the centre of the tumour islands have shown a strong positive reaction for involucrin staining.²⁷ In the latter study, ameloblastomas showing acanthomatous and follicular patterns with foci of squamous differentiation were also positive. Transmission electron microscopy^{2 18} of tumour cells reveal mature squamous epithelial cells with intercellular oedema as seen in the stratum spinosum of the oral mucosa. A case of SOT was reported to have transformed into intraosseous squamous cell carcinoma. In this case, p53 positive was reported as a prognostic indicator. Jwa-Young Kim et al’s²⁸ case study also showed p53 immunopositive. Expression of p53 in many reactive lesions is unlikely to be the consequence of gene mutations.

Since the SOT is considered a benign odontogenic neoplasm, therapy mainly consists of enucleation, curettage or local excision. Tumours located to the maxilla may, as mentioned previously, need more radical treatment because of a more aggressive biological behaviour.

Learning points.

• Care should be taken not to misdiagnose this condition histologically as acanthomatous ameloblastoma or well-differentiated squamous cell carcinoma thus avoiding mutilating surgery.

• Acanthomatous or desmoplastic ameloblastoma may be misdiagnosed as a SOT with the unfortunate result of inadequate treatment.

• Radiographically, the typical picture of SOT is a triangular radiolucency with a sclerotic border in between the roots of teeth and causing divergence of teeth, where as typical picture of ameloblastoma is a multi-locular radiolucency with either honeycombed or soap bubble appearance.