A rare case report of acanthomatous ameloblastoma based on aspiration cytology with histopathological confirmation

INTRODUCTION

Ameloblastoma is a benign progressively growing intraosseous epithelial odontogenic tumor. It is characterized by expansion and a tendency for a local recurrence if inadequately removed. It accounts for 1% of all tumors of jaw and 18% of all odontogenic tumor.^[1] FNAC has an advantage of rapid diagnosis, cost-effectiveness, and it is a minimally invasive procedure. However, cytological diagnosis is challenging when it mimics as a salivary gland tumor.

Since it has an aggressive clinical course, surgical removal and histopathological examination should be done for appropriate management.^[2]

CASE REPORT

A 52-year-old female patient came to our institute with a complaint of swelling in lower midline of gum with extension to posterior gum and discharge for 8 months [Figure 1].

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Figure 1:

Patient photograph showing diffuse swelling on lower gum.

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The patient has a history of bleeding gum and swelling 25 years back, for that she underwent tooth extraction in a private clinic. Subsequently, swelling subsided partially. The patient again developed gum swelling and pus discharge for which the patient was again subjected to tooth extraction in a private clinic. However, this time her symptoms persisted, and for this reason, the patient visited our institute.

Examination revealed a swelling in the right lower alveolus which extended from the right posterior region to midline. On palpation, the lesion was firm and non-tender, appeared to arise from mandibular bone. Overlying skin was normal in color.

Radiological examination: Magnetic resonance imaging (multiplanar and multisequence) was done and it was found that heterogeneous expansile T2 hyperintense T1 hypointense mass lesion measuring ~ 24 × 27 × 32 mm in maximum AP × TR × CC dimension involving symphyseal and parasymphyseal region of mandible. Focal expansion of overlying buccal and lingual cortex of mandible present. Multiple intralesional bony septa present. T2 hyperintense cystic area measuring ~ 6 × 8 mm was seen in this mass lesion. Mild thickening of adjacent lower gingivolabial sulcus and oral aspect of lip present. Intense heterogeneous enhancement of solid component was seen on post-contrast images. It was reported as expansile multiseptate mandibular symphysis mass, possibly ameloblastoma with no significant cervical lymphadenopathy.

Before visiting to our institute, FNAC was done from the right lower alveolus in a private pathology laboratory, a yellow-colored blood mixed fluid was aspirated which was reported as pleomorphic adenoma with focal squamous metaplasia. These slides were reviewed in our institute which showed moderately cellular smears having cohesive clusters of basaloid cells with focal peripheral palisading on an hemorrhagic background. Cells displayed oval to round hyperchromatic nuclei, fine chromatin, inconspicuous nucleoli, and scant amount of basophilic cytoplasm. Clusters of squamous epithelial cells were also seen. There were no nuclear atypia, necrosis, or mitosis evident in the smears examined [Figures 2-4]. It was reported as suggestive of odontogenic tumor, favoring ameloblastoma. Biopsy and histopathological examination were advised for confirmation.

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Figure 2:

FNAC photomicrograph showing cells with stromal tissue (H&E ×10 view).

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Figure 3:

FNAC photomicrograph showing sheets of cells in cohesive clusters of basaloid cells with peripheral palisading (H&E ×20 view).

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Figure 4:

FNAC photomicrograph showing squamous cells centrally (H&E ×20 view).

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On the basis of FNAC report, clinicoradiological correlation and after taking consent from the patient tumor were surgically enucleated along with curettage of the site. Excised specimen was sent for histopathological examination to pathology department of our institute.

Specimen comprised multiple gray-white soft-tissue pieces, in aggregate measured 4.5 × 2.5 × 1.0 cm. It was entirely submitted. A small bony bit was also received measuring 1.8 × 1.5 × 0.8 cm, grossly unremarkable. Bony bit was entirely submitted.

Hematoxylin and eosin-stained sections were prepared which showed tumor composed of epithelial islands lined by columnar cells displaying peripheral palisading of nuclei with reverse polarization. These cells have hyperchromatic nuclei and pale eosinophilic cytoplasm. Central zone of the stellate reticulum-like cells showed squamous metaplasia and keratinization. Tumor cells were surrounded by fibrocollagenous stroma. No atypical or malignant cells seen [Figures 5 and 6]. Based on the clinical, radiological, cytological, and histopathological examination, a final diagnosis of acanthomatous ameloblastoma was made.

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Figure 5:

Photomicrograph showing odontogenic epithelial islands with cystic changes and squamous metaplasia surrounded by connective tissue stroma (H&E ×10 view).

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Figure 6:

Photomicrograph showing areas of ameloblast-like cells, individual cell keratinization, and squamous metaplasia (H&E ×20 view).

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DISCUSSION

Ameloblastoma originates from dental lamina. A mean age of presentation is 34 years in cases with BRAFV600E mutation as compared to 54 years for BRAF wild type.^[3] It accounts for 1% of all cysts/tumors of jaws and 18% of all odontogenic neoplasms.^[1,4,5] Frequency of occurrence in male and female is equal when the tumor is not associated with an unerupted tooth; however, male-to-female ratio is of 1:1.8.^[4]

Ameloblastoma accounts for only 1% of all oral tumors and 11–18% of odontogenic tumors.^[6] Acanthomatous type is more commonly occurs on the mandible than the maxilla, mandibular ramus region involvement is seen in 80% of cases.^[6] Acanthomatous ameloblastoma is a slow-growing and painless expansile lesion, which can later exhibit accelerated growth and associated with complications. It can be fatal if not treated appropriately.^[7]

On radiology, ameloblastoma is mostly a radiolucent lesion although it is not a pathognomonic finding. Radiological appearance is also known as soap bubble or honeycomb.^[7] Grossly, ameloblastomas range from entirely solid, solid cystic, to entirely cystic.

Variants that are seen in most of the solid lesions are follicular ameloblastoma (64.9%), followed by the plexiform (13.0%), desmoplastic (5.2%), and acanthomatous (3.9%).^[8]

FNAC is a well-established minimally invasive and low-cost investigation for early and accurate diagnosis of jaw lesions.^[9] When characteristic components present on FNAC smears, diagnosis is not difficult. However, in our case, FNAC of mandibular lesion done outside reported as Pleomorphic Adenoma. Due to the presence of basaloid cells in smears, pathologist might get confused with cellular pleomorphic adenoma; however, our careful search showed absence of chondromyxoid fibrillary stroma and plasmacytoid myoepithelial cells and presence of basaloid cells with focal peripheral palisading and clusters of bland squamous cells.^[10] This finding has resolved the issue. Nevertheless considering solid cystic lesion, location of tumor and basaloid cell population in cytosmears possible differentials other than cellular pleomorphic adenoma could be adenoid cystic carcinoma and basal cell adenoma. The absence of basement membrane material and hyaline globules and the presence of squamous cells in ameloblastoma have resolved the problem. Adenoid cystic carcinoma shows basaloid cells with hyperchromatic nuclei and nuclear molding. The presence of hyaline globules and the absence of squamous cells help in differentiating it from acanthomatous ameloblastoma.^[11] A careful search should be made for the presence of mucous cells, atypical squamous cells, and dirty background to rule out mucoepidermoid carcinoma.^[12] The presence of bland squamous cells, basaloid cells, and stromal component favor diagnosis of acanthomatous ameloblastoma over metastatic squamous cell carcinoma. The presence of all the components helps in differentiating acanthomatous ameloblastoma from the other benign cystic lesions of the jaw^[13] [Table 1].

Histopathological findings of acanthomatous ameloblastoma show tumor islands composed of peripheral cells which are columnar to cuboidal (ameloblast-like) with hyperchromatic nuclei arranged in a palisading pattern with reverse polarity. Squamous metaplasia and formation of keratin are seen in the central stellate reticulum. Ki-67 index assessment for cellular proliferative activity is done which is an important prognostic marker for behavior, tumor recurrence, and invasiveness.^[14]

On genetic analysis, MAPK pathway is seen in almost 90% of all ameloblastomas with BRAFV600E being the most common mutation. Non-MAPK tends to co-occur with the MAPK pathway includes SMO, SMARCB1, CTNNB1, and PIK3CA mutations, among which SMO mutations are very common.^[15]

The current treatment protocol is wide surgical excision, including an area of bone beyond radiographical margins. A high recurrence rate (60–80%) is often associated with conservative surgery. A novel option to complement surgery for aggressive and/or recurrent ameloblastoma is BRAF-targeted therapy.^[16]

SUMMARY

To the best of our knowledge, very few cases of acanthomatous type of ameloblastomas have been diagnosed on aspiration cytology followed by excision and histopathological confirmation. Early diagnosis by cytology helps in early treatment by surgical excision of this locally aggressive tumor.