Plasmacytoid Myoepithelioma of the Hard Palate: Case Report.

BACKGROUND: Myoepitheliomas are uncommon salivary gland neoplasms consisting entirely or predominantly of cells with myoepithelial phenotype. They commonly involve the parotid gland and the minor salivary glands of the palate. AIM: A case of plasmacytoid myoepithelioma of the hard palate is described. CASE DESCRIPTION AND
RESULTS: A 55-year-old woman presented to her oral surgeon with a tumor on the hard palate. Microscopic examination showed a well-circumscribed but non-encapsulated tumor, consisting mostly of plasmacytoid cells in a loose fibrovascular stroma. Neoplastic myoepithelial cells showed immunoreactivity for S-100 protein, CK AE1/AE3 (Figure 5b), GFAP, calponin, and CD138/Syndecan-1. Total excision of the tumor under local anesthesia was performed and no recurrence was noted 14 months after treatment.
CONCLUSIONS: Since plasmacytoid myoepithelioma is uncommon, minor salivary glands, its immunohistochemical features, management and prognosis should be further investigated.

Introduction

Myoepitheliomas are benign salivary gland neoplasms consisting entirely or predominantly of cells with myoepithelial phenotype (, ). They represent approximately 1.5% of all salivary glands tumors () and usually involve the parotid gland or the minor salivary glands of the palate (-). They show no age or gender predilection, but they are more common in middle-aged persons (-). They present as asymptomatic, slowly growing and well-circumscribed tumors of normal color that may be soft to hard on palpation [2-4], and are not associated with neurological symptoms (, ).

Microscopically, one or all types of neoplastic myoepithelial cells, i.e. spindle, plasmacytoid (hyaline), epithelioid, clear, polygonal, basaloid or oncocytic, may be seen, arranged in solid, myxoid, reticular, microcystic or cribiform growth patterns (, , ). The preponderant cell type defines the tumor’s subtype, although neither cell type nor growth pattern correlate with the clinical presentation or biologic behavior of the lesion (). The spindle cells subtype is more common in the parotid gland and the plasmacytoid in the minor salivary glands of the palate (). Whether myoepithelioma is a distinct entity or a variant of pleomorphic adenoma with a preponderance of cells with myoepithelial phenotype is disputable.

Since the review of the English literature by Zormpa et al () in 2011, where 19 cases of plasmacytoid myoepithelioma of the hard palate were included, three more cases have been published (-). An additional case of plasmacytoid myoepithelioma of the hard palate is described.

Case report

A 55 year-old woman was referred for diagnosis and management of a painless swelling on the hard palate that had gradually enlarged during the last few months. Family and past medical histories were non-contributory.

Oral examination revealed a round, well-circumscribed mass covered by normal mucosa on the right posterior hard palate, between the premolar teeth and the midline (Figure 1). It measured approximately 2.5x2x1.5cm and was compressible and non-tender on palpation. The first molar tooth did not react to pulp testing, but adjacent teeth were vital. There was no regional lymphadenopathy. Panoramic radiograph and dental scan showed a hypodense mass that did not involve the maxillary cortical bone, and identified a cystic lesion apically to the first molar tooth (Figure 2). With the clinical diagnosis of a salivary gland tumor, an incisional biopsy was done that rendered the diagnosis “consistent with a pleomorphic adenoma”. Consequently, total excision of the tumor under local anesthesia was performed. Postsurgical healing was uneventful and no recurrence has been recorded 14 months after treatment (Figure 3).

Figure 1

Oral examination. A round, well-circumscribed mass covered by normal mucosa on the right posterior hard palate, between the premolar teeth and the midline.

Figure 2

Dental scan. The hypodense mass does not involve the maxillary cortical bone. A cystic lesion is seen apically to the first molar tooth (asterisk).

Figure 3

Follow up. Postsurgical healing 14 months after treatment.

Microscopic examination of 5μm thick formalin-fixed and paraffin-embedded tissue sections showed a well-circumscribed but non-encapsulated tumor, consisting of solid sheets and nests of neoplastic epithelial cells embedded in a loose fibrovascular stroma (Figure 4a). Most cells showed plasmacytoid (hyaline) myoepithelial features, i.e. abundant eosinophilic neoplasm and an oval, slightly dense, eccentric nucleus (Figure 4b). Small groups of spindle-shaped cells with dense nuclei were also observed (Figure 4c). There was a minimal cellular and nuclear pleomorphism and there were no atypical mitoses. Rare ductal structures were seen, but constituted <2% of the total tumor parenchyma, while acinar differentiation was absent. Hemorrhage, inflammation and pseudoepitheliomatous hyperplasia of the covering parakeratinized mucosa were seen in the site of the incisional biopsy.

Figure 4

Microscopic features. (a) Solid nests of neoplastic epithelial cells embedded in a loose vascular connective tissue stroma (hematoxylin and eosin stain, original magnification x200). (b) Plasmacytoid and (c) spindle-shaped myoepithelial cells (hematoxylin and eosin stain, original magnification x400).

Immunohistochemistry was performed with a standard avidin-biotin peroxidase technique after pretreatment with high temperature citrate buffer, with antibodies against S-100 protein (polyclonal, 1:100, Dako, Glostrup, Denmark), cytokeratin (CK AE1/AE3, 1:50, Dako), glial fibrillary acidic protein (GFAP, 6F2, 1:100. Dako), calponin (CALP, 1Q50, Dako), a-SMA (HHF-35, 1:50, Dako), p63 (VS38c, 1:100, Dako), CD138/Syndecan-1 (B-A38, 1:50, Abcam, Cambridge, MA, USA), and Ki-67 (Mib1, 1:500, Dako). Plasmacytoid and spindle cells showed intense cytoplasmic positivity for S-100 protein (Figure 5a), and plasmacytoid cells diffuse cytoplasmic positivity for CK AE1/AE3, GFAP (Figure 5b), and calponin, scant cytoplasmic positivity for a-SMA and p63, and strong, membranous positivity for CD138/Syndecan-1 (Figure 5c). Ki-67 proliferation index was <1%. The diagnosis was plasmacytoid myoepithelioma.

Figure 5

Immunohistochemical features of plasmacytoid cells. (a) An intense cytoplasmic positivity for S-100 protein and (b) GFAP. (c) Strong, membranous positivity for CD138/Syndecan-1 (original magnification x400).

Discussion

The case presented herein showed clinical, microscopic and immunohistochemical features consistent with plasmacytoid myoepithelioma. Panoramic radiograph and dental scan found no sign of erosion of the adjacent bone that, however, may be seen in benign salivary gland tumors, including plasmacytoid myoepithelioma (). Microscopically, differentiation from a common pleomorphic adenoma was based on limited ductal differentiation that in our case was <2%, lack of chondroid or osteoid stroma, and predominance of myoepithelial cells (, ).

The immunophenotype of neoplastic cells in myoepitheliomas may vary among different tumors (, , ) and among different cell types of the same tumor (, , ). In our case, both plasmacytoid and spindle cells were S-100 positive, and plasmacytoid cells, unlike spindle cells, showed diffuse cytoplasmic positivity for CK AE1/AE3, GFAP, and calponin. Expression of those antibodies in plasmacytoid myoepithelioma is a constant finding (, , ), while SMA and GFAP positivity may vary (, , ). In our case, α-SMA expression was very limited, but GFAP expression was diffuse. GFAP expression could possibly facilitate differentiation from myoepithelial carcinoma, as neoplastic myoepithelial cells of pleomorphic adenoma are GFAP positive, but of polymorphous low-grade adenocarcinoma GFAP negative (). Limited p63 expression was also seen in our case and p63 was found in some cutaneous myoepitheliomas () and salivary myoepithelial carcinomas (, ), therefore, its application in differentiation of plasmacytoma from its malignant variant should be further evaluated. CD138/Syndecan-1 is expressed by normal and neoplastic plasma cells, as well as neoplastic cells with plasmacytoid features (). Strong, membranous positivity for CD138/Syndecan-1, as seen in the present case, was reported in a single case of plasmacytoid myoepithelioma (). Lack of cytological atypia, cellular pleomorphism, necrosis, hemorrhage, infiltration into adjacent tissue, as well as low mitotic rate were highlighted by Ki-67 labeling index of <1% and they precluded a diagnosis of malignant myoepithelioma (, , ).

Complete surgical excision on tumor-free margins is usually curative (), although myoepithelioma is reported to have more aggressive behavior to pleomorphic adenoma (). Recurrence is associated with positive margins and this may occur in myoepitheliomas of minor salivary glands, especially of the palate, where tumor encapsulation is uncommon (). Recurrence rate of plasmacytoid myoepitheliomas of the palate is estimated to 15-18% (, ). Malignant transformation may ensue, particularly in the spindle cell variant ().

CONCLUSIONS

Since plasmacytoid myoepithelioma is an uncommon tumor of minor salivary glands, its immunohistochemical features, management and prognosis should be further investigated through the publication of more documented cases.