Clear cell myoepithelioma of the hard palate

Abstract

Clear cell myoepithelioma arising from the minor salivary glands of hard palate is a rare entity. Most of the cases of palatal myoepitheliomas reported so far are either plasmacytoid or spindle cell type. Our literature search revealed only one case report of clear cell myoepithelioma of the palate. We report a case in a 21-year-old female who presented to us with a non-ulcerating, painless firm swelling of the left side of the hard palate. The tumor was excised with a healthy rim of surrounding tissue. Immunohistochemistry was performed on formalin-fixed and paraffin-embedded tissue with a panel of immunohistochemical markers. Tumor cells showed positivity for S100, cytokeratin and Muscle specific actin. Histological and immunohistochemical analyses revealed the tumor to be a myoepithelioma of the clear cell variety.

Myoepitheliomas are very rare tumurs of the salivary glands constituting less than 1.5% of all salivary gland tumors.¹ The most common site of occurrence is the parotid gland.^1–3 Although, clear cell myoepitheliomas have been reported at the other sites, most of the cases of palatal myoepitheliomas reported are either of the plasmacytoid or spindle cell type.⁴ A review of published reports revealed only one case of clear cell myoepithelioma of the hard palate reported by Agarwal et al. ⁵ The extreme rarity of clear cell myoepithelioma of the hard palate prompted us to report this case with the objective of contributing to a better understanding of this neoplasm. We discuss the clinical, radiological and pathological characteristics.

CASE

A 21-year-old female was referred to the Head and Neck Unit at Prince Aly Khan hospital, Department of Surgical Oncology, with recurrent painless swelling of the hard palate. She had undergone excision of the a similar swelling in the same region 4 months previously in another facility. Histopathological analysis of the previous surgical specimen showed it to be clear cell myoepithelioma. There was history of a progressive but slow increase in the size of this recurrent swelling. On intraoral examination there was smooth, fleshy, firm mass on left side of the hard palate approximately 3×3 cm in size (Figure 1).

Figure 1.

Clinical picture showing the smooth swelling on the left side of the hard palate.

CT scan of the paranasal sinuses showed a well defined lobulated enhancing soft tissue mass, 3.4×2.5 cm in the transverse plane causing expansion of the left maxillary sinus. Erosion of the medial wall and floor of the maxillary sinus, including a portion of the maxillary alveolar process (in the molar region), was also noted. Mild focal erosion of the posterior wall of the left maxillary sinus was also seen without extension of the soft tissue into the masticator space. Widening of the ptery-go- palatine fissure was also noted on CT scan with the presence of enhancing soft tissue within it (Figure 2). With these observations and previous histopathology, a diagnosis of recurrent/residual clear cell myoepithelioma was made.

Figure 2.

CT scan axial view showing the well-defined lobulated mass eroding the palate with extension into the maxillary antrum.

An enbloc infrastructure maxillectomy was done with adequate margins. Histpathological examination of the surgical specimen showed grossly infiltrative fleshy white circumscribed mass measuring 3.8×3.7×2 cm with adequate cut margins. The overlying mucosa was opaque, white and smooth. The tumor was composed of large polyhedral cells with clear eosinophilic cytoplasm and these cells were arranged in closely packed interlacing bundles. The nuclei were round to oval, eccentrically placed and were vesicular with small nucleoli (Figures 3, 4).

Figure 3.

Photomicrograph under low power showing sheets and cords of ovoid to spindle component with moderate amount of pale to eosinophilic cytoplasm (hematoxylin-eosin stain ×100).

Figure 4.

Photomicrograph under high power showing ovoid to spindle cells with uniform, mildly hyperchromatic nuclei and inconspicuous nucleolei (hematoxylin-eosin stain ×100).

Immunohistochemically, the clear cells were immunoreactive for cytokeratin, S-100 and muscle-specific actin (MSA). A diagnosis of clear cell type myoepithelioma of minor salivary gland origin was made on microscopy and tumor was invasive in nature. The postoperative period was uneventful. Since she had a recurrence and the tumor was invasive in nature, adjuvant treatment of postoperative adjuvant radiotherapy was also given to the primary site. After 3 years of follow up she was free of disease without any signs of recurrence.

DISCUSSION

Myoepitheliomas are rare benign tumors, with the parotid gland and palate accounting for three-fourths of all cases.⁶ The plasmacytoid variant is usually seen on the palate in slightly younger individuals, while the spindle cell type tends to occur in the parotid gland of older individuals.^7,8 Clear cell tumors of the salivary glands are almost invariably malignant in nature, with rare exceptions in the form of myoepitheliomas and oncocytomas.⁸ Our literature search revealed only one case of clear cell myoepithelioma of the hard palate reported by Agarwal et al.⁵ In our review, 20 cases of ultrastructurally or immunohistochemically confirmed myoepithelioma of the palatal minor salivary gland were found (Table 1). Of the 20 cases of palatal myoepitheliomas reported, 17 cases were of the plasmacytoid cell type, two were of the clear cell type, one belonged to the spindle cell variety and one was of mixed cellularity.

Table 1.

Reported cases of myoepithelioma of minor salivary gland tumor of palate.

Report	Age/Sex	Site	Cell type
Kahn and Schoub9	17/F	Hard palate	Plasmacytoid
Luna et al10	30/F	Hard palate	Spindle
Sciubba and Goldstein1	22/M	palate	Plasmacytoid
Nesland et al11	18/F	Soft palate	Plasmacytoid
Barnes et al6	24/F	Hard palate	Plasmacytoid
Enomoto et al12	57/F	Soft palate	Plasmacytoid
Ellyn and Gnepp13	8/F	Soft palate	Plasmacytoid
Kawabe et al14	53/M	Soft palate	Plasmacytoid
Stromeyer et al15	14/M	Anterior maxilla	Cytological pleomorphism
Harusachi et al16	42/F	Hard palate	Plasmacytoid
Agarwal et al5	40/F	Hard palate	Clear cell type
Rastogi et al17	33/M	Hard palate	Plamacytoid
Lins JE et al18	8/F	Soft palate	Plasmacytoid
Lopez JI et al19	46/M	Soft palate	Plasmacytoid
Acikalin MF et al20	32/M	Soft palate	Plasmacytoid
Zelaya et al21	28/F	Hard palate	Plasmacytoid
Kanazawa H et al4	42/F	Hard palate	Plasmacytoid
Katsuyama E et al22	67/F	Soft palate	Plasmacytoid
Nwoku et al23	11/M	Palate	Plasmacytoid
Present case	21/F	Hard palate	Clear cell type

Electron microscopic and immunohistochemical examinations are useful for accurate identification and characterization of myoepithelial cells. Immunohistochemically, a large number of markers have been used for establishing the diagnosis of myoepitheliomas. The myoepithelial cells are usually found to be immunoreactive to S-100, actin and prekeratin, and are nonreactive to desmin and EMA. Other markers that have been used in some reports are CEA, secretory piece, high molecular weight keratin, cytokeratin and factor VIII antigen.⁴ The reactivity to MSA and vimentin has been reported variably in different reports.^6,10–12 In our patient, the tumor cells were found to be immunoreactive to MSA, S-100 and cytokeratin and negative for vimentin and EMA.

In conclusion, myoepitheliomas are rare benign tumors arising from minor and major salivary glands. Immunohistochemical staining is extremely important for diagnosis. The clear cell variants of myoepitheliomas are extremely rare on the palate with only one case reported. Adequate excision with clear margins is usually curative.