The term myoepithelioma was first introduced by Sheldon in 1943. ¹ Myoepitheliomas are generally benign neoplasms composed exclusively of myoepithelial cells. A myoepithelioma classically presents as an asymptomatic mass that slowly enlarges over a long period of time. However, recurrent and malignant ones have also been described. ² They are more likely to occur in the parotid gland, but rare cases arising from minor salivary glands are also on record. ³ , ⁴ , ⁵ The authors present an exceptionally rare case of sinonasal myoepithelioma with recurrent and resistant to treat clinical course. To our knowledge and regarding the Medline database, this is the seventh case of its kind until 2012.

The patient was a 57-year-old Iranian male living in Dargaz, a small district in north east of Iran. He was admitted to the E.N.T ward, Ghaem Hospital, Mashhad University of Medical Sciences in June 2010. He presented with a 17-year history of recurrent and resistant to treat sinonasal mass. First, at 1993, he accidentally discovered a small painless mass of the palate, which was excised in an outpatient care facility. After 4 years, the lesion recurred and the patient underwent medial maxillectomy. Thereafter, the patient had five more debulking surgeries.

At the time of admission, the patient was complaining of nasal fullness, hyponasal speech, snoring, oral breathing, and loss of hearing on left side.

On oral examination, the hard palate in the right side had been removed and a nasal mass was obvious through it. Uvula was in the midline. Both nasal cavities were completely filled with the mass. There was no cervical lymphadenophaty. The right tympanic membrane was retracted and the Weber test was shifted to right. All other physical examinations were unremarkable.

Laboratory tests were within normal limits. Family and past medical histories were unremarkable and the patient was non-smoker.

Both computed tomography scan and magnetic resonance imaging demonstrated a huge bilateral lobulated sinonasal mass with extension to cribriform plate and near the dura, medial walls of both orbits, and upper gingival area Figure 1. The plain chest x-ray was normal. Figure 1

Magnetic resonance imaging. T1-weighted transverse (left) and sagittal (right) planes show a huge bilateral sinonasal mass

Myoepithelioma is a generally benign neoplasm that constitutes approximately 1-1.5% of all salivary gland neoplasms. ² They are more likely to occur in the parotid gland, but rare cases arising from minor salivary glands such as in palate, gingiva, lips, nasopharyngeal space, orbit, and middle ear are also on record. ³ , ⁴ , ⁵ Exceptionally, they can occur in the sinonasal cavity, with only seven reports, including our case, present in the literature. ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹

Histologically, myoepitheliomas are classified as spindle, plasmacytoid, reticular, epitheliod, and clear cell types. The most common is the spindle type, whereas the others are very rare. ² All the previously reported sinonasal myoepitheliomas were composed mostly of spindle cells and to a lesser extent of plasmacytoid/ epithelioid cells. ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ However, the present case was almost entirely composed of plasmacytoid cells, with dispersed foci of spindle cells.

The most important differential diagnosis of myoepitheilomas includes pleomorphic adenoma. There is a general tendency to put myoepitheliomas at one end of the histologic spectrum that includes pleomorhic adenomas at the other end. This claim is supported by the fact that these two entities are very similar in biologic course, distribution, morphology, and even immunohistochemical aspects. Indeed, the criteria to differentiate these lesions are mainly subjective. World Health Organization proposes that if the neoplasm contains less than 5% of ductal components they must be named myoepithelioma. ² , ⁵

Furthermore, several mesenchymal neoplasms such as fibrosarcoma, leiomyoma, schwannoma, and myxoma are in the differential diagnoses of the spindle cell myoepitheliomas, but they all lack the immunoreactivity for cytokeratin. ⁶ However, in the presented case, the non- spindle (plasmacytoid) appearance of the neoplastic cells can easily rule out these entities. Moreover, the specimen expressed cytokeratin.

Immunohitochemcal examination is a proper technique to facilitate the diagnosis of myoepithelioma. Myoepitheliomas are characteristically immunoreactive for cytokeratins, S-100, Calponin, smooth muscle actin, myosin, vimentin, glial fibrillary acidic protein, and carcinoembryonic antigen. ⁶ , ¹² In the presented case, the smooth muscle actin, vimentin, cytokeratin, and S-100 protein were positive.

Conventional myoepitheliomas are not more aggressive than pleomorphic adenomas, but invasive myoepitheliomas that infiltrate the adjacent tissues have been described. In addition, malignant/metastatic ones are also on record. The malignancy is assisted by the infiltrative margins, necrosis, marked pleomorphism, and prominent mitotic activity, ⁶ , ¹³ , ¹⁴ which were all absent in the presented case. Moreover, high recurrence rates among benign myoepitheliomas have been described. Recurrence is associated with involved surgical margins at the first excision. ² In the present case, the tumor had a benign morphology with a progressive, invasive, and recurrent clinical course, as the tumor has been recurred six times within 17 years. Therefore, the surgical excision must be done with wide healthy margins and an appropriate follow-up is also recommended.

Myoepitheliomas are generally well-circumscribed solid masses that measure less than 3 cm in diameter. ² However, in our case, the tumor was a huge bilateral sinonasal mass, which showed prominent extension to cribriform plate, dura, both orbits, and upper gingiva, so that complete surgical removal was practically impossible. To our knowledge, this is the most extensive myoepithelioma of its kind.

Some of the malignant myoepitheliomas have been arisen from pleomorphic adenomas. ⁶ , ¹⁴ Regarding our case, the primary lesion had been diagnosed as a myoepithelioma; thus, the further transform of a pleomorphic adenoma seems improbable.

We reported the seventh case of myoepitheliom of the sinonasal cavity ever diagnosed regarding the Medline database until 2012. Immunohistochemical investigation may be a useful tool in order to reach to the appropriate diagnosis while a myoepithelioma is in the differential diagnosis of a tumor of the oral/sinonasal cavity. Moreover, complete surgical excision is necessary.