Myoepithelioma of the parotid gland: A report of two cases.

Abstract

We describe the clinical and pathologic features of two benign myoepitheliomas of the parotid gland. Through 1985, only 42 other cases had been reported in the literature--39 benign and three malignant. Fewer than 100 cases had been reported through 1993. Since then, two other reports have been published. But are these tumors really rare, or are they simply not well recognized? It is our opinion that they are not as rare as is generally believed because the number of case reports is increasing as pathologists have become more aware of their existence.

Introduction

Myoepithelial cells are ectodermally derived contractile cells that act as smooth muscle cells. They can be routinely identified in many normal tissues that have secretory functions, such as the major and minor salivary glands, lacrimal glands, sweat glands, breasts, and the prostate. [1,2] In a normal gland, myoepithelial cells are located between the epithelial cells and the basal lamina of acini and intercalated ducts. [3] Some authors believe that myoepithelial cells play a definite role in several salivary gland tumors, such as mixed tumors, adenoid cystic carcinomas, and salivary duct carcinomas. [2] Others contend that these cells play only a minor part in these tumors, and they are still undecided as to whether this role is active or passive. [4]

Myoepitheliomas consist exclusively of myoepithelial cells, and they are likely to represent one of the two cellular components of mixed tumors.

In this report, we describe two cases of a benign myoepithelioma of the parotid gland. We do not believe that this entity is rare. Rather, we believe that in general, physicians are simply unaware of its existence.

Case reports

Case 1. A 48-year-old man complained of a swelling and pain on the right side of his face that had been present for approximately 10 months. Examination revealed an 8 x 5-cm swelling in the area of the right parotid gland (figure 1). The growth was well demarcated, and it had a smooth external appearance. It had a moderately firm consistency, was fixed to neighboring tissues, and was slightly painful on palpation. Despite the large size of the lesion, the facial nerve remained intact, and there was no cervical lymphadenopathy.

Ultrasonography revealed a well-demarcated 78 x 60 x 43-mm mass. A solid component and internal echoes were found in the posterior portion of the growth. It was determined to be a cystic lesion because of the acoustic enhancement in its posterior. There was a limited normal parenchyma echo in the anterior portion of the lesion (figure 2).

The patient underwent a total parotidectomy with preservation of the facial nerve. The main facial nerve trunk had passed through the mass. The mass was unencapsulated and consisted of cystic and solid components (figure 3). It filled the peripharyngeal space. Postoperatively, there was no facial paralysis. Followup 9 months later revealed no evidence of disease.

Case 2. A 25-year-old man complained of a swelling on the right side of his face of approximately 3 years' duration. On examination, a 6 x 5-cm swelling was observed in the area of the right parotid. It was a smooth, mobile mass that was lobulated at the posterior. Ultrasonography revealed a solid, well-demarcated 45 x 30-mm mass.

The patient underwent a right partial parotidectomy. The tumor was encapsulated. Two lymph nodes were found around the tail of the parotid gland. The pathologist reported them as a reactive lymphoid hyperplasia. At 6 months' followup, the patient was still free of disease.

Pathologic findings

Following surgery in both cases, the specimens were fixed in 10% formalin and then embedded in paraffin. They were sectioned at 5 [micro]m and stained with hematoxylin and eosin. For the immunohistochemical study, the strept-avidin method was performed on the paraffin-embedded tissues. The antibodies used were those against cytokeratin, vimentin, S-100 protein, actin, and desmin.

Macroscopic examination. Both tumors were located in the parotid gland. In the first case, the tumor was unencapsulated and measured 7 x 4 x 3 cm. In the second case, the tumor was encapsulated and measured 4 x 4 x 3 cm. Both tumors were soft in consistency and grayish-white in color.

Microscopic examination. The encapsulated tumor was composed entirely of spindle-shaped cells with small, fusiform extensions of eosinophilic cytoplasm. The shapes of the nuclei ranged from oval to elongated and vesicular; some were hyperchromatic. The tumor had a fascicular cellular arrangement with poor intercellular stroma (figure 4).

The unencapsulated tumor was histologically similar and consisted primarily of spindle-shaped cells. In some areas, the tumor cells had a distinct plasmacytoid appearance. The plasmacytoid cells were characterized by eosinophilic cytoplasm and eccentric, round to oval nuclei. The nuclei were generally hyperchromatic, occasionally vesicular, and with inconspicuous or absent nucleoli. Few ductal or acinar structures were present in some areas, and some of the ducts contained eosinophilic material (figure 5).

There was no necrosis, cellular atypia, or mitosis in either tumor. Chondroid differentiation was not identified in either neoplasm.

Immunoistocemical findings. Both tumors were strongly immunoreactive for cytokeratin, vimentin, and S-100 protein and negative against antibodies to actin and desmin. Staining was diffuse and strong in both cases (figure 6).

Discussion

Myoepitheliomas account for fewer than 1% of all salivary gland tumors. Only 42 cases-39 benign and three malignant-were reported in the English-language literature through 1985. [5] A review of the literature through 1993 yielded approximately 100 cases. Since then, two other reports have been published. [67] In 1982, Sciubba and Brannon reported cases of myoepithelioma of the head and neck in 23 patients (mean age: 53 years). [8] In 1985, Barnes et al reported a new case and reviewed the literature on the 41 previously reported cases (mean age: 39). [5] In both reports, the sexes were equally affected. The most common locations of myoepithelioma of the head and neck are the parotid gland and the palate. [5] In our two patients, both tumors originated in the parotid.

Parotid lesions never cause facial nerve dysfunction or cervical lymphadenopathy, and those of the palate rarely ulcerate. [5] Neither of our patients had any cervical metastasis or facial nerve dysfunction, despite the large size of their masses.

Myoepitheliomas usually range from 1 to 5 cm in diameter, and they are well demarcated, smooth, and uniformly white, tan, or gray. They can develop with or without a capsule. Parotid myoepitheliomas are surrounded by a thin fibrous capsule; palatal lesions are not. Both of our patients had large masses, and one of them was unencapsulated.

Microscopically, myoepitheliomas usually consist of spindle-shaped cells. Some consist of plasmacytoid cells and others are made up of a combination of the two cell types. Stellate and clear cell variants are not common. Collagen is sparse and chondroid areas are never found. [5] In our two cases, the first tumor was made up of a combination of spindle-shaped and plasmacytoid cells, and the second tumor consisted entirely of spindle-shaped cells. Both tumors had poor intercellular stroma, but necrosis, cellular atypia, and mitosis were not present. In the first tumor, few ductal or acinar structures were present in some areas, and some of the ducts contained eosinophilic material. [9]

In developing salivary glands, a common stem cell is responsible for the development of luminal epithelial cells and myoepithelium. Therefore, it should be expected that proliferating neoplastic cells in a myoepithelioma would occasionally differentiate these luminal cells. [10] With immunocytochemical techniques, myoepithelial cells stain positive for cytokeratin, S-100 protein, and actin, and they stain negative for desmin. [10] Normal salivary gland myoepithelial cells stain negative for vimentin, [10] while neoplastic myoepithelial cells stain positive for it. [11,12] The tumors in both of our patients were strongly immunoreactive for cytokeratin, vimentin, and S-100 protein and negative against antibodies to actin and desmin. To identify myoepithelial tumors as either benign or malignant on histologic grounds is difficult. The criteria for a diagnosis of malignancy are the presence of cytologic abnormalities, an increased mitotic rate, and particularly an invasive growth pattern. [13] These criteria wer e not met in our two cases.

Cell type is not related to differences in biologic behavior, recurrence rate, or the patient's age. [8,14] Spindle-cell myoepithelioma must be distinguished from an extracranial meningioma, a pleomorphic adenoma, a nerve sheath tumor, and a leiomyoma. A plasmacytoid myoepithelioma must be distinguished from a plasmacytoma.[5] According to the World Health Organization's classification of salivary gland tumors, "Myoepitheliomas are characterized by a more aggressive growth pattern than pleomorphic adenomas." [9] But some authors have found that the biologic behavior of myoepitheliomas appears to parallel that of the pleomorphic adenoma. [5] Others believe that myoepitheliomas are monomorphic variants of mixed tumors. [3,8]

It is our opinion, as well as that of Barnes et al [5] and Dardick et al [15] that salivary adenomas are part of a spectrum, with monomorphic adenoma and myoepithelioma at the extremes and a wide range of pleomorphic adenomas in between, depending on the type and degree of gene expression that is coupled with neoplastic transformation.

Myoepithelioma occurs in most major and minor salivary gland tissues, and it is generally a biologically benign lesion. By means of these two case reports, we have attempted to scrutinize this entity. While myoepithelioma has no specific clinical features, it is accepted pathologically as a distinct entity. We do not consider it to be as rare a condition as is generally believed because the number of case reports is increasing as pathologists have become more aware of its existence.

From the ENT Clinic, Sisli Etfal Hospital, Istanbul (Dr. Turgut), the 2nd ENT Clinic, Ankara Numune Hospital (Dr. Cekic and Dr. Ozdem), and the Department of Pathology (Dr. Ergul, Dr. Aksoy, and Dr. Seckin), Ankara Numune Hospital, Turkey.

Reprint requests: Dr. Suat Turgut, Buyukdere Cad. 18/15, 80220 Sisli, Istanbul, Turkey. Phone: [phone]; fax: [phone]; e-mail: [e-mail]

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