Myoepithelioma of Parotid Gland: A Cytological, Histological and Immunohistochemical Study
Myoepitheliomas of salivary glands are extremely rare benign tumors of head and neck, comprising approximately 1% of all salivary gland tumors. The present case describes the clinical, cytological and histopathological features of a benign myoepithelioma of parotid gland. A 26 year old female presented with symptoms of a gradually progressive swelling at left preauricular region. Examination revealed a 4x3 cm mass with smooth surface in left parotid area. Cytological, histopathological and immunohistochemical findings established the diagnosis of spindle cell variant of myoepithelioma of parotid gland.
Keywords: Benign; Cytology; Histopathology; Immunohistochemistry; Myoepithelioma; Parotid Gland.
Vani D, Sandhya M, Ashwini NS, Bharathi M. Myoepithelioma of Parotid Gland: A Cytological, Histological and Immunohistochemical Study. International Journal of Oral & Maxillofacial Pathology; 2014:5(3):28-30. ©International Journal of Oral and Maxillofacial Pathology. Published by Publishing Division, Celesta Software Private Limited. All Rights Reserved.
Myoepitheliomas of salivary glands were first described by Sheldon et al., in 1943. They were considered as a variant of pleomorphic adenoma. The World Health Organization in 1991 distinguished myoepithelioma from pleomorphic adenoma, classifying it as an independent entity.1 Most are benign and are monomorphic tumors composed of sheets and islands of myoepithelial cells. Diagnosis of myoepithelioma through light microscopy is possible and immunohistochemistry is done to facilitate the diagnosis. The tumor occurs over a wide age range and has no sex predilection. It usually presents as a painless, slow growing mass of benign nature, but sometimes it may be locally aggressive. The parotid gland is preferentially involved as compared to the other salivary glands.2
A 26 year old woman presented with a slow growing painless swelling in left preauricular region for duration of one year. Local examination showed a 4x3 cm swelling in the area of left parotid gland. The lesion was well demarcated and had a smooth external appearance. It was a firm, slightly movable and non-tender mass. No evidence of facial nerve palsy or cervical lymphadenopathy was observed.
Fine needle aspiration cytology smears were fairly cellular. It showed bundles of spindle shaped cells as well as plasmacytoid cells in sheets and dissociated forms over a proteinaceous background. These spindle cells had bipolar cytoplasm and distinct cell membrane. The nucleus was small and oval with homogenous chromatin (Fig 1a). Differential diagnosis of myoepithelioma and pleomorphic adenoma was given. Gross tissue examination showed a grey white to grey brown solid tumor with thin fibrous capsule measuring 5x3x1.5 cm with surrounding normal appearing glandular tissue was observed (Fig 1b). Microscopically, the tumor was composed of spindle and ovoid cells arranged in streaming fascicular pattern and interlacing bundles. These cells had central oval shaped nucleus and eosinophilic cytoplasm (Fig 1c). There was no necrosis, cellular atypia or mitosis in the tumor. Absence of glandulo-ductal differentiation and chondromyxoid / chondroid foci ruled out pleomorphic adenoma from the differential diagnosis. Immunohistochemically the tumor cells showed strong positivity to cytokeratin, S-100 and smooth muscle actin and negative to desmin (Fig 1d). Histological and immunohistochemical findings confirmed the cytodiagnosis of Myoepithelioma of spindle cell type of parotid gland.
Myoepithelioma account for less than 1%of all salivary gland tumors.2 The most common locations for myoepithelioma of head and neck area are the parotid gland and palate.3 Both sexes are affected with equal frequency. Myoepithelial cells are contractile cells that originate from the ectoderm. Several normal tissues that have secretory function have myoepithelial cells. They are present in major and minor salivary glands, sweat glands, lacrimal glands, prostate, breast, nasopharynx, lung, skin and soft tissue. Though myoepithelioma can develop in any of these regions, its frequency is much less. Myoepithelioma develop preferentially in the parotid gland.4 Approximately, 50% of salivary gland myoepithelioma involve the parotid gland, 33% arise from sublingual glands and 13% affect the submandibular gland.1 A common stem cell with a bidirectional differentiation into epithelial and myoepithelial cell is hypothesized to be the cell of origin in this tumor.4 Myoepithelial cells stain positive for cytokeratin, S-100 protein, actin and they stain negative for desmin.2
Figure 1: The photomicrograph of Hematoxylin and eosin stained fine needle aspiration cytological smear (a) shows spindle shaped cells admixed with plasmacytoid cells in dissociated forms over a proteinaceous background (x400). The gross macroscopic examinations of the surgically excised tissue (b) shows a grayish-white to grayish-brown solid tumor mass, measuring 5x3x1.5 cm with surrounding normal appearing glandular tissue. The photomicrograph of hematoxylin and eosin stained tissue section at low power (c) showing bundles of spindle and ovoid cells and at high power view of immunohistochemical staining (d) showing positivity for smooth muscle actin (x400).
Sciubba and Brannon consider myoepithelioma as a subtype or final spectrum of pleomorphic adenoma, because of their biological behavior and distribution. On the other hand, World Health Organization classifies myoepithelioma as an independent entity. From histological point of view the myoepitheliomas classified into the following types-spindle, plasmacytoid, reticular, epitheloid and clear. The spindle cell type is the most common overall, while plasmacytoid type is less common.2 Myoepitheliomas can present several architectural patterns which are non-myxoid (solid), myxoid (pleomorphic adenoma-like), reticular (canalicular-like) and mixed.4 The present case showed a solid pattern. Due to their infrequency and multiplicity of histopathology, myoepithelioma present difficulties in diagnosis and classification. Cellular varieties can be misdiagnosed as malignancies.2 To identify myoepithelioma tumors as either benign or malignant on histologic grounds is difficult. The criteria for a diagnosis of malignancy are the presence of cytological abnormalities, an increase in mitotic rate, invasive growth pattern, pleomorphism and necrosis.3 These criteria were not met in the present case. The assessment of cell proliferative activity may be helpful in the differentiation between benign and malignant myoepitheliomas, and Ki-67 labeling index of more than 10% is diagnostic of myoepithelial carcinoma.5
Spindle cell myoepithelioma must be distinguished from extra-cranial meningioma, pleomorphic adenoma, nerve sheath tumor and leiomyoma1.An awareness of the wide morphologic range of the myoepithelioma and immunohistochemical stains are necessary to make the correct diagnosis1. Cytological diagnosis of myoepithelioma is difficult with fine needle aspiration cytology. Erroneous results often arise because of the variations in cytomorphology of myoepithelioma and because tumors arise by proliferation of myoepithelial cells and show some features similar to pleomorphic adenoma.6
Myoepithelioma tend to exhibit benign behavior.2 The treatment of myoepithelioma should be designed as for any benign salivary gland tumor. The first choice is surgery. The mass should be totally removed with a margin of normal gland tissue. Myoepitheliomas seem to replicate slowly, for decades, and they carry the possibility of the scattered tumor cells growing alongside the incision area in the adjacent tissues, after a parotid surgery1. Recurrence rate of 15-18% is observed.4 Thus a long-term follow up is needed.
Myoepithelioma of parotid gland is rare in daily medical practice. However it should be kept in mind in the differential diagnosis of salivary gland tumors. Whilst myoepithelioma has no specific clinical features, it is accepted pathologically as a distinct entity. To avoid unnecessary extensive surgical resection, an accurate diagnosis is of utmost importance. The prognosis for benign myoepithelioma is quite well, but patients should undergo regular follow up examinations to rule out local recurrence.
We would like to thank all the staff members of the department for their support and guidance.
1.Dr.Vani D, Associate professor, 2.Dr.Sandhya M, Post Graduate, 3.Dr.Ashwini NS, Post Graduate, 4.Dr.Bharathi M, Professor and Head, Department of Pathology, Mysore Medical College and Research institute, Rajiv Gandhi University of Health Sciences, Karnataka, India.
1. Mochizuki Y, Omura K, Tanaka K, Sakamoto K, Yamaguchi A. Myoepithelioma of the Parotid Gland presenting as a Retro-auricular Cutaneous nodule: A case report. Journal of Clinical and Diagnostic Research 2013;7(6):1165-8.
2. Yenidunya S, Haltas H, Bayrak R, Aktas D. Myoepithelioma of the Parotid Gland: A Rare Case. Inonu Universitesi Saglik Bilimleri Dergisi 2012;1:49-51.
3. Karli R. Myoepitheliomas are rare tumors of the parotid gland: A case report and review of the literature. Universal Journal of Medicine and Dentistry 2012;1(1):1-4.
4. Nair BJ, Vivek V, Sivakumar TT, Joseph AP, Varun BR, Mony V. Clear cell myoepithelioma of palate with emphasis on clinical and histological differential diagnosis. Clinics and Practice 2014;4(628):17-21.
5. Ferri E, Pavon I, Armato E, Cavaleri s, Capuzzo P, Ianniello F. Myoepithelioma of a minor salivary gland of the cheek: case report. Acta Otorhinolaryngol Ital. 2006;26(1):43–6.
6. Karli R. Myoepitheliomas are rare tumors of the parotid gland: A case report and review of the literature. Universal Journal of Medicine and Dentistry 2012;1(1):1-4.
Dr. Sandhya M,
Post Graduate Student,
Department of Pathology,
Mysore Medical College and Research Institute,
Rajiv Gandhi University of Health Sciences,
Source of Support: Nil, Conflict of Interest: None Declared.