Salivary glands

Primary salivary gland neoplasms

Benign

Myoepithelioma



Topic Completed: 8 September 2021

Minor changes: 8 September 2021

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PubMed Search: Myoepithelioma salivary glands[title]

Xiaofeng Zhao, M.D., Ph.D.
Shuanzeng (Sam) Wei, M.D., Ph.D.
Page views in 2020: 12,672
Page views in 2021 to date: 13,462
Cite this page: Zhao X, Wei S. Myoepithelioma. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/salivaryglandsmyoepithelioma.html. Accessed October 25th, 2021.
Definition / general
  • Rare, benign tumor composed almost exclusively of myoepithelial cells
Essential features
  • Epithelial component should be less than 5% (some consider even focal epithelial differentiation sufficient to label the tumor as pleomorphic adenoma) (J Oral Maxillofac Pathol 2013;17:257)
  • Monomorphic histology and rare or absent ductal structures in myoepithelioma differentiate it from pleomorphic adenoma
Terminology
ICD coding
  • ICD-O: 8982/0 - myoepithelioma
  • ICD-10: D11.0 - benign neoplasm of parotid gland
Epidemiology
Sites
Pathophysiology
  • Arising from neoplastic myoepithelial cells located between the basement membrane and the basal plasma membrane of acinar or ductal / luminal cells
  • Accumulation of p53 and overexpression of KIT receptor have been implicated in malignant progression (J Clin Pathol 1998;51:552, Cancer Lett 2000;154:107)
Etiology
  • Myoepithelial cells are contractile cells originating from the ectoderm, found in normal tissues with secretory function that aids in excreting glandular secretions (Ear Nose Throat J 2011;90:E9, Clin Pract 2014;4:628)
  • Common stem cell with a bidirectional differentiation into epithelial or myoepithelial cell is hypothesized to be the cell of origin; the varied histological types (spindle, plasmacytoid, epithelioid, clear and oncocytic) exhibited by myoepitheliomas can be attributed to the various stages in the differentiation from a cell that has the potential to differentiate into epithelial cells (J Clin Diagn Res 2013;7:1165)
  • Myoepitheliomas and pleomorphic adenomas are hypothesized to exist along a continuum, with a shared chromosome 12q cytogenetic abnormality (Cancer Genet Cytogenet 1999;113:49)
Clinical features
Diagnosis
  • Diagnosis is often rendered in resection specimen
Radiology description
  • Located chiefly in the superficial lobe and abutted on the capsule of the gland
  • CT: well circumscribed, enhancing mass lesion with smooth or lobulated margins (AJNR Am J Neuroradiol 2008;29:1372)
  • May contain enhancing nodules and nonenhancing areas of linear bands, slit-like shaped or of cystic configuration
  • MRI: well circumscribed, lobulated, inhomogeneous enhancement, apparent diffusion coefficient, different from Warthin tumor and similar to pleomorphic adenoma (AJNR Am J Neuroradiol 2009;30:591)
Prognostic factors
Case reports
Treatment
  • Complete surgical excision with free margin recommended (Clin Pract 2014;4:628)
  • Regular follow up to evaluate for local recurrence
Clinical images

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Plasmacytoid myoepithelioma of minor salivary glands

Gross description
  • Up to 5 cm; encapsulated, may have cystic change
  • Absence of grossly myxoid or chondroid areas
  • Solid, tan or yellow-tan glistening cut surface
  • Usually encapsulated in parotid gland and no capsule for those in minor salivary glands (Ear Nose Throat J 2001;80:155)
Frozen section description
  • Nonspecific, may show atypical cells, usually need to defer to permanent for further characterization
Microscopic (histologic) description
  • Spindled, epithelioid, plasmacytoid, clear or oncocytic cells; most tumors are composed of a single cell type but combinations may occur (Clin Pract 2014;4:628)
  • Well circumscribed or encapsulated with a thin capsule
  • Stroma, when present, may be hyalinized, fibrous, myxoid or mucoid; lipomatous stroma may rarely been seen.
  • Encapsulation uncommon in minor salivary gland tumors
  • Architectural pattern: solid (non-myxoid), myxoid, reticular (canalicular-like) or mixed patterns of myoepithelial cells with no ductal differentiation
  • Mucinous myoepithelioma is rarely reported; contains abundant intracellular mucin material, which may be mistaken for signet ring cell adenocarcinoma of salivary gland with positive myoepithelial markers and should be distinguished from secretory myoepithelial carcinoma (Head Neck Pathol 2013;7:S85)
Microscopic (histologic) images

Contributed by Xiaofeng Zhao, M.D., Ph.D. and Shuanzeng Wei, M.D., Ph.D.

Spindle cell type

Epithelioid type

Nest / trabecular pattern

Cytology description
  • Monotonous spindle cell, plasmacytoid cells or epithelioid cells (J Oral Maxillofac Pathol 2014;18:131)
  • No glandular / ductal cells
  • Can have nuclear grooves, intranuclear cytoplasmic inclusions, no marked pleomorphism, no mitoses
Cytology images

Contributed by Shuanzeng Wei, M.D., Ph.D.

Cellular specimen

Monotonous epithelioid cells

Negative stains
  • Plasmacytoid variant may be negative for all myoepithelial markers
Electron microscopy description
Molecular / cytogenetics description
Sample pathology report
  • Parotid, right, parotidectomy:
    • Myoepithelioma, 3.6 cm, completely excised
Differential diagnosis
Board review style question #1

A 46 year old man presents with swelling close to his right ear. Imaging reveals a well circumscribed 1.8 cm nodule in the parotid gland. Biopsy findings are shown in the picture above. Which of the following statements about this tumor is true?

  1. Monomorphic tumor cells and absence of ducts help to differentiate myoepithelioma from pleomorphic adenoma
  2. Mucinous features should rule out this entity
  3. Sometimes the epithelial component can be as high as 30%
  4. This lesion is most common in minor salivary glands of palate
  5. This tumor is completely benign and no malignant transformation has been reported
Board review style answer #1
A. Monomorphic tumor cells and absence of chondromyxoid stroma and ducts help to differentiate myoepithelioma from pleomorphic adenoma. The presence of chondromyxoid stroma is consistent with pleomorphic adenoma rather than myoepithelioma.

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Reference: Myoepithelioma
Board review style question #2
Which of the following is correct about myoepithelioma?

  1. Negative for keratin
  2. Negative for nuclear beta catenin
  3. Negative for S100
  4. Negative for SMA and HHF35
  5. Positive for PLAG1 translocation
Board review style answer #2
E. Positive for PLAG1 abnormalities. Detection of PLAG1 rearrangement by cytogenetics or other methods can be helpful. NTF3-PLAG1, FBXO32-PLAG1 and GEM-PLAG1 fusions have been detected in oncocytic myoepithelioma (Ann Diagn Pathol 2017;28:19, Hum Pathol 2020;103:52).

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Reference: Myoepithelioma
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