These neoplasms arise from myoepithelial cells, the contractile elements that wrap around secretory units in exocrine glands such as the salivary, sweat, mammary, and lacrimal glands. When these cells proliferate into a mass showing predominantly myoepithelial differentiation, pathologists apply the term myoepithelioma, a distinct salivary gland neoplasm category recognized by the World Health Organization. Diagnosis hinges on correlating histology, immunohistochemistry, and molecular findings, because the lesion mimics several look-alikes under the microscope.
This reference covers how pathologists define myoepithelial tumors, where they appear, the stains and fusion tests used to confirm them, and what your clinical team will likely do once a diagnosis is confirmed.
The Cellular Origins and Definition of Myoepithelial Tumors
Myoepithelial cells sit between the secretory epithelial lining and the basement membrane of glandular tissue, contracting in response to neural and hormonal signals to push secretions toward a duct. Tumors that arise from these cells are uncommon, and the WHO Classification of Head and Neck Tumours formally lists myoepithelioma as a separate entry, distinct from mixed tumors and other salivary gland neoplasms.
From contractile cell to neoplasm
When a myoepithelial cell escapes normal growth control, it can form a mass composed almost entirely of cells showing myoepithelial differentiation, with little or no ductal component. That defining absence of ducts separates myoepithelioma from its closest relative, pleomorphic adenoma, which characteristically combines myoepithelial, ductal, and chondromyxoid stromal elements.
Tissue distribution across the body
Because myoepithelial cells populate exocrine glands, tumors can theoretically arise anywhere those glands exist:
- Salivary glands: parotid, submandibular, and minor mucosal sites of the palate, buccal cavity, and lip
- Sweat glands: skin of the head, neck, extremities, and trunk
- Mammary gland: rare reports in the breast, often within the adenomyoepithelial spectrum
- Lacrimal gland: orbital presentations that mimic other epithelial lesions
Soft tissue myoepithelioma, a variant arising without a clear glandular connection, has its own entry in soft tissue tumor references and behaves somewhat differently from its salivary counterpart.
Because these tumors arise from cells that normally wrap secretory acini, their clinical behavior reflects the anatomy of the gland they inhabit.
Common Anatomic Sites and Clinical Presentation
The parotid gland accounts for the majority of salivary cases, while soft tissue variants cluster in the extremities, trunk, and deep head and neck region. Knowing the likely location helps narrow the differential before a needle even touches the mass.
Typical presentation and demographics
Patients usually notice a slow-growing, painless, mobile lump that has been present for months to several years. The demographic skew varies by site: salivary cases peak between the third and sixth decades, while soft tissue myoepithelioma is reported across a wider age range, including pediatric patients. There is no strong sex predilection in most series, though some reports note a slight female predominance for cutaneous lesions.
Most myoepithelial tumors are benign, slow-growing, and asymptomatic, but malignant forms can recur or metastasize, so any rapidly enlarging or fixed mass deserves prompt evaluation by a specialist.
Histological Patterns and Cellular Morphology
Under the microscope, architectural and cytologic variability is striking, and that variability is exactly what makes these tumors a diagnostic challenge. A single case may combine solid nests, trabecular cords, and cystic spaces within the same field.
Architectural diversity
Growth patterns include solid sheets, trabecular ribbons, reticular or pseudo-glandular arrangements, and pseudocystic areas where degenerative change creates fluid-filled spaces. The supporting stroma ranges from chondromyxoid and hyaline to densely collagenous, sometimes within the same lesion.
Cytomorphologic spectrum
The tumor cells themselves come in several shapes, and most tumors show at least two:
- Plasmacytoid (Hürthle-like): round, eccentric nuclei with abundant eosinophilic cytoplasm, common in palate and parotid lesions
- Spindled: elongated cells resembling smooth muscle or fibroblast-like elements, dominant in soft tissue cases
- Epithelioid: polygonal cells with more abundant cytoplasm and visible nucleoli
- Clear cell: glycogen-rich cytoplasm that requires careful exclusion of metastatic renal cell carcinoma
Features that raise concern for malignancy
Cytologic atypia, increased mitotic activity (especially atypical mitoses), infiltrative borders, and tumor necrosis shift the diagnosis toward malignant myoepithelioma or myoepithelial carcinoma. Grading criteria codified in the WHO classification separate malignant salivary tumors by stage and behavior, even when the cell of origin matches its benign counterpart.
Immunohistochemical Markers Confirming Myoepithelial Differentiation
No single stain is specific, so the diagnosis rests on a panel. A practical myoepithelial tumor pathology workup typically combines epithelial, myoepithelial, and basal markers.
| Marker | Type | Typical result | Diagnostic note |
|---|---|---|---|
| S100 protein | Nuclear and cytoplasmic | Positive | Sensitive but nonspecific; also stains nerve sheath and adipose tumors |
| p63 | Nuclear | Positive | Reliable basal/myoepithelial marker; lost in some carcinomas |
| p40 | Nuclear | Positive | More specific isoform of p63, helpful against squamous mimics |
| Calponin | Cytoplasmic | Positive | Smooth muscle marker reinforcing contractile phenotype |
| Smooth muscle actin | Cytoplasmic | Often positive | Confirms myoepithelial lineage; may be focal |
| GFAP | Cytoplasmic | Variable | Supportive when positive, especially in plasmacytoid variants |
| AE1/AE3 cytokeratin | Cytoplasmic | Positive | Confirms epithelial lineage and argues against pure mesenchymal tumors |
| CK14 | Cytoplasmic | Often positive | Highlights basal/myoepithelial cells in glandular tumors |
Real diagnostic confidence comes from the combination, not any single result. An S100-positive, p63-positive, calponin-positive, cytokeratin-positive tumor fits myoepithelial differentiation, while loss of cytokeratin should push the pathologist toward a nerve sheath or smooth muscle origin.
Molecular Alterations and Genetic Signatures
Molecular testing has become the tie-breaker when histology and immunohistochemistry leave ambiguity. Two rearrangements dominate the literature, and a third category overlaps with pleomorphic adenoma.
EWSR1 and FUS rearrangements
Roughly half of soft tissue myoepithelioma cases in published series harbor EWSR1 rearrangements, making them the most frequent molecular event. FUS rearrangements occur in a smaller subset, sometimes as an alternative partner when EWSR1 is wild-type. Both fusions can be detected by FISH break-apart probes or by targeted next-generation sequencing panels that include fusion detection.
Overlap with pleomorphic adenoma
PLAG1 and HMGA2 alterations link salivary myoepitheliomas to pleomorphic adenoma, fueling the view that they sit along one morphologic spectrum rather than as entirely separate entities. Identifying the same rearrangement in both supports the concept of a biological continuum.
Recognizing that these lesions sit on a shared genetic spectrum reframes the daily task of telling them apart under the microscope.
If immunohistochemistry leaves the diagnosis open, ask whether molecular testing for EWSR1, FUS, PLAG1, and HMGA2 is appropriate, since fusion results can reframe an ambiguous case into a confident myoepithelial tumor diagnosis.
Distinguishing Myoepithelioma from Other Salivary Gland Neoplasms
Several common salivary gland tumors enter the differential, and separating them is the daily work of head and neck pathology. The table below summarizes the most useful contrasts.
| Tumor | Key distinguishing feature | IHC clue |
|---|---|---|
| Myoepithelioma | Pure myoepithelial population, little ductal component | S100+, p63+, CK+, calponin+ |
| Pleomorphic adenoma | Mixture of ductal, myoepithelial, and chondromyxoid stroma | S100+, p63+, GFAP+ in stromal cells |
| Basal cell adenoma | Basaloid nests with peripheral palisading, scant stroma | CK5/6+, p63+, S100 often negative |
| Adenoid cystic carcinoma | Cribriform pattern, perineural invasion, small uniform basaloid cells | CD117+, MYB+, S100 variable |
| Carcinoma ex pleomorphic adenoma | Malignant transformation in a pre-existing mixed tumor | Cytologic atypia, infiltrative growth, high Ki-67 |
Adenoid cystic carcinoma is the most important malignant mimic because of its tendency for perineural invasion and late recurrence. A uniform small-cell population with cribriform architecture and MYB overexpression favors adenoid cystic carcinoma over myoepithelioma, even when both express p63 and S100.
Once the differential narrows to one diagnosis, the next decisions shift from morphology to surgical margins, adjuvant therapy, and follow-up cadence.
Treatment Approach, Prognosis, and Diagnostic Workflow
Most cases follow a predictable clinical course, but a thoughtful diagnostic sequence and disciplined follow-up are what protect patients from the rare aggressive variant.
Standard treatment and prognosis
Wide local excision with clear margins remains the cornerstone of therapy for localized disease at either salivary or soft tissue sites. Benign lesions rarely recur after complete resection, and prognosis is excellent. Malignant myoepithelioma carries a real risk of local recurrence and distant metastasis to lung, bone, and soft tissue, which is why margin status and tumor grade drive follow-up intensity.
Diagnostic workflow in practice
Most patients move through the same set of steps:
- Imaging: ultrasound or MRI to characterize the mass, evaluate depth, and look for satellite nodules
- Fine-needle aspiration or core biopsy: provides cytology or limited histology before definitive surgery
- Histopathology: full evaluation of architecture, cytology, and stromal patterns
- Immunohistochemistry panel: S100, p63, p40, cytokeratin, calponin, and selected basal or myoepithelial stains
- Molecular confirmation: FISH or NGS for EWSR1, FUS, PLAG1, or HMGA2 when the case is ambiguous
- Multidisciplinary review: pathology, radiology, surgical oncology, and, for malignant cases, medical oncology
Follow-up and special considerations
For benign lesions, clinical surveillance at intervals decided by your surgical team is usually sufficient. Malignant myoepithelioma warrants longer and more frequent imaging, particularly of the chest, because pulmonary metastases are the most common distant site. You should discuss follow-up imaging schedules and warning signs such as new lumps, pain, or facial nerve changes with your treating specialist, who can tailor surveillance to your specific tumor grade and margin status.
Bottom Line
its are uncommon but well-defined neoplasms whose recognition depends on integrating histology, immunohistochemistry, and selective molecular testing. Most behave benignly after complete excision, and the rare malignant variant warrants longer follow-up coordinated by a multidisciplinary team familiar with salivary gland neoplasm management.
FAQ
What is a myoepithelial tumor?
A it is a neoplasm composed primarily of myoepithelial cells, which normally wrap secretory units in exocrine glands such as the salivary and sweat glands. The pure form is called myoepithelioma, and it is recognized as a distinct salivary gland neoplasm in the WHO classification.
Are myoepithelial tumors cancerous?
Most are benign and cured by complete surgical excision, but a subset classified as malignant myoepithelioma can recur locally or spread to distant sites such as the lung or bone. Tumor grade and margin status, not the diagnosis alone, drive prognosis.
Where do myoepithelial tumors occur?
The parotid gland is the most common site, with additional salivary locations including the submandibular gland and minor salivary tissues of the palate and oral cavity. Soft tissue variants arise in the extremities, trunk, and head and neck region without an obvious glandular origin.
How is a myoepithelial tumor diagnosed?
Diagnosis combines imaging, fine-needle aspiration or core biopsy, histopathology, and an immunohistochemistry panel that typically includes S100 protein, p63, p40, calponin, and cytokeratins. Molecular studies for EWSR1, FUS, PLAG1, or HMGA2 rearrangements help resolve ambiguous cases.
How are myoepithelial tumors treated?
Surgical excision with clear margins is the standard treatment for localized disease, and benign tumors rarely recur after complete removal. Malignant myoepithelioma may require wider margins, longer imaging follow-up, and multidisciplinary coordination, with management guided by a specialist in head and neck or soft tissue tumor care.
