Myoepithelial Carcinoma

Epidemiology and Etiology:

• About ½ develop within pre-existing pleomorphic adenoma or myoepithelioma
• Multiple recurrences of PA or myoepithelioma are a risk factor
• <2% of malignant salivary gland neoplasms
• Wide age range (mean 6th decade)
• Rare in children

Common sites:

• Parotid (~75%)
• Submandibular gland and minor salivary glands

Gross features:

• Rapidly expanding, painless mass
• 2-10 cm
• Locally destructive
• Bone destruction
• Unencapsulated (most)
• Nodular
• Gray-white, firm, “glassy” cut surface
• Cystic degeneration common
• Necrosis maybe

Histologic features:

• Exclusively myoepithelial differentiation
• Variable growth patterns
• Nodular to diffuse
• Nests, sheets, or cords
• Highly cellular usually, but hypocellular lesions can be seen
• Invasive growth (most helpful feature in separation from benign tumour)
• Adjacent bone
• Perineural and vascular invasion frequent
• Myoepithelial cells
• Plasmacytoid
• Epithelioid
• Spindled
• Pleomorphism usually limited (may be marked)
• Clear cytoplasm maybe
• Myxoid or mucoid stroma
• Proteoglycan accumulation
• Basal lamina may be present
• Cartilaginous rarely
• Mitotic activity easy to find usually
• Necrosis maybe
• No ducts identified (by definition)
• Adjacent PA or myoepithelioma maybe
• Rare squamous differentiation

Immunophenotype:

Marker:	Sensitivity:	Specificity:
CK5/6
S100		Negative in LMS
GFAP (isolated cells)		Synovial sarcoma and LMS negative
Vimentin
Calponin	Not as frequent as S100
SMHC
Actin-sm
CK14	Variable
CK7	Variable

Molecular features:

• Cytogenetic aberrations in up to ½
• Chromosome 8 alterations most frequent

Other features:

• ~1/3 mortality rate
• Marked pleomorphism and high proliferation index suggest worse outcome
• ~1/3 recurrence, frequently multiple
• Regional and distant metastases late in disease course maybe
• Cervical metastases
• Lung
• Others
• Locally aggressive maybe
• Intermediate to high-grade tumour clinically

References:

• Thomson & Wenig, eds. Diagnostic Pathology: Head and Neck (2^nd ed.) (2013)