Mucoepidermoid carcinoma (MEC)

• Mucoepidermoid carcinoma (MEC):
  • Is the most common malignant neoplasm of the:
    • Mayor and minor salivary glands
• Epidemiology:
  • They encompass between 2.8% to 15.5% of all salivary gland tumors
  • Among 12% to 35% of malignant salivary gland tumors
  • Among 6.5% to 41% of all minor salivary gland tumors:
    • Representing the most common type of malignant minor salivary gland tumor in most series
  • Approximately half the cases occur in the major salivary glands:
    • 65% to 80% of these occur in the parotid
    • 8% to 13% occur in the submandibular gland
    • 2% to 4% involve the sublingual gland
  • MEC of the minor salivary glands:
    • Ordinarily arises on the palate:
      • But a number may also be found in the:
        • Retro molar area
        • Floor of the mouth
        • Buccal mucosa
        • Lip
        • Tongue
  • Its prevalence is highest in:
    • The fourth to fifth decade of life (35 to 65 years of age):
      • With a female preponderance as high as 4:1
• Grossly:
  • The tumor is poorly circumscribed and measures from 3 to 5 cm
• Histologically:
  • They are characterized by a mixed population of cells, including:
    • Mucin-producing cells
    • Epidermoid cells with squamoid differentiation
    • Clear cells
    • Intermediate cells:
      • That may predominate in numbers
      • Are believed to be the progenitor of the other types of cells
    • No myoepithelial cells are present
• The clinical behavior of MEC has proved to be difficult to predict:
  • But correlations to tumor grade and stage have been reported
  • The histologic features that are most useful in predicting the aggressive nature of these tumors are:
    • A minor cystic component (less than 20%)
    • Tumor necrosis
    • Neural invasion
    • Cellular anaplasia
    • Brisk mitotic activity
  • Based on the presence or absence of these features and the clinical behavior, MEC are classified as:
    • Low grade
    • Intermediate grade
    • High grade
  • Low-grade MEC are:
    • Well circumscribed, with pushing margins and dilated cystic areas containing mucin
    • Mucin producing, intermediate, or epidermoid cells make up the lining of these cystic structures
  • Intermediate-grade MEC:
    • As the grade worsens:
      • The tumors become more infiltrative, poorly circumscribed
      • Cystic formations are lost
      • Nests of tumor become more solid and irregular with intermediate or epidermoid cells dominating
  • High-grade MEC are characterized by:
    • The invasion of adjacent structures
    • Atypical mitoses
    • Necrosis
    • Perineural invasion
    • Lymph node metastasis:
      • 40% to 50%
    • Distant metastases
  • Differential diagnosis of these high-grade lesions are:
    • Primary of metastatic squamous cell carcinoma:
      • MEC is differentiated from metastatic SCC by:
        • The presence of intracellular mucin
    • Sebaceous carcinomas
    • Clear cell carcinomas
• Histologic grade and tumor stage:
  • Appear to have profound effects on survival
  • Aro et al:
    • Found a statistically significant difference in disease free survival (DFS) by grade:
      • Between low-grade MEC and intermediate / high-grade MEC (P = 0.001)

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