Mucoepidermoid Carcinoma of the Parotid Gland

Overview

• Cummings Otolaryngology, p. 1531 | Scott-Brown's Otorhinolaryngology, p. 1149 | Bailey and Love's Surgery, p. 862

Etiology / Risk Factors

• Radiation exposure is the major known risk factor. Prior therapeutic radiation carries a latent period of 7-32 years before MEC development.
• MECs are known to occur following radiation or chemotherapy in childhood.
• Scott-Brown's Otorhinolaryngology

Clinical Presentation

• Typically a painless, slow-growing mass in the parotid region.
• Low-grade tumors are soft and often cystic, sometimes with a bluish hue resembling a mucocele (especially intraoral minor gland tumors).
• High-grade tumors may present with skin or bone involvement, pain, or facial nerve involvement.
• Distant metastases (mainly to lungs) are seen predominantly with high-grade disease.

Gross Pathology

• Tumors are circumscribed or infiltrative, predominantly solid with tan-white to pink cut surface.
• Often have cystic components filled with viscous mucinous (brown) fluid.
• Low-grade: macrocystic, well-circumscribed.
• High-grade: solid, infiltrative.

Microscopic Pathology (Histology)

True keratinization is rare in MEC; its presence should raise the alternative diagnosis of adenosquamous carcinoma.

• Cummings Otolaryngology, p. 1531

Histologic Grading

• Cummings Otolaryngology, p. 1531 (Table 85.6)

• Low-grade: Predominantly cystic, well-circumscribed, rich in mucous cells, minimal atypia.
• Intermediate-grade: Less circumscribed, more solid, predominant intermediate cells; most challenging for pathologists.
• High-grade: Solid, infiltrative, nuclear atypia, necrosis, perineural invasion, lymphovascular emboli; resembles SCC - diagnosis requires demonstration of at least focal intracellular mucin.

Molecular Genetics

• Scott-Brown's Otorhinolaryngology, Table 26.9

Differential Diagnosis

• Necrotizing sialometaplasia - nonneoplastic reactive change in minor salivary glands (hard palate); can mimic MEC.
• Squamous cell carcinoma - especially for high-grade MEC; mucin stains are key to differentiate.
• Mucocele - for low-grade intraoral MEC with bluish hue.
• Adenosquamous carcinoma - when true keratinization is present.

FNA Cytology

• Mucous cells: abundant vacuolated cytoplasm
• Intermediate cells: small, round, scant cytoplasm, open chromatin
• Squamoid cells: moderate dense homogeneous cytoplasm; appear orange on Papanicolaou staining

Treatment

• Complete surgical excision with wide margins is the treatment of choice for all grades.
• Parotidectomy (superficial or total depending on tumor location and extent) with facial nerve preservation when possible.
• Adjuvant radiotherapy is indicated for intermediate- to high-grade MEC.
• Neck dissection is performed when there is clinical or radiographic evidence of nodal metastasis; elective neck dissection is considered for high-grade tumors.
• Distant metastases (predominantly to lungs) are treated with systemic therapy.
• Bailey and Love's Surgery, p. 861

Prognosis

• Clinical stage is as important as histologic grade and the two should always be considered together.
• MEC of the submandibular gland has a more aggressive course than parotid MEC.
• Cummings Otolaryngology, p. 1531

Key Summary Points

• Most common malignant salivary gland tumor; most common site = parotid
• Most common salivary gland carcinoma in children and adults
• Triphasic: mucous + squamoid + intermediate cells
• Graded low/intermediate/high (Auclair or Brandwein systems)
• Molecular signature: CRTC1-MAML2 translocation in >80%
• Treatment: surgery ± adjuvant radiotherapy for intermediate/high grade
• Prognosis directly correlates with histologic grade and clinical stage