Definition
Mucoepidermoid carcinoma is a malignant salivary gland tumour composed of mucinous, intermediate (clear-cell) and squamoid/epidermoid tumour cells forming cystic and solid patterns. It is the most common single type of malignant salivary neoplasm and the most frequently encountered malignant salivary gland tumour in children and young adults.
Etiology and Pathophysiology
Contributory factors :
- Caused by a t(11;19) translocation bringing the MECT1 gene together with the MAML2 gene, producing a novel fusion gene that activates the notch signalling pathway (a developmental pathway deranged in several cancers).
- May develop secondary to radiation or chemotherapy during childhood (median latency period of ~8 years).
Prevalence
- Most common malignant salivary gland tumour overall, yet accounts for less than 10% of all salivary gland neoplasms.
- Most frequent malignant salivary gland tumour in children and young adults.
- Wide age range with a peak incidence in the second decade of life; female predilection.
Histological Features
- Numerous mucin-filled cysts of varying shape and size and cribriform structures set within a dense desmoplastic stroma; mixture of solid and cyst-like areas.
- Three essential cell types lining the cystic spaces:
- Mucocytes (mucous cells): pale cytoplasm with peripherally displaced nuclei.
- Intermediate cells.
- Epidermoid (squamoid) cells: polygonal with central, mildly pleomorphic nuclei and abundant cytoplasm; no definite keratinisation.
- Variable numbers of clear cells, columnar cells and oncocytes may also be present.
- Surrounding stroma shows extensive sclerosis and a prominent peritumoral inflammatory infiltrate (lymphocytes, plasma cells, lymphoid aggregates with reactive germinal centres); usually non-encapsulated, infiltrative, and perineural invasion may be present.
- Mucin demonstrated by mucicarmine or Alcian blue; clear cells are diastase-sensitive PAS-positive (glycogen).
- Graded histologically into low-, intermediate- and high-grade types, which predicts (though not accurately) metastatic risk.
Clinical Features
Location
- Parotid gland is the most common site (about half arise in a parotid gland); all glands, including minor glands, can be affected.
- Other sites: palate, submandibular gland and intraoral minor salivary glands; minor-gland sites also include tuberosity, retromolar region, tongue and floor of mouth.
- Rarely primary intraosseous (central) lesions.
Appearance
- Generally a slow-growing mass; presentation varies by site, size and grade.
- Cystic intraoral lesions can be easily mistaken clinically for a mucocele / mucous extravasation or retention cyst; mucinous lesions may fluctuate in size if a cyst ruptures.
- May present with asymptomatic swelling, pain and/or facial nerve palsy.
Differential Diagnoses
- Mucocele / mucous extravasation or retention cyst (cystic intraoral mucoepidermoid carcinoma closely mimics these).
- Other salivary gland neoplasms (e.g. pleomorphic adenoma, adenoid cystic carcinoma, acinic cell carcinoma).
Relevant Clinical Investigations
- History and clinical examination.
- Imaging: ultrasound, CT, MRI (MRI to confirm the swelling lies within the gland).
- Fine needle aspiration (FNA) cytology — high-grade lesions are readily recognised on FNA, but lower-grade types are often only diagnosed after excision.
- Biopsy / histological examination is required for diagnosis; identification of the t(11;19) translocation can aid diagnosis if required.
Patient Management
- Surgical excision with clear margins is the treatment of choice; the tumour grows slowly and infiltrates surrounding tissue, so wide excision is used.
- Radiotherapy may play a role in local control in selected cases.
- Prognosis (10-year survival): low-grade ~90%, intermediate-grade ~70%, high-grade ~25%. Low- and intermediate-grade tumours metastasise in only a few percent of patients and are almost never fatal; ~40% of high-grade carcinomas metastasise and ~30% of high-grade patients die of the disease.