Mucoepidermoid carcinoma of salivary glands

Mucoepidermoid carcinoma is a tumour that usually occurs in the salivary glands. It can mimic most other tumours of the glands, and therefore is often considered in the differential. 

Epidemiology

Mucoepidermoid carcinomas are seen throughout all adult age groups, but are most common in middle age (35-65 years of age) ⁵. However, it is the most common malignant salivary gland tumour of childhood ^4-5. Overall, mucoepidermoid carcinomas account for ^1,5:

• 2.8-15.5% of all salivary gland tumours:
  • 1-10% of all major salivary gland tumours
  • 6.5-41% of minor salivary gland tumours

In the parotid gland they are the most common malignant primary neoplasm. A slight female predilection has been described, and radiation has been implicated as a risk factor ⁵.

Clinical presentation

Mucoepidermoid carcinomas most frequently arise in the parotid gland, and presents as a painless swelling, with or without facial nerve involvement. These tumours can however be found anywhere there are salivary glands. Overall distribution across various glands is as follows ⁵:

• major salivary glands: ~50% *
  • parotid gland: ~40%
  • submandibular gland: ~7%
  • sublingual gland: ~3%
• minor salivary glands: ~50%
  • palate: most common
  • retromolar trigone
  • floor of the mouth
  • buccal mucosa
  • lip
  • tongue
  • other: anywhere in the proximal aerodigestive tract, the lacrimal glands and even in the bronchi ^2,5

As such, presentation will depend on the anatomic location.

Pathology

The tumours are composed of a mixture of:

1. mucus secreting cells (muco-)
2. squamous cells (-epidermoid)
3. lymphoid infiltrate often also present ³

Histology

Histology will often show clear mucin containing cells, which stain reddish pink with the mucicarmine stain. Mucoepidermoid tumours are graded histologically into:

• low grade:
  • well-differentiated cells with little cellular atypia
  • high proportion of mucous cells
  • prominent cyst formation
• intermediate grade: intermediate features
• high grade
  • poorly differentiated with cellular pleomorphism
  • high proportion of squamous cells
  • solid with few if any cysts

Radiographic features

Radiographic appearances largely depend on grade, making preoperative imaging important in planning and counselling. 

Ultrasound

Typically a well-circumscribed hypoechoic lesion, with a partial or completely cystic appearance.  The lesion stands out against the relatively hyperechoic normal parotid gland.

CT

Low-grade tumours appear as well-circumscribed masses, usually with cystic components. The solid components enhance, and calcification is sometimes seen. They have appearances similar to benign mixed tumours. 

High-grade tumours on the other hand, have poorly defined margins, infiltrate locally and appear solid.

MRI

Again, imaging is dependent on grade. 

Low-grade tumours have similar appearances to benign mixed tumours:

• T1: low to intermediate signal; low signal cystic spaces
• T2: intermediate to high signal; cystic areas will be high signal
• T1 C+ (Gd): heterogeneous enhancement of solid components

High grade tumours on the other hand have lower signal on T2 and poorly defined margins and infrequent cystic areas:

• T1: low to intermediate signal
• T2: intermediate to low signal

It is essential to image the cranial nerves with fat saturated post contrast T1 sequences to assess for perineural spread, and as such the base of skull should be imaged up to and including the cavernous sinus and inner ear. 

Treatment and prognosis

Treatment is dependent on grade and location:

• low grade (well circumscribed) can usually be treated with wide local excision and preservation of the facial nerve, without the need for neck dissection or adjuvant radiotherapy
• high grade (poorly circumscribed) usually requires complete parotidectomy, often with sacrifice of the facial nerve, neck dissection (as nodal metastases are common) and adjuvant radiotherapy

Prognosis is also very dependent on grade, with low grade tumours having a 90-98% survival and a low local recurrence rate, compared to 30-54% surviving, and a very high local recurrence rate for high grade tumours ^1,5.

Additionally this tumour has a predilection for perineural spread, and careful and long term follow-up is therefore required.

Differential diagnosis

The differential is therefore different according to appearance.

For well circumscribed lesions consider:

• benign mixed tumour (BMT): can be indistinguishable
• Warthin tumour
• adenoid cystic carcinoma

For infiltrative lesions consider:

• adenoid cystic carcinoma
• lymphoma (NHL)
• metastasis (especially squamous cell carcinoma metastases to intra-parotid nodes)