Description
Mucoepidermoid carcinoma is a type of cancer that most commonly arises from the salivary glands, including the parotid gland, submandibular gland, and minor salivary glands. It is the most common malignant tumour of the salivary glands, accounting for around 5-15% of all salivary gland neoplasms.
Pathogenesis
Mucoepidermoid carcinoma originates from the excretory duct cells in salivary glands. It arises from genetic mutations, with a common aberration being a translocation between chromosomes 11 and 19. This leads to the fusion of MECT1 and MAML2 genes. This fusion dysregulates the NOTCH signalling pathway, resulting in uncontrolled cell growth and tumour formation.
The tumour comprises a mixture of mucus-secreting cells, squamous cells, and intermediate cells. The ratio of these cells and their level of differentiation vary, contributing to the different grades of the carcinoma.
- Low-grade mucoepidermoid carcinomas have well-differentiated cells, form cystic and glandular structures, grow slowly and have a good prognosis.
- High-grade variants contain more poorly differentiated cells, grow rapidly, are invasive, and have a higher likelihood of metastasis, leading to a poorer prognosis.
Epidemiology, Risk Factors & Associations
- Mucoepidermoid carcinomas can occur at any age but are most common between the ages of 20 and 60 (mean age: 40-50 years).
- There’s a slight female predominance.
- Risk factors are not clearly defined, but previous radiation exposure is a recognised risk.
Clinical Features
The clinical presentation depends on the location of the tumour. For parotid gland mucoepidermoid carcinoma, symptoms may include a slow-growing, painless mass, facial nerve weakness (seen more often in malignant cases), and pain. Minor salivary gland tumours can present as a slowly enlarging mass or ulcer in the palate.
Complications
Higher-grade tumours can invade surrounding structures leading to local complications. Metastasis can occur, typically to the regional lymph nodes and the lungs.
Pathological Features
Histopathology
- Mucoepidermoid carcinoma is characterised by the presence of mucous-secreting, epidermoid, and intermediate cells. The tumour grade (low, intermediate, high) is determined by the relative proportions of these cells, cellular differentiation, and growth pattern (cystic vs solid).
Genetics
- A translocation between chromosomes 11 and 19 leading to the fusion gene MECT1-MAML2 is often identified in low and intermediate-grade tumours.
Radiological Features
General Features
- Generally, these tumours appear as well-defined masses, and higher-grade tumours may demonstrate more aggressive features such as ill-defined borders or invasion into surrounding structures.
CT
- Typically heterogeneous due to a mix of solid and cystic components.
- Higher-grade tumours tend to have more solid components.
- Contrast enhancement is variable.
MRI
- Low-grade tumours may appear with high signal intensity on T2-weighted images due to cystic components.
- Solid components show intermediate signal intensity on T2-weighted images and enhance with contrast.
Grading and Staging
Tumour grade is based on the proportion of cell types (mucinous, squamoid, intermediate), growth pattern, neural or vascular invasion, necrosis, and mitotic activity. The tumour-node-metastasis (TNM) staging system is used for staging.
Differential Diagnosis
- Pleomorphic adenoma: A benign mixed tumour that commonly presents as a painless, slow-growing mass. It has a ‘pseudo-capsule’ which can be identified radiologically and can help differentiate it from mucoepidermoid carcinoma. Histologically, it shows a mixture of myoepithelial and epithelial cells in a chondromyxoid stroma.
- Adenoid cystic carcinoma: It typically presents with perineural invasion and a ‘Swiss-cheese’ appearance histologically due to cribriform pattern. This can differentiate it from mucoepidermoid carcinoma, which lacks perineural invasion and the characteristic histological pattern.
- Warthin tumour: Typically bilateral and has a strong association with smoking. Unlike mucoepidermoid carcinoma, it often presents as a cystic mass with hypoattenuating areas on CT due to its high lipid content.
- Sialadenosis: This is a non-inflammatory, non-neoplastic enlargement of the salivary gland usually associated with systemic diseases like diabetes, alcoholism. It lacks the distinct tumour mass seen in mucoepidermoid carcinoma.
- Lymphoma: It typically presents with salivary gland enlargement without a discrete mass, often associated with systemic symptoms (fever, weight loss, night sweats), and lymphadenopathy. The absence of a discrete mass can help differentiate it from mucoepidermoid carcinoma.
- Squamous cell carcinoma: This aggressive malignancy often has a history of surface ulceration or leukoplakia, and may invade adjacent structures. High-grade mucoepidermoid carcinoma can mimic squamous cell carcinoma but usually lacks the preceding history of surface changes.
Management
- Surgical excision with clear margins is the mainstay of treatment.
- Neck dissection may be considered in the presence of neck lymph node metastasis.
- Postoperative radiotherapy may be considered in high-grade or advanced-stage tumours.
- Regular follow-up is required given the risk of local recurrence and distant metastasis.