The incidence of the various salivary gland tumours is not well understood. Reporting is complicated by:
- Varying rates of the different pathologies throughout the world
- Variable reporting of benign tumours
- Metastatic and haematological diseases frequently involve the major salivary glands
Most tumours arise in the parotid gland (two thirds); the next most common site is the minor salivary glands (one fifth); ten percent arise in the submandibular glands and fewer than one percent arise from the sublingual gland. Most tumours are benign; only one third are malignant; tumours arising in sites other than the parotid are far more likely to be malignant.
The most common tumour is the pleomorphic adenoma (50%). After this, the most common benign tumour is Warthin tumour (5-10%) and the most common malignant tumour is mucoepidermoid carcinoma (15%)
In order of relative incidence, they are:
- Pleomorphic adenoma
- Mucoepidermoid carcinoma
- Adenocarcinoma, not otherwise specified
- Warthin tumour, acinic cell carcinoma, adenoid cystic carcinoma
- Rare carcinomas (e.g. squamous cell carcinoma)
In general, the tumours are all more common in women (slightly) except for Warthin tumour which has a striking male predominance.
The varied salivary gland tumours will be summarised on separate pages.
Pleomorphic adenoma, as the name suggests, contains a variable population of tissues. It has a bosselated, encapsulated surface. The cut surface may be glistening if there is a large myxoid component. The three important components are:
- A capsule, which is often breached microscopically by tumour and increases the risk of local recurrence if local resection alone is attempted
- An epithelial/myoepithelial component, which consists of epithelial cells in solid or glandular patterns. The epithelial cells have a variety of appearances (cuboid, columnar, spindle, plasmatcytoid etc)
- A mesenchymal component, which forms a myxoid stroma or less commonly cartilaginous or fibrous areas. The myxoid stroma contains scattered myoepithelial cells.
Warthin tumour is the second most common benign salivary tumours. It occurs mostly in men and more commonly in smokers (8:1). Macroscopically, it is encapsulated and contains cystic areas in an otherwise tan tumour.
Microscopically, the solid areas are formed by lymphoid tissue with germinal centres. These are lined by a bilayered epithelium; the upper layer forms columnar cells with ciliated epithelium and oncocytic cytoplasm (pink and granular!). The lower layer is cuboidal. The cystic spaces are filled with mucinous or serous material.
An uncommon tumour of the major salivary glands, adenoid cystic carcinoma is notable for being responsible for about 30% of minor salivary gland tumours. It is formed by basaloid cells which are arranged in cribriform, tubular or solid patterns. Perineural invasion is often present and extensive beyond the macroscopic boundary of the tumour, as is bony invasion. Immunohistochemistry is not well defined.