Tumours of salivary glands

What is the general distribution of salivary gland tumours across the body's major and minor glands?

● They represent less than 2\% of all human tumours.

● ​65% to 80% arise within the parotid gland.

● ​10% arise in the submandibular gland.

● ​The remainder occur in the minor salivary glands, including the sublingual glands.

Hint: Most common in the parotid gland.

How are salivary gland tumours broadly classified into benign and malignant categories in this curriculum?

Benign: Pleomorphic adenoma and Warthin tumour.

​Malignant: Mucoepidermoid carcinoma and adenoid cystic carcinoma.

Hint: Two major benign types and two major malignant types.

What are the gross and histological features of a pleomorphic adenoma?

● ​It is the most common salivary gland neoplasm, representing about 60% of tumours in the parotid.

● ​It is a benign tumour consisting of a mixture of ductal (epithelial), myoepithelial, and mesenchymal cells (hence "mixed tumor").

● ​Known risk factor: Radiation exposure increases the risk.

What are the gross and histological features of a pleomorphic adenoma?

Gross: Rounded, well-demarcated, encapsulated masses. Cut surface is gray-white with myxoid and blue translucent areas of chondroid (cartilage-like) stroma.

​Histology: Marked heterogeneity. Epithelial/myoepithelial cells form ducts, acini, tubules, or sheets dispersed within a loose myxoid and hyaline stroma containing islands of cartilage, rare foci of bone, and well-differentiated squamous epithelium.

Hint: Cartilage-like (chondromyxoid) stroma mixed with epithelial/myoepithelial elements.

What are the typical clinical features and recurrence/malignant transformation risks of pleomorphic adenoma?

Presentation: Painless, slow-growing, mobile, discrete masses in the parotid, submandibular, or buccal areas.

​Recurrence: Rate is about 4% following parotidectomy, occurring months to years later.

​Malignant Transformation: Risk correlates with the age/duration of the lesion.

Hint: Slow-growing, mobile, painless mass with a small recurrence rate.

What are the unique epidemiological and anatomical characteristics of a Warthin tumour?

● It is the second most common salivary gland neoplasm.

● ​Arises almost exclusively in the parotid gland.

● ​Benign tumour occurring more commonly in males than females, usually in the 5th to 7th decades.

● ​Smokers have an 8-fold increased risk.

● ​Mostly unifocal, but about 10% are multifocal and 10\% are bilateral.

What are the gross and histological features of a Warthin tumour?

Gross: Round to oval, encapsulated mass (2\text{ to }5\text{ cm}) in the superficial parotid. Cut surface shows a pale gray background with narrow cystic or cleft-like spaces filled with mucinous/serous secretions, narrowed by polypoid lymphoepithelial projections.

​Histology: Lined by a double layer of oncocytic cells (innermost layer is columnar, outer layer is cuboidal) with abundant eosinophilic cytoplasm, set within a dense lymphoid stroma containing germinal centers.

Hint: Double-layered oncocytic epithelium resting on a dense lymphoid stroma.

What is the frequency and primary site of mucoepidermoid carcinoma?

● It is the most common primary malignant tumour of salivary glands, representing about 15\% of all salivary gland tumours.

● ​60% to 70% occur in the parotid glands.

● ​Hint: Most frequent malignant salivary gland tumor, mostly in the parotid.

What are the gross and microscopic features of mucoepidermoid carcinoma?

Gross: Can grow up to 8\text{ cm}. Appears circumscribed but lacks a true well-defined capsule, showing infiltrative borders. Cut surface is pale, gray-white with small mucin-containing cysts.

​Histology: Composed of cords, sheets, or cystic configurations of squamous (epidermoid) cells, mucous cells (mucocytes), and intermediate cells containing small to large mucus-filled vacuoles.

Hint: Mixture of epidermoid cells, intermediate cells, and mucocytes around cystic/mucin spaces.

How does the histological grade of mucoepidermoid carcinoma impact its clinical course and prognosis?

Low-grade tumors: May invade locally and recur in ~15% of cases, but rarely metastasize; 5-year survival rate is >90\%.

​High-grade and intermediate-grade tumors: Invasive and difficult to excise; up to 30% recur and 30% metastasize to distant sites, yielding a 5-year survival of only 50\%.

​Hint: Prognosis relies heavily on histological grade (low vs. high/intermediate).

Where do adenoid cystic carcinomas commonly arise, and what are their gross features?

​Commonly affect the parotid and submandibular glands, with 50% occurring in minor salivary glands (particularly the palatine glands).

​Gross: Small, poorly encapsulated, infiltrative, gray-pink lesions.

What are the three major histologic patterns and key histological hallmarks of adenoid cystic carcinoma?

Cribriform Pattern (Intermediate grade/prognosis): The classic "swiss cheese" pattern where "holes" are pseudocysts lined by replicated basement membrane produced by neoplastic cells.

​Tubular Pattern (Lowest grade, best prognosis): Two distinct cell populations arranged into ducts within a hyaline stroma, where luminal epithelial cells are completely surrounded by myoepithelial cells.

​Solid Pattern (Highest grade, worst prognosis): Composed of non-cystic sheets or nests of neoplastic cells making up \ge 30\% of the tumour.

​Classic Characteristic: Prominent perineural and/or intraneural invasion.

​Hint: Cribriform (swiss cheese), tubular, solid patterns, and strong perineural invasion.

What is the clinical behavior and long-term prognosis of adenoid cystic carcinoma?

Unpredictable, slow-growing tumors that aggressively invade perineural spaces and regularly recur.

​50% or more eventually disseminate to distant sites including bone, liver, and brain, sometimes decades after attempted removal.

​Tumors arising in minor salivary glands have a poorer prognosis than those in the parotid.

​Hint: Slow-growing, late distant metastasis (decades later), and prominent perineural spread.