Malignant Salivary Gland Tumours

Malignant salivary gland tumours are a histologically diverse group of malignancies arising from the major and minor salivary glands. Mucoepidermoid carcinoma is the commonest specific malignant tumour overall.

Other types include:

• adenoid cystic carcinoma, characterised by perineural spread
• acinic cell carcinoma
• carcinoma ex pleomorphic adenoma, which is malignant transformation within a pre-existing, longstanding pleomorphic adenoma (see benign salivary gland tumours)
• several rarer histological subtypes

Malignancy risk is proportionally higher in the smaller glands (submandibular, sublingual, and minor salivary glands) than in the parotid. The parotid still harbours the largest absolute number of malignant tumours, because its overall tumour burden is much greater. The proportion of benign to malignant tumours differs across glands.

Classification and pathophysiology

• Mucoepidermoid carcinoma shows a spectrum of histological grade. Low grade tumours are generally indolent, with a favourable prognosis approaching that of benign disease. High grade tumours behave more aggressively, with a greater propensity for local invasion and distant metastasis. Grade is a central determinant of both prognosis and treatment.
• Adenoid cystic carcinoma characteristically invades and spreads along nerves, often well beyond the grossly visible margin of the primary tumour. This explains the pain or facial or other cranial nerve dysfunction that is disproportionate to tumour size, and it creates surgical and radiotherapy planning challenges.
• Adenoid cystic carcinoma also tends towards late distant metastasis, particularly to the lungs, sometimes many years after apparently successful initial treatment. Its long term natural history is generally indolent but relentless, and frequently fatal.
• Facial nerve involvement, presenting as facial weakness or paralysis, is a red flag strongly suggestive of malignancy in any parotid mass, because it is essentially never seen in benign parotid disease (see benign salivary gland tumours). It bears directly on diagnosis and on the extent of surgical resection required.

• A mass within the affected salivary gland is the common initial feature, and early on it may be indistinguishable from a benign tumour.
• Features raising concern for malignancy: rapid growth; pain (particularly characteristic of adenoid cystic carcinoma, given its perineural spread); facial nerve weakness (parotid tumours); fixation of the mass to surrounding structures or overlying skin; and cervical lymphadenopathy.
• Facial nerve weakness or paralysis in a parotid mass is an essentially pathognomonic feature of malignancy, because it is near universally absent in benign parotid disease. It warrants urgent assessment, and should never be attributed to a benign process without thorough investigation excluding malignancy.
• Submandibular or minor salivary gland masses are proportionally more likely to be malignant than parotid masses. A mass at these sites warrants a heightened index of suspicion and prompt investigation.
• Adenoid cystic carcinoma may present with pain, numbness, or cranial nerve dysfunction disproportionate to the size of the primary tumour. This reflects its perineural spread and is an important clue to the diagnosis.

• Initial diagnosis and staging use ultrasound with fine needle aspiration cytology, following similar principles to those under benign salivary gland tumours. Magnetic resonance imaging characterises tumour extent and assesses perineural spread, which is critical for suspected adenoid cystic carcinoma because it informs surgical and radiotherapy planning. It also shows any facial nerve or other adjacent structure involvement.
• Formal histopathological assessment of the surgically excised specimen gives the definitive diagnosis, the tumour grade, and the adverse pathological features: perineural invasion, lymphovascular invasion, and margin status. These directly inform whether adjuvant treatment is needed and how extensive it should be.
• Staging includes cross-sectional imaging of the neck to assess regional and distant disease. Chest imaging is added where tumour grade, size, or histological subtype indicates, particularly for adenoid cystic carcinoma, given its propensity for late pulmonary metastasis.

• Surgical excision is the primary treatment across the spectrum of malignant salivary gland tumours. Its extent is determined by tumour size, grade, and location.
• Facial nerve preservation is attempted wherever oncologically feasible. Nerve sacrifice may be needed where the tumour directly involves or encases the nerve, and grafting is considered at the time of resection or as a later, staged procedure.
• Elective neck dissection is considered according to tumour grade, size, and histological subtype, given the variable but recognised risk of occult regional lymph node metastasis, particularly in high grade tumours. The approach and extent are individualised through multidisciplinary team discussion.
• Adjuvant radiotherapy is indicated for high grade tumours and for tumours with significant adverse pathological features: perineural invasion, close or positive surgical margins, and regional lymph node metastasis.
• Adjuvant radiotherapy is also indicated for adenoid cystic carcinoma, generally regardless of grade, because of its perineural spread. Radiotherapy addresses microscopic residual disease along nerve fibres beyond the surgical margin.
• Systemic therapy has a limited, generally palliative role for recurrent or metastatic disease not amenable to further local treatment, because most histological subtypes have limited chemosensitivity. Targeted therapy for selected, molecularly characterised subtypes is an area of ongoing research.
• Long term structured follow up is needed, with attention to the pattern of recurrence and metastasis of the individual histological subtype. Adenoid cystic carcinoma needs prolonged surveillance, because distant metastasis can be very delayed, sometimes occurring many years after apparently successful initial treatment.
• Supportive and rehabilitative care addresses any facial nerve dysfunction and its functional and cosmetic impact.