Carcinoma ex pleomorphic adenoma

Carcinoma ex pleomorphic adenoma is the most common of three malignant mixed tumours of salivary glands, and are thought to arise from pre-existing pleomorphic adenomas (historically also called benign mixed tumours, but see below) ^1,3.

Epidemiology

These tumours usually occur in older patients (6^th to 8^th decade), who have had a pleomorphic adenoma in situ for many years (typically 10-15 years) ¹.

Terminology

Historically the benign form, pleomorphic adenomas have also been called benign mixed tumours, however this name is now discouraged. It was previously thought that these were a form of teratoma, however it is now clear that the neoplasm forms from a single layer of germ cells and is purely epithelial in nature ³.

Clinical presentation

Patients will describe having had a stable parotid mass for many years, before experiencing rapid growth, often associated with painful facial nerve palsy due to tumour infiltration ¹.

Radiographic features

As these tumours are aggressive and infiltrating, they present as irregular masses extending into the gland parenchyma and beyond.

MRI

• T1: heterogeneous signal due to haemorrhage, necrosis and calcification ¹
• T2: heterogeneous but generally low signal
• ADC: low values (compared to pleomorphic adenomas which typically have elevated values)

For staging see malignant salivary tumour staging.

Treatment and prognosis

These tumours have a high rate of metastasis at the time of diagnosis (25-76%) typically to brain, bone, lungs, and of course local lymph nodes ².

Differential diagnosis

The differential is essentially that of other malignant salivary gland tumours including:

• mucoepidermoid carcinoma
• adenoid cystic carcinoma
• myoepithelioma
• parotid adenocarcinoma (not otherwise specified)
• acinic cell carcinoma of salivary glands
• squamous cell carcinoma  of salivary glands
• intraparotid metastasis