Head and Neck Imaging

Pleomorphic adenoma

most common benign tumour of the salivary glands, predominantly encountered in the parotid gland; it occurs less commonly in the submandibular gland and minor salivary glands. Pleomorphic adenoma is more often seen in women and the majority present in the fifth decade. Histologically, both epithelial and mesenchymal elements are found (hence the name mixed tumours).

The CT and MRI appearance of these tumours is variable. They usually appear well circumscribed, but may be lobulated (parotid gland (VI:2), Fig. 2). Tumours of the deep lobe of the parotid gland extend into the parapharyngeal space and become symptomatic when they compress the pharyngeal wall (parapharyngeal space (VI:2), Fig. 2). Sometimes a less sharp demarcation is noted, making the distinction from a low-grade malignant salivary gland tumour difficult. Intratumoral calcifications may be seen (Fig.1).

Incomplete surgical resection is the principal reason for tumour recurrence; rupture of the tumour capsule during surgery is another cause. Such a recurrence is typically multicentric and may only become clinically evident after several decades. MRI is the preferred modality for confirming the presence of recurrent tumour; these nodular tumour recurrences are usually very bright on T2-weighted images (Fig.2).

Malignant transformation of the epithelial component of a pleomorphic adenoma (called 'carcinoma ex pleomorphic adenoma' or 'malignant mixed tumour') is the principal reason why all pleomorphic adenomas should be surgically removed. It occurs typically in tumours that have already existed for a very long time, and manifests itself clinically by a sudden increase in tumour volume, sometimes accompanied by pain and facial nerve paralysis. The appearance of a malignant mixed tumour on CT and MRI may be unremarkable compared to the 'benign' mixed tumour, or it may show infiltration into the surrounding structures (Fig.3).

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Fig.3

a, b. Axial contrast-enhanced CT images. Giant soft tissue tumour in left side of the neck, originating from the parotid region, invading the masseter muscle. The lesion contains numerous calcifications and shows inhomogeneous contrast enhancement. Malignant mixed tumour.
Pleomorphic adenoma, Fig.1
Pleomorphic adenoma, Fig.2 (a)
Pleomorphic adenoma, Fig.2 (b)
Pleomorphic adenoma, Fig.3 (a)
Pleomorphic adenoma, Fig.3 (b)