Question
Pleomorphic adenoma
Answer
Pleomorphic adenoma is the most common salivary gland tumour, most frequently arising in the parotid gland, typically as a benign, slow-growing, painless mass.
Gross: Well-circumscribed but incompletely encapsulated (has small pseudopod-like extensions into surrounding tissue, important for surgical planning), lobulated, grey-white, with a glistening, mucoid/myxoid cut surface.
Microscopy: Characteristically shows a mixed (“pleomorphic”) pattern of epithelial and mesenchymal-appearing elements — ductal epithelial cells and myoepithelial cells arranged in nests, cords, and ducts, set within a variable stroma that can show myxoid, chondroid (cartilage-like), and hyalinized areas, all derived from the myoepithelial cell component (hence a true mixed tumour of a single origin, not a true mesenchymal admixture).
Behaviour: Benign, but has a tendency to recur locally if incompletely excised (due to pseudopod extensions/capsular breach) — hence complete surgical excision with a margin of normal tissue (not simple enucleation) is required. Long-standing or recurrent tumours carry a small risk of malignant transformation (carcinoma ex pleomorphic adenoma).