Lipomatous pleomorphic adenoma of the parotid gland. Classification of lipomatous tissue in salivary glands.

Lipomatous pleomorphic adenoma is an unusual subtype with a lipomatous stromal component of more than 90% of the tumour tissue. This special type of pleomorphic adenoma must be distinguished from other types of lipomatous tumours or non-tumourous lipomatosis of the salivary glands. Until now only two cases of lipomatous pleomorphic adenoma have been reported in the literature. We report of a 36-year old woman who developed a well circumscribed nodule measuring 3.5 x 2.5 x 2 cm in the right parotid gland. The cut surface was grey-yellowish. Histologically, more than 90% of the tumour tissue was fatty tissue with univacuolar adipocytes. The pleomorphic epithelial elements were duct-like cells forming small lumina and spindle-shaped myoepithelial cell with surrounding mucoid stroma. Components of pleomorphic adenoma were intermingled with mature adipose tissue which was more concentrated in the central portion of the adenoma. Some compressed epithelial cords in the adipose tissue formed a septa-like pattern. The differential diagnosis to other lipomatous tumours (lipoadenoma, lipoma) and to non-tumourous interstitial lipomatosis as well as the possible pathogenesis as metaplastic change or epithelial-mesenchymal transdifferentiation are discussed.