Abstract
We describe a case of pleomorphic adenoma associated with the formation of a large oval cyst in the right parotid gland in a 54-year-old Japanese man. The proliferative activity of the tumor cells was evaluated by counting the number of argyrophilic nucleolar organizer regions (AgNORs). The tumor, measuring 17×12×12 mm, was a clearly demarcated mass with an almost cystic appearance. It contained whitish cheese-like material. Histologically, the cyst cavity was filled with keratin and was lined by keratinizing squamous tumor cells. Flattened tumor cells were found in the hyalinized stroma. Adjacent islands of squamous metaplastic tumor cells and glandular tumor cells were present. The mean number of AgNORs per nucleus of tumor cells in the glandular portion, mildly keratinizing lining, highly keratinizing basal lining, highly keratinizing superficial lining, and flattened portion was 2.2±0.9, 1.7±1.1, 1.6±1.2, 1.2±0.9, and 1.3±0.8, respectively. Our results suggest that in some pleomorphic adenomas, small squamous metaplastic tumor islands might aggregate and lead to the formation of a large cyst in which the keratinizing tumor cells of the lining have higher proliferative activity than the flattened tumor cells in the hyalinized stroma.
