Cytological features of cystadenocarcinoma in cyst fluid of the parotid gland: Diagnostic pitfalls and literature review.

Cystadenocarcinoma is a rare malignant tumor, with an estimated incidence of 2% of malignant salivary gland tumors. Cytological diagnosis of cystadenocarcinoma is important for differential diagnosis between benign lesions and malignant tumors with cystic growth. We report a case of cystadenocarcinoma causing difficulty in cytological diagnosis. A 23-year-old man presented with an asymptomatic mass in the left parotid gland that had been present for 2 years. The mass was elastic hard, measuring 30 x 35 mm in diameter. Preoperative fine-needle aspiration cytology (FNAC) showed a small number of tumor cell clusters in the cystic fluid. The cluster was arranged in a ball-like structure and was cohesive with overlapping. Tumor cells had a small vacuolated, soap-bubble appearance in the cytoplasm. The papillary-cystic variant of acinic cell carcinoma (ACC-PCV) was suggested from these findings on FNAC. Histologically, the tumor was not encapsulated, but formed large cystic spaces against a background of fibrous connective tissue. The tumor cells in the cystic dilated duct showed papillary structures, which were continuous with the lining cuboidal cells. There was neither a definite double-layered arrangement in cystic ducts and solid islands nor histological findings characteristic of the papillary-cystic or follicular pattern of ACC-PCV. As tumor cells with a small vacuolated, soap-bubble appearance of the cytoplasm are common findings of both cystadenocarcinoma and ACC-PCV, they are of little use for differentiation; however, they are so characteristic that the majority of benign salivary gland lesions with cystic structures can be excluded, if enough attention is paid.