Differential Diagnosis of Adenoid Cystic Carcinoma from Pleomorphic Adenoma of the Salivary Gland on Fine Needle Aspiration Cytology

Abstract

To evaluate the distinguishing morphologic features of pleomorphic adenoma and adenoid cystic carcinoma on fine needle aspiration cytology. A retrospective study was undertaken to compare the cytomorphology of 9 adenoid cystic carcinomas with that of 12 pleomorphic adenomas of the salivary gland among histologically proven cases. The most consistent finding for differentiating adenoid cystic carcinoma from pleomorphic adenoma in our study was the amount of cytoplasm of individual tumor cells. Plasmacytoid appearance is a reliable finding for pleomorphic adenoma, and it enables us to rule out adenoid cystic carcinoma. All 12 cases of pleomorphic adenoma showed tumor cells containing abundant cytoplasm in clusters and isolated cells, with plasma cell-like appearance in 9 cases, especially in isolated cells. In contrast with pleomorphic adenoma, most adenoid cystic carcinomas showed little cytoplasm. Hyaline spherical globules were found in four of nine adenoid cystic carcinomas but were not found in any case of pleomorphic adenoma. It was specific but not sensitive to adenoid cystic carcinoma. Fibrillary chondromyxoid ground substance and a mixture of epithelial cells with stroma were found in two-thirds of the pleomorphic adenomas in our cases. The pattern of cell clusters was helpful to differentiate pleomorphic adenoma from adenoid cystic carcinoma. Large, loose clusters with a spindle cell core suggested pleomorphic adenoma. Small, dense trabeculae with a smooth margin and dense clusters containing clear, round spaces were more suggestive of adenoid cystic carcinoma. Overall cellularity, proportion of isolated cells, orientation of cellular clusters and degree of cellular overlapping gave almost no help in making the distinction between the lesions. A plasmacytoid appearance of individual tumor cells with abundant cytoplasm was a reliable finding in pleomorphic adenoma for differentiating it from adenoid cystic carcinoma. Practically, nuclear features were not enough to distinguish the two lesions in isolated cases.