Atypia in fine-needle aspiration cytology of the breast: a histologic follow-up study of 301 cases.

A gray area of uncertainty exists in fine-needle aspiration (FNA) cytology of the breast in which common criteria to distinguish benign from malignant lesions overlap. Aims of this study were to define this area and to evaluate statistically cytomorphologic criteria in a semiquantitative analysis. In a test set of specimens from well-differentiated carcinomas and benign proliferative lesions, signs of malignancy were cell dissociation, arrangement in small clusters, nuclei greater than 16 microns, anisonucleosis, irregular nuclear borders, nucleoli, and necrosis. Features in favor of benignancy were large monolayers, nuclei less than 16 microns without variation in size, smooth nuclear borders, and bipolar nuclei in the monolayers. Originally the term "atypia" had been applied to 956 (12%) of all FNAs of the breast performed at our institute from 1974 to 1985. Using these criteria in a review of all 301 cases in which histologic follow-up and cellular smears were available, much better results were obtained than originally; specificity increased from 80% to 95%, and sensitivity increased from 60% to 90%. The number of overdiagnoses decreased from 24 to seven, and underdiagnoses decreased from 57 to nine. In this selected series, the area of uncertainty was restricted to 16% of the cases; the number of these cases that proved to be malignant and benign was equal. In such cases of indistinct cytomorphologic criteria, a surgical biopsy is indicated for histologic studies.