Atypia in fine needle aspiration cytology of nonpalpable and palpable mammographically detected breast lesions.

Widespread use of fine needle aspiration (FNA) in conjunction with mammography may lead to increased atypical or suspicious cytologic diagnoses. To assess the incidence and cytologic criteria of atypia in a mammographically screened population, we reviewed 220 FNA samples in this diagnostic category (5.8% incidence) from 3,798 FNAs performed in 1990-1991. The atypical category (134 cases) included 72 benign and 62 malignant confirmed cases. Common benign diagnoses were fibroadenoma and fibrocystic changes (33 cases, or 46%). Features responsible for an atypical diagnosis in benign cases were increased cellularity, single epithelial cells and "reactive nuclear atypia," the last defined as a finely granular, uniform chromatin pattern with small, prominent nucleoli. The suspicious category included 86 cases; only 10 cases (12%) were confirmed benign lesions, and the remainder were malignant. These benign cases included 3 fibroadenomas, 3 fibrocystic changes, 3 papillomas and 1 scar after radiotherapy. Two cases had prominent inflammation, a finding that should prompt caution. Malignant diagnoses in both categories included infiltrating and in situ cancers. A combination of limited cellularity, drying and/or bland nuclear features precluded a definitive diagnosis of cancer in many cases. For optimal management of mammographically screened patients, an acceptable "atypical" diagnostic rate should be defined for FNA. We conclude that a rate of less than 5% can be achieved by recognition of the following: (1) reactive atypia in the setting of fibrocystic and fibroadenomatous patterns, (2) significance of atypia in the setting of inflammation, and (3) improved sampling and sample preparation.