Stratification of risk of malignancy in atypical breast fine needle aspiration: a cytomorphological approach

Although atypical (C3) breast cytology is a legitimate reporting category, it is not very useful clinically because it yields benign as well as malignant histological outcomes. Therefore, a cytomorphological approach to minimise inappropriate use of the C3 category may improve the clinical usefulness of this category. In a previous training set of 180 atypical breast cytology (C3) cases we had identified the best discriminating cytological criteria to predict the histological outcomes (malignant, benign proliferative and benign non-proliferative). Using a selection of these previously identified statistically significant criteria (cystic background, cohesiveness, myoepithelial cells or bare bipolar nuclei, papillary fragments and tubules) we tested their ability to predict the same outcomes on 182 subsequent C3 cases (validation set). The diagnostic accuracy for each outcome was compared with the training set. A probability calculator with nominated cut-points was developed to stratify and re-grade C3 cases. Cases outside the cut-points were either up-graded to suspicious (C4) or down-graded to benign (C2). Re-graded cases were reviewed to identify reasons for over or under-interpretation. Statistical analysis showed comparable diagnostic accuracy between the training set and the validation set for malignant and benign proliferative outcomes but not for the benign non-proliferative outcome. The probability calculator resulted in an up-grade of 18% (33/182) ofthe cases. Malignant histology was seen in 25/33 (76%) of the up-graded cases but 8/33 (24%) produced a proliferative non-malignant outcome. A meaningful lower cut-point could not be established without including malignant cases. The C3 category remains a legitimate with a heterogeneous mix of pathological entities. Attempts to minimise inappropriate allocation of C3 cases into this category have met with limited success.