Benign and malignant papillary lesions of the breast (PBL) can be difficult to distinguish in fine‐needle aspirates (FNA). This study evaluates the use of smooth muscle actin (SMA) and a new smooth muscle‐specific protein, calponin, for identifying myoepithelial cells (MEC) by immunohistochemical methods in paraffin‐embedded cell blocks of FNA of PBL.

Formalin‐fixed, paraffin‐embedded cell blocks of 40 cases of PBL were stained using SMA and calponin, steam heat‐induced epitope retrieval, and an avidin biotin‐complex technique. Staining was evaluated in MEC, epithelial, and stromal cells. The diagnosis of benign vs. malignant papillary lesion was made by using cytomorphological criteria and the presence/absence of MEC in the cell block. These results were compared to the original cytologic and subsequent histologic diagnoses.

Of 40 cases of FNA diagnosed as PBL, there were 27 intraductal papillomas (IP), 6 papillary lesions with atypical features (PLAF), and 7 papillary carcinomas (PC). In all of the IP, MEC stained both with SMA and calponin. None of the PC cases was positive for MEC with calponin, and 2 out of 7 cases were weakly positive by SMA. In 6 cases of PLAF, 2 were negative for MEC, both by SMA and calponin, and a malignant papillary lesion was confirmed by histology. The remaining 4 cases were positive for MEC with both markers and were confirmed to be benign by histology. SMA stained stromal cells strongly in all of the cases where stroma was present (18 of a total of 40 cases of PBL), while calponin stained stroma focally in only 7 cases. More than half of all cases had nuclear staining of epithelial cells with SMA; calponin did not show any nuclear staining. Diagn. Cytopathol. 2000;23:151–155. © 2000 Wiley‐Liss, Inc.