Ultrastructural analysis of fine needle aspirates from benign breast lesions

Distribution of p63, a novel myoepithelial marker, in fine-needle aspiration biopsies of the breast: an analysis of 82 samples

BACKGROUND

The presence of myoepithelial cells (MECs) in fine-needle aspiration biopsies (FNAB) of the breast constitute an important criterion used to diagnose benign breast lesions. However, MECs sometimes have a distorted cytomorphology, and most of the previously evaluated myoepithelial markers do not have satisfactory sensitivity and specificity. p63, a recently characterized p53 homolog, is a nuclear transcription factor that is expressed in basal cells of multilayered epithelia and myoepithelial cells of the breast. The authors analyzed the immunocytochemical distribution of p63 in a series of 82 breast FNABs (30 benign lesions and 52 malignant breast lesions).

METHODS

Eighty-two archival, Papanicolaou-stained smears of breast lesions were retrieved from the files of the authors' institutions. Immunocytochemistry was performed according to the streptavidin-biotin-peroxidase complex technique using the antibody 4A4 (against all p63 isoforms). Two pathologists evaluated the distribution of p63 positive cells. Only nuclear reactivity was considered specific.

RESULTS

In benign lesions, p63 decorated the nuclei of MECs in all samples. p63 also stained naked nuclei in fibroadenomas. In malignant lesions, p63 was positive in MECs overlying malignant cell clusters in all 8 samples of ductal carcinoma in situ (DCIS), in 9 of 16 samples of pure invasive carcinomas (IC), and in 16 of 20 samples that contained both DCIS and IC. In 18 samples (36%), a variable population of p63 positive, malignant cells was observed. p63 failed to decorate stromal, neural, adipocytic, and smooth muscle cells in all samples.

CONCLUSIONS

p63 is a reliable nuclear marker of MECs in breast aspirates. Regardless of the fact that variable proportions of p63 positive, malignant cells were observed in 36% of breast carcinoma aspirates, p63 may be a useful adjunct antibody to confirm the presence of MECs in FNABs of benign breast lesions.

Naked nuclei revisited: p63 Immunoexpression

The presence of naked nuclei (NN) in cytological preparations of fibroadenomas is a well-known finding. Regardless of their importance on the differential diagnosis of fibroadenoma and other benign lesions of the breast, the origin of NN remains elusive. Despite previous efforts to characterize them, the lack of a reliable nuclear marker for myoepithelial cells impaired definitive conclusions. We performed a systematic evaluation of p63 expression in cytological and histological preparations of 10 fibroadenoma specimens. We observed that in histological sections, p63 was restricted to the nuclei of myoepithelial/basal cells in lobules and ducts of normal breast. In fibroadenomas, p63 decorated the nuclei of myoepithelial cells in the periphery of epithelial duct-like formations and slit-like formations. No p63 immunoreactivity was observed in stromal or epithelial cells. In cytological samples, almost all NN and cells surrounding epithelial cell clusters were stained; no stromal cell admixed with fibrillary matrix or epithelial cell was stained with p63. Based on our findings, we strongly suggest that most, if not all, NN are myoepithelial in origin.