Correct histological assessment of papillary lesions of the breast is sometimes fraught with difficulties and even fallacies. Nevertheless, this is important as the management of benign and malignant (either in situ or invasive) papillary lesions is different, with vastly different prognostic implications. The observation that papillomas are frequently involved in other epithelial proliferative processes, ranging from florid epithelial hyperplasia and atypical duct hyperplasia to DCIS further complicates this issue. The major problems lie in the differentiation between papilloma and papillary carcinoma in situ, as well as between papillomas that are involved in either florid epithelial hyperplasia or DCIS (particularly low‐grade disease). In the literature, apart from the well‐established histological criteria,9,11,21
immunohistochemistry to investigate myoepithelial cell numbers is the most common approach that has been reported, under the presumption that there are far fewer or no myoepithelial cells in malignant papillary lesions compared with their benign counterparts.6,21
As an increasing number of needle core biopsies are performed for each breast lesion, the correct diagnosis of papillary lesions becomes more difficult as the amount of material is reduced. Although many authors have reported a good degree of accuracy in diagnosing benign papillary lesions in needle core biopsy specimens,22,23,24,25
both benign and malignant papillary lesions may show up as papilloma with atypical epithelial features in needle core biopsy specimens.24
Immunohistochemistry becomes a useful tool in assessing such specimens.
SMA, in addition to staining smooth muscle or myoepithelial cells, also stains stromal myofibroblasts and pericytes, and this is particularly problematic in cases at the base of an enlarged epithelial cluster, where stromal myofibroblasts may be compressed to form an apparently continuous layer, mimicking a myoepithelial cell layer. In this series, we found strong staining of the stromal cells, thus reducing the specificity of the stain as a myoepithelial marker. This problem is accentuated in papillary carcinoma in situ, in which the presence of myoepithelial cells is sparse, and cross‐reactivity with stromal myofibroblasts may give an erroneous impression of abundant myoepithelial cells (fig 4).
Figure 4Papillary carcinoma in situ (A) Haematoxylin and eosin staining, 200×), highlighting that smooth‐muscle actin staining of the stromal cells may give a false impression of myoepithelial cell layer (B).
p63 is one of the p53 homologues and related genes, and it seems to play a crucial part in the regulation of epithelial proliferation and differentiation. Mice that are p63−/−
have no hair follicles, teeth or mammary, lachrymal and salivary glands.26,27
In humans, immunohistochemical analysis shows p63 expression in the epithelial cells of stratified epithelium, including skin, oesophagus, exocervix, tonsil and bladder, and the basal cells in glandular organs including breast, bronchi and prostate.28
Furthermore p63 is expressed in the myoepithelial cells in the breast15,16
and as a marker of “undifferentiation,”29
and may be useful as a myoepithelial marker in assessing breast lesions.17,18,19
suggest that p63 is also expressed in sarcomatoid or metaplastic carcinoma of the breast and may be a marker for this specific mammary carcinoma subtype. In our study, p63 proved to be the most sensitive marker for the detection of myoepithelial cells, and thus a negative or sparse p63 staining is quite specific for malignant papillary lesion. Interestingly, epithelial staining was present in a single case of papillary carcinoma in situ in our series, and this fact has also been observed in other series, in which epithelial staining was present in a small proportion of the papillary lesions.21
Howoever, evaluation in combination with the morphological characteristic obtained on haematoxylin an eosin staining, is unlikely to create confusion in the assessment of myoepithelial cells. This probably relates to the fact that p63 is also a marker of undifferentiation,29
thus the more basal epithelial cells may be positive for the stain.
CD10, or the common acute lymphoblastic leukaemia antigen, is a cell‐surface neutral endopeptidase, and is expressed by lymphoid precursor cells and some B lymphoid cells.32
It has been recently demonstrated in normal tissues including myoepithelial cells of the breast and salivary glands, in apocrine metaplastic cells of the breast33
and in endometrial stromal sarcoma,34,35
and is useful to differentiating endometrial stromal sarcoma from leiomyoma.34
In the breast, CD10 has been assessed as a myoepithelial cell marker,33,36
and overexpressed in malignant mammary phyllodes tumours.37
In our series, CD10 was shown to be a sensitive myoepithelial cell marker, although there seems to be considerable cross‐reactivity with epithelial and stromal cells.
CKs belong to one of five classes of intermediate filaments; they form a dense network radiating from the nucleus to the plasma membrane, and act as cytoplasmic scaffolding. The CK family is a highly complex multigene family of polypeptides; some are simple epithelial keratins (CK7, CK8, CK18 and CK19), expressed by the ductal epithelium of the breast, and the myoepithelial cells express basal‐type cytokeratins (CK5, CK14 and CK17).38,39
More recently, the expression of basal cell keratins (CK5/6 and CK14) has been reported as indicative of a worse prognosis in invasive breast cancer.40
Furthermore, CK14 was also reported to be useful in differentiating between benign and malignant epithelial lesions of the breast and in papilloma.12,41,42
In our series, we found CK14 to be useful in two respects. Firstly, it can identify myoepithelial cells with high sensitivity, thus rendering it a useful marker in assisting differentiation between benign and malignant papillary lesions. Secondly, in the group of papillary lesions with prominent epithelial proliferation, which includes papillomas with florid epithelial hyperplasia or DCIS involving papillomas, CK14 has the additional benefit of selectively staining the epithelium of the hyperplastic (fig 2) but not malignant lesions (fig 5). As morphological differentiation is occasionally difficult, CK14 provides an additional criterion for this differentiation. If we use a cut‐off of moderate to strong staining of
50% of the epithelial cells, then CK14 becomes 100% specific in identifying benign epithelial proliferation within a papilloma.
Figure 5Ductal carcinoma in situ involving papilloma. (A) Haematoxylin and eosin staining, 200×) showing negative staining of the epithelial component by cytokeratin 14, (B) in contradistinction with the epithelial cells in florid epithelial (more ...)
- Immunohistochemistry is useful in differentiating mammary papilloma and papillary carcinoma.
- P63 and CD10 are useful to highlight myoepithelium with papillary lesions.
- CK14 can differentiate benign and neoplastic epithelium within a papilloma.
In this study, we have shown that all myoepithelial markers —SMA, p63, CD10 and CK14—were sensitive in identifying myoepithelial cells in papillary lesions. They are thus useful in assisting the differentiation of benign (more myoepithelial cells with continuous staining) from malignant (much less or absent staining, discontinuous if present) lesions. p63 is probably superior to SMA and CD10 because the staining is nuclear, and there is no cross‐reactivity with the stromal cells or stromal myofibroblasts. However, we should remember that p63 may rarely be expressed by epithelial cells. In those cases with prominent epithelial proliferation, when either florid epithelial hyperplasia or DCIS involving pre‐existing papillomas enters into the differential diagnosis, CK14 may provide an added advantage in highlighting the benign epithelial proliferation.
To conclude, a simple panel of p63 and CK14 may be a useful and sufficient immunohistochemical tool for assisting the characterisation of papillary lesions of the breast.