Uses and Limitations of Immunohistochemistry in Breast Pathology
Moderator: Dr. Anna Sapino and Dr. Nour Sneige
Section 3 -
Are Immunostains Useful Diagnostic Tools in Breast Pathology?
European Institute of Oncology and University of Milan
Milan , Italy
Immunostains are useful in selected instances, being not routinely performed to aid in the
differential diagnosis of breast diseases -at least in the Departments dealing with a high number of
breast biopsies or surgical samples- because the peculiar morphological features of benign and malignant
lesions have been precisely identified since a long time and thoroughly described in several papers and
comprehensive textbooks. The unfortunately high prevalence of breast cancer has facilitated spreading of
knowledge and expertise among pathologists in most Institutions. Experts or specialists in breast
pathology are commonly encountered in large Pathology Departments, and breast pathology is one of the key
issues in the post-graduate or residency programmes for younger pathologists.
Still, there are some specific fields of breast pathology where immunohistochemical data are sought
even by expert pathologists to facilitate -or at least to confirm- the histopathological diagnosis.
This is especially true in the setting of the differential diagnosis between well-differentiated ductal
carcinoma (either invasive or in situ) and benign proliferative lesions, and between pure intraductal
carcinoma and (micro)-invasive cancer. Also, immunostains may be an useful adiunct in the correct
identification of special types of breast carcinomas. 
Distinguishing well-differentiated invasive ductal carcinoma from benign lesions.
As with most human malignancies, a specific positive marker of breast
carcinoma is not available for immunohistochemical purposes, with the possible exception of HER2/neu
over-expression. This oncogene, however, is over-expressed only in a fraction of breast carcinomas, and
most commonly in high-grade carcinomas, where issues of differential diagnosis with benign lesions are
unlikely to occur. Therefore, the correct diagnosis of breast cancer must rely upon a constellation of
data, which include gross and microscopic features and only a limited number of immunohistochemical
Well differentiated invasive ductal carcinomas, including tubular and cribriform variants, may be
difficult to be distinguished from some benign proliferative diseases (e.g., tubular adenosis, radial
scar) on purely morphological grounds. In these circumstances, highlighting of the myoepithelial cell
component, which is invariably present in the benign proliferative lesions (with the possible exception
of microglandular adenosis) and absent in invasive breast cancer, may be the diagnostic key.
Luckily, there are several sensitive and specific immunohistochemical markers for mammary
myoepithelial cells, which are suitable for staining routinely fixed and embedded tissues, and
cytological preparations as well. These include, among others, p63, smooth muscle myosin heavy chain,
calponin, and caldesmon
I would strongly recommend not to use smooth muscle actin any longer for
the identification of myoepithelial cells, because this protein is also expressed by myofibroblasts which
may surround the glands or nests of invasive breast carcinomas and lead to an underdiagnosis of
malignancy. S-100 protein, on the other hand, may be expressed also by normal and neoplastic epithelial
cells of the breast, again leading to a false positive identification of myoepithelial cells.
Among the more consistent and effective markers of myoepithelial cells, I most often rely on p63,
because it has a nuclear location -which facilitates labelling of myoepithelial cells even in case of
extreme clearing of their cytoplasm- and it may be also safely used to decorate the naked nuclei of
myoepithelial cells in cytological specimens. 
Lack of a myoepithelial cell component is a most reliable diagnostic feature for distinguishing
well-differentiated invasive duct carcinoma from non malignant breast proliferations. It should be
noted, however, that to rely on negative immunohistochemical findings is always risky, because of the
possible appearance of false-negative results due to technical artefacts. For this reason it is highly
recommended that the sections to be immunostained always include benign breast tissue surrounding the
suspicious proliferation, that may function as an internal positive control.
Also, it should be considered that some special types of invasive breast cancer have a myoepithelial
component (e.g., adenoid-cystic carcinomas, low-grade adenosquamous carcinomas), or show a myoepithelial
differentiation (e.g., so-called basal-type carcinomas), and that some metaplastic carcinomas exhibit a
variable proportion of neoplastic cells with definite nuclear immunoreactivity for p63.
Differentiating ductal carcinoma in situ (DCIS) from florid epithelial hyperplasia.
Here again we have to complain of the lack of a positive marker for breast cancer. Indeed, the
immunophenotypic differences between DCIS and ductal hyperplasia consist mainly in the different
expression of basal-type cytokeratins (especially cytokeratins 5 and 14). These are consistently
co-expressed by a variable fraction of epithelial cells in case of florid epithelial hyperplasia, but not
by DCIS cells, which only express "luminal" type cytokeratins (mainly cytokeratins 8,18 and19)
It should be noted that some high-grade DCIS may co-express cytokeratins 5 and 14 (so-called
basal-type DCIS): the nuclear features of these neoplasms, however, allow easy recognition of their
malignant nature .
Identifying Microinvasion in DCIS
The correct identification of microinvasive foci in DCIS is in my opinion one of the most challenging
tasks in breast pathology, and I am afraid that inter-observer disagreement in case of microinvasive
breast cancer is exceedingly common. Unfortunately, the over-estimation of microinvasion may have very
harmful therapeutic implication, including undue axillary surgery and adjuvant interventions. This is
particularly true in case of high-grade tumors, most often lacking any estrogen and progesterone
receptors, for whom adjuvant chemotherapy would be recommended, in case of microinvasion being reported.
I believe that we should be very accurate in diagnosing microinvasion and use stringent morphological
criteria. The attitude to diagnose microinvasion just because "this DCIS is too large and too bad: it
must be invasive somewhere" has to be discontinued.
It is very unfortunate that immunohistochemistry is only marginally useful in this scenario. Indeed,
many pathologists rely on immunostaining for basal membrane proteins and/or myoepithelial cell markers to
ascertain or rule out microinvasion, but this may be useless and sometimes very misleading. Invasive
breast cancer may indeed express basal membrane components leading to under-estimation of microinvasion.
On the other hand, lack of a myoepithelial cell layer is not a marker of invasion, because the
myoepithelial cells disappear in many high-grade DCIS, leading to a possible over-diagnosis of invasion.
Identifying Special Types of Breast Cancer
Immunostainings may facilitate recognition of special types of breast carcinomas. Lobular carcinomas
are characterized by the lack of e-cadherin immunoreactivity, apocrine carcinoma express androgen
receptors and GCDFP-15, adenoid cystic carcinomas are immunoreactive for myoepithelial cell markers and
myoepithelial carcinomas may be identified using myoepithelial cell markers, and so-called
basal-type breast cancer variably express cytokeratins 5,14, and 17, EGFR and p63.
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