33 Special Types of Invasive Breast Carcinoma: Diagnostic Criteria ...

squamous carcinoma of the breast is a subtype of metaplastic carcinoma and is
reserved for tumors composed entirely of keratinizing or non-keratinizing squamous
carcinoma cells (3-10, 15, 21-25). It is important to rule out adjacent cutaneous or
metastatic squamous carcinoma to the breast from a distant site before making the
diagnosis of primary disease. Like other metaplastic breast carcinomas, squamous cell
carcinomas are negative for ER/PR/HER2. The squamous differentiation is retained in
metastatic foci. Squamous cell carcinoma can be graded based mainly on nuclear
features and, to a lesser degree, cytoplasmic differentiation.
Spindle-cell transformation of squamous carcinoma is common but usually focal and
inconspicuous. Acantholytic or pseudoangiomatous change has been reported as well
and may lead to a mistaken diagnosis of angiosarcoma, and when present, acantholytic
squamous carcinoma may follow a very aggressive clinical course (26). Similar
aggressive behavior has been noted in primary cutaneous and oral acantholytic
squamous carcinomas (27, 28). The most bland appearing and well differentiated
cells often line cystic spaces; as the tumour cells emanate out to infiltrate the
surrounding stroma, they become spindle shaped and lose their squamous features. A
pronounced stromal reaction is often admixed with the spindled squamous carcinoma.
The spindle-cell and acantholytic variants require confirmation of their epithelial
nature, which are positive high molecular weight cytokeratins (CK5, CK5/6, CK14,
and CK34betaE12) but negative for vascular endothelial markers. Squamous tumor
cells immunostain for keratin, especially for high molecular weight keratins such as
CK5/6 and for p63, which is good marker for myoepithelium, basal cells, reserve
cells, and squamous cells.
Adenosquamous carcinoma of the breast is very rare invasive carcinoma with areas of
well developed tubule/gland formation intimately admixed with often solid nests of
squamous differentiation (WHO 2003). Since focal squamous differentiation can
occur in typical infiltrating duct carcinomas of no special type (i.e., in up to 5% of
cases), there should be a prominent admixture of invasive ductal and squamous
carcinoma before the term adenosquamous carcinoma is used. Unlike other
metaplastic breast carcinomas, the adenomatous component may be ER/PR/HER2
positive and prognosis is rougly proportional to size and grade of the tumor.
A very rare variant has been reported as low-grade mucoepidermoid carcinoma of the
breast, which is similar to those occuring in the salivary glands (WHO 2003). They
behave as low-grade carcinomas. Furthermore, a second rare variant has been reported
as low-grade adenosquamous carcinoma or syringomatous squamous tumour – that is,
a metaplastic breast carcinoma morphologically similar to adenosquamous carcinoma
of the skin. The same lesion has been interpreted as an infiltrating syringomatous
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