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

size (14).
Metaplastic breast carcinomas showing mesenchymal differentiation with or without
heterologous elements originate from carcinomas that undergo sarcomatous
neometaplasia as a result of further genetic instability or mutations (15, 16).
Abundant ultrastructural and immunohistochemical studies have indicated that the
spindle-cell components show variable myoepithelial differentiation, akin to that in
mixed tumors (pleomorphic adenomas) of the salivary glands (17). It is fascinating
that some myoepithelial-like differentiation has been found in so-called basal-like
breast carcinoma. Thus, it is interesting that Sarrio and coworkers (18) studied
metaplastic breast carcinomas and found that the epithelial-mesenchymal transition
(EMT), as defined by the loss of epithelial characteristics and the acquisition of a
mesenchymal phenotype, can be associated with increased aggressiveness, and
invasive and metastatic potential.
Classification: It had been customary to separate metaplastic breast carcinomas into
squamous, heterologous (i.e., cartilage, bone, and myoid differentiation), and
pseudosarcomatous types. These morphologic distinctions are somewhat arbitrary
because some metaplastic tumors exhibit multiple types of growth; thus, some authors
have found little reason to make these distinctions. These adherents refered to all
mixed carcinomas of the breast as metaplastic carcinomas, regardless of whether the
metaplastic element is epithelial or mesenchymal (19, 20).
The WHO (2003) has classified metaplastic breast carcinomas into two basic types,
each with subcategories. The two basic types are pure epithelial carcinoma and mixed
epithelial and mesenchymal carcinoma. The pure epithelial group includes: 1)
squamous carcinoma (large-cell type with or without spindle-cell metaplasia or
acantholysis), 2) adenocarcinoma with spindle-cell metaplasia, 3) adenosquamous
(“mucoepidermoid”) carcinoma, and 4) low-grade adenosquamous carcinoma. The
mixed epithelial and mesenchymal carcinoma (carcinosarcoma) group includes: 1)
carcinoma with chondroid differentiation, 2) carcinoma with osseous differentiation,
and 3) carcinoma with rhabomyosarcomatous differentiation. This discussion will
follow the recommendations of the WHO classification of tumors (2003).
A common metaplastic pattern is squamous metaplasia in an otherwise typical
invasive duct carcinoma, and the metaplastic component usually constitutes less than
10% of the tumor, but may comprise the entire pattern. A spectrum of squamous
differentiation may be found ranging from mature keratinizing epithelium to poorly
differentiated carcinoma with spindle-cell, acantholytic, or sarcomatous areas,
including various combinations of these features. Yet, the specific diagnosis of
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