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

studied, this taxonomy identifies subgroups of breast cancer that were to some extent
already known and the stability or utility of the assigned subtypes has been
questioned. Indeed, this taxonomy is not be used by UCSD oncologists. Nonetheless,
the most robust distinction observed by microarray is between estrogen receptorpositive
and ER-negative breast cancers, and none has generated as much interest as
the basal-like group. Although there is no clear-cut histologically defined basal cell in
breast epithelium, the myoepithelial cell is well defined.
So-called basal-like breast cancer has been extensively reviewed, and there remains no
internationally accepted definition for basal-like breast cancer. Some use microarraybased
expression profiling to define basal-like breast cancers, whereas others use
immunohistochemical markers. Unfortunately, direct comparisons between the
proposed immunohistochemical markers and microarray-defined molecular subtypes
are scarce. Immunohistochemical marker panels that have been proposed to define
basal-like breast cancers include: (1) lack of ER, PR, and HER2 expression (“triplenegative”
immunophenotype); (2) expression of one or more high-molecular-weight/
basal cytokeratins (CK5/6, CK14, and CK17); (3) lack of expression of ER and HER2
in conjunction with expression of CK5/6 and/or epidermal growth factor receptor
(EGFR); and (4) lack of expression of ER, PR, and HER2 in conjunction with
expression of CK5/6 and/or EGFR. These high-molecular-weight keratins also
highlight myoepithelial cells.
Despite the different definitions for basal-like breast cancers, these tumors may have
somewhat distinctive clinical presentations, histological features, response to therapy,
sites of distant relapse, and outcome. Basal-like tumors are heterogeneous, account for
up to 15% of all breast cancers, affect younger patients, are more prevalent in African-
American women, and often present as interval cancers. The majority classify as IDC-
NST type, are high histological grade, have high mitotic indices, show central necrotic
or fibrotic zones, have pushing borders, have conspicuous lymphocytic infiltrate, and
often display medullary-like features.
However, not all basal-like cancers are of the IDC-NST type. Indeed, the majority of
medullary and atypical medullary, metaplastic, secretory, myoepithelial, and adenoid
cystic carcinomas also have a basal-like phenotype. A subgroup of lobular
carcinomas express high-molecular-weight cytokeratins, but may not show a basal-
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