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

SPECIAL TYPES OF INVASIVE BREAST CARCINOMA
INTRODUCTION
Depending on the series cited, up to 35% of invasive breast carcinomas can be
considered of “special” type (1,2). Invasive breast carcinomas not meeting the criteria
of “special” type are usually designated by the generic term “invasive duct carcinoma,
not otherwise specified” (InvDC,NOS) or of no special type (InvDC,NST). This term
can be useful for distinguishing these tumors from other specific forms of invasive
breast carcinoma (e.g., tubular, lobular, medullary, metaplastic, colloid, adenoid cystic
carcinoma, etc.). Many of the invasive breast carcinomas of special type (InvC,ST)
have a relatively favorable prognosis, but this only applies to those tumors composed
entirely or in very large part (i.e., 90% or more) of the special pattern (1). Also,
InvDC,NOS may be used to designate tumors that express in small part, rather than
purely, one or more characteristics of the specific types of breast carcinoma. More
specifically, InvCa,NOS can have limited microscopic foci (i.e., less than 10%) of
special types of differentiation. Examples of InvCa,NOS, containing 10 to 90% of a
special type of carcinoma, can be diagnosed as “mixed ductal and special-type
carcinomas” (see table below). Mixed patterns are quite common, and in one review
of 1000 breast carcinomas, ~33% of invasive breast carcinoma expressed
combinations of features (3). The discussion that follows focuses primarily on
invasive carcinomas, although selected in situ carcinomas are also included.
Although certain types of breast cancer have a better prognosis than others, Elston et
al. (4,5) and Periera et al. (6) continue to grade all histologic types of invasive
carcinoma; a practice which is still encouraged. Indeed, their data on grading include
grades of the various special types (6), regardless of the fact that special types of
invasive breast carcinoma that have a favorable prognosis usually (but not always) fall
into the SBR grade 1 category. This association suggests that, in most cases, the
correlation of tumor grade with outcome can be explained by the fact that most of
these special tumor types are SBR grade 1 tumors. Indeed, ~20% of breast carcinomas
are grade 1 by the modified Bloom-Richardson (mBR) criteria (5,6), and some suggest
that "special-type" carcinomas comprise about the same number of all invasive breast
carcinomas. However, Clayton and Hopkins. (7) were able to show prognostic significance
of histologic grading within the category of infiltrating ductal (no special type)
carcinoma, and Pereira et al. (6) have found that histologic grade and tumor type,
when used together, more accurately predict prognosis (see table below). In this latter
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