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

potentially aggressive carcinomas and treated appropriately.
Ductal carcinoma in situ is present in 75% of cases. Occasionally, mucinous differentiation
is evident in the intraductal component, but any pattern of DCIS can be
found. One pattern is an intracystic papillary carcinoma (21), in which multiple cysts
are distended with mucin and lined by papillary carcinoma. "Mucin leakage" into the
stroma surrounding cysts resembles the mucin extravasation seen in mucocele-like
tumors.
In invasive mucinous carcinoma, tumor cells are arranged in a variety of patterns,
including strands, alveolar nests, papillary clusters, and micropapillary growth (39), as
well as larger sheets that may have cribriform areas or focal necrosis. Nuclei are
usually grade 1, but examples with grade 2 nuclei occur. Periera et al. (22) have
defined mucinous breast carcinoma as a tumor composed of small islands (10-20
cells) of uniform small cells in lakes of extracellular mucin; any amount of invasive
ductal (NOS) carcinoma places the lesion in the mixed ductal and mucinous type.
They reported that the 10-year overall survival for all pure mucinous carcinomas was
81%; but, it was 86% for those examples that were mBR grade 1 and 75% for those
mBR grade 2. They had no cases of mBR grade 3 tumors. I have seen occasional
grade 2 pure mucinous carcinomas and even rarer examples that some pathologists
might consider grade 3, and I believe additional long-term follow-up studies are
needed to determine the behavior of these higher grade examples of pure mucinous
carcinoma. Often, when extensively sampled, these higher grade examples of “pure”
mucinous carcinoma are found to contain areas of InvDC,NOS, thus making them
mixed mucinous and ductal tumors. As expected, pure mucinous carcinomas are
almost always diploid (~96%)(10), but only ~40% of mixed mucinous and ductal
carcinomas are diploid, with most being aneuploid.
Argyrophilic granules have been detected in 25% to 50% of mucinous carcinomas
(3,4,9,14); these occur more frequently in elderly women and the tumor cells often
grow in clumps, sheets, or trabeculae sometimes suggesting an endocrine growth
pattern. The granules can contain immunohistochemically detectable serotonin,
somatostatin, and gastrin (23). The presence of argyrophilic granules has not been
prognostically significant in pure mucinous tumors or in infiltrating duct carcinomas
with focal mucinous differentiation (3,24). Maluf et al. (25) have recently described a
low-grade form of DCIS that shows both endocrine and mucinous differentiation.
They postulated that these lesions are the pre-invasive counterpart of mucinous
carcinoma with endocrine differentiation. Tsang et al. (26) have described a very
similar lesion with both endocrine differentiation and mucinous carcinoma in 80% of
those with an invasive component. The fine-needle aspirate from mucinous carcinoma
reveals isolated cells and small clusters in a background of mucin. Occasionally,
myxoid material from an edematous fibroadenoma results in an aspirate that resembles
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