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

mucinous carcinoma. A fine-needle aspiration sample, however, is not reliable for
distinguishing between mucocele-like tumor, pure mucinous carcinoma, and
infiltrating duct carcinoma with a mucinous component (27).
Differential diagnosis: Mucinous carcinoma should be distinguished from the benign
mucocele-like tumor of the breast, as well as cystic hypersecretory hyperplasia and
cystic hypersecretory duct carcinoma. Rosen initially described mucocele-like tumor
of the breast as a benign condition characterized by extravasated mucin in the
mammary stroma (a constant feature) accompanied by multiple cysts filled with mucin
and lined by flat or cuboidal columnar epithelium devoid of significant papillary or
other proliferative features (28). The epithelium in the typical mucocele-like tumor is
largely flat or cuboidal, but columnar and focal papillary elements may be present.
Large, granular calcification are also seen in the mucin in these benign lesions.
Detached epithelial cells are not found in the secretion within cysts or when it is
discharged into the stroma. The resultant picture resembles the mucocele of salivary
gland origin commonly found in the oral cavity. (Apparently, true connective-tissue
myxomas can arise in the breast (29), and these myxomas are distinct from mucocelelike
tumors.)
Subsequently, it was shown that the typical mucinous cysts can be lined with
hyperplastic ducts and could contain foci of low-grade papillary carcinoma (30-32).
Mucocele-like tumors appear to represent a morphologic continuum from benign
lesions to carcinoma in situ with abundant mucus production to invasive mucinous or
colloid carcinoma (33,34). Benign mucocele-like lesion is characterized by
extracellular mucin extravasation but without epithelial elements. The ducts associated
with benign mucocele-like lesions are dilated and lined by bland epithelium without
stratification. Ro et al. (35) described a group of mucocele-like tumors with foci of
more extensive epithelial proliferation, including atypical hyperplasia, intraductal
carcinoma, and focal invasive mucinous carcinoma. The secretion in various
mucocele-like tumors and mucinous carcinomas had similar immunohistochemical
properties and the authors proposed that mucocele-like tumor and mucinous
carcinoma may be two extremes of a spectrum of lesions. Clearly, these mucocele-like
lesions must be carefully and completely examined, and the patients followed because
these patients may already have or develop low-grade papillary and mucinous
carcinomas (30-34). While available evidence suggests that some mucinous
carcinomas resemble and may arise from mucocele-like tumors, the majority appear to
arise from conventional forms of intraductal carcinoma.
Cystic hypersecretory hyperplasia is characterized by ectatic ducts filled with
eosinophilic, colloid-like secretion. The colloid-like secretion does not extravasate
into adjacent stroma (36-38). The cysts of hypersecretory carcinoma are lined by
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