Microscopic description and IHC:

Mucinous carcinomas are characterized by accumulation of abundant extracellular mucin around the invasive tumor cells.

The tumor cells are typically clustered and dispersed within pools of extracellular mucin, that can be transversed by thin fibrous septae with thin walled blood vessels.

The cells can have a variety of arrangements such as trabecular, sheet-like, or papillary.

The nuclei is often of low or intermediate grade. High nuclear grade is extremely rare.

The histologic features should compromise at least 90% of the tumor to qualify for the diagnosis.

Immunohistochemical features:
  • CK7 positive
  • CK20 typically negative
  • PAS highlights intracellular mucin
  • ER and PR have variable reactivity, but usually positive
  • HER2 usually not overexpressed

Discussion:

Mucinous carcinoma is also known as colloid carcinoma.

It is uncommon and accounts for less than 2% of invasive breast cancers.

They occur more commonly in older woman.

It is a well circumscribed or lobulated mass rarely associated with calcifications.

Pure mucinous carcinoma generally have a favorable prognosis.

References:

Schnitt SJ, Collins LC. Biopsy interpretation of the breast. Philadelphia: Wolters Kluwer; 2013

Gattuso P. Differential diagnosis in surgical pathology. Philadelphia, PA: Saunders/Elsevier; 2010.

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