Pure Mucinous Carcinoma of the Breast

Definition

A breast carcinoma of which at least one half of the volume of the tumor is extracellular mucin throughout
- Invasion is in the form of pools of stromal mucin containing neoplastic cells

Pools of extracellular mucin make up at least one half of volume throughout
- If focal areas are not at least 50% mucinous, designate as mixed mucinous/ductal
  - i.e. if only some areas are 50% mucinous, call it mixed
- If mucin does not attain 50% threshhold in any area, designate as infiltrating ductal carcinoma with mucinous differentiation
  - i.e. if no area makes it to 50% mucinous, call it carcinoma with mucinous differentiation
Detached epithelial elements present floating in the mucin
- May be trabecular, cribriform, micropapillary, sheet like or clumps
No areas of the usual type of invasion of stroma in the absence of mucin
- If present, designate as mixed mucinous/ductal
Usually low grade cytology
- May have any higher grade of atypia
- Must have at least the cytologic features of the cells of low grade ductal carcinoma in situ
  - Must not be cytologically identical to normal cells
In situ component may be present
- May be mucinous also (mucinous DCIS)
May be seen in association with neuroendocrine differentiation
Should always be diagnosed with the qualifier "pure" or "mixed"

Kristin C Jensen MD
Robert V Rouse MD
Richard L Kempson MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342