Pure Mucinous Carcinoma of the Breast

Differential Diagnosis

A number of low grade or benign lesions are characterized by pools of mucin

Mucinous DCIS	Extracellular, intralumenal mucin in a duct lined by DCIS, no mucin in contact with stroma
DCIS with mucocele-like lesion	Typical DCIS with discrete areas of rupture resulting in mucin pools in stroma, no groups of cells floating in mucin
ADH with mucocele-like lesion	Some but not all features required for diagnosis of DCIS with discrete areas of rupture resulting in mucin pools in stroma, no groups of cells floating in mucin
Mucocele-like lesion	Pools of mucin in stroma variably lined by bland epithelium, usually no groups of cells floating in mucin (if present they are normal ductal cells and are not complex), almost always a microscopic lesion
Mucinous carcinoma	Mucin in contact with stroma, neoplastic cells floating in mucin
Nodular mucinosis	Nodules of stromal mucin, no epithelial component, subareolar

Mucinous carcinomas are characterized by cells floating in pools of mucin
Signet ring carcinomas are characterized by cells with intracellular mucin that indents the nucleus
Signet ring cells can be found in otherwise mucinous carcinomas
- If at least 20% of cells are signet ring, designate as signet ring carcinoma
  - Prognosis is that of usual infiltrating carcinoma of identical grade
  - Signet ring carcinoma is usually a variant pattern of lobular carcinoma
- If <20% of cells are signet ring, designate as mucinous carcinoma with signet ring cells
  - Prognosis may not be worse than pure mucinous carcinoma