Low Grade Neuroendocrine Carcinoma of the Breast

Definition

Low grade breast carcinoma expressing predominant neuroendocrine differentiation

Diagnosis requires all of the following four criteria
- At least 50% of cells are neuroendocrine marker positive
  - Most cases positive for chromogranin and/or synaptophysin
  - Neuron specific enolase is not sufficiently specific for this diagnosis
  - Lesser numbers of positive cells are seen in usual type breast carcinomas
- Growth in solid sheets or insular pattern
  - Peripheral palisading common
- Stippled chromatin
- Low grade cytologic features
  - Round, regular to mildly irregular nuclei up to 2-3x the size of a RBC
  - No comedo necrosis
Extracellular mucin is common
- May merge with mucinous carcinoma
Frequently associated with endocrine DCIS
Spindle cell variant
- Above criteria required
- Predominantly composed of spindle cells
- Subset of cells may be cuboidal and form glands
- Reported cases all low grade
High grade neuroendocrine carcinoma and neuroendocrine carcinoma NOS are considered separately