Lobular Carcinoma in Situ of the Breast

Definition

Uniform population of cells with round, sometimes eccentric nuclei
- Two cytologic types (mixtures and intermediate forms may occur)
  - Classic
    - Small round nucleli
    - Fine chromatin, smooth nuclear contours
    - Nucleoli small, inconspicuous
    - Mitotic figures infrequent
    - Has been subdivided as:
      - Type A - small completely bland cells
      - Type B - slightly larger, slightly irregular, small nucleoli
  - Pleomorphic
    - Moderately large nuclei
    - Coarse chromatin
    - Promininent nucleoli
    - Mitotic figures not uncommon
    - Central necrosis may be seen
    - Requires confirmation of lobular nature with a negative E-cadherin stain
- Lightly eosinophilic to vacuolated cytoplasm
  - Signet ring cells may be present
Discohesive pattern
Uniform cells (above) must fill all the acini in at least one lobular unit and half the acini must be expanded
- Expansion of acini apparent at low magnification
- Expanded acini typically balloon-like
May form tiny acini but must not form cribriform spaces or micropapillae
Pagetoid spread into ducts is common
- May completely fill ducts
- May involve sclerosing adenosis
Lesions fulfilling some but not all required features of LCIS are considered Atypical Lobular Hyperplasia