Invasive Lobular carcinoma.

Invasive carcinoma composed of tumor cells which are non cohesive and E-cadherin negative
Incidence is 5 to 15% of all invasive breast carcinomas
Age – from 26 – 86 years with peak incidence at 50 years
This tumor is more frequently multicentric and bilateral
Presents as mammographic density with irregular borders or palpable mass

Gross – varies from firm nodular mass with irregular borders to micronodularity and subtle thickening of fibrous parenchyma
Microscopic features
- Dyscohesive infiltrating tumor cells arranged often in single file pattern (Indian file pattern)
- Tumor cells surround and invade the stroma surrounding residual duct in a “targetoid” pattern
- Tumor cells are arranged in single file pattern as they spread along the prelymphatic channels due to lack of cohesion because of loss of E-cadherin.
- Cell morphology:
  - Tumor cells are monotonous having round to oval nuclei and pale eosinophilic cytoplasm.
  - They may show intracytoplasmic lumina
  - Signet ring morphology with eccentrically placed nuclei and intracytoplasmic mucin droplet can be present

Variants
- Alveolar variant
- Tubulo alveolar variant
- Solid variant
- Pleomorphic variant
- Other rare variants: Apocrine cell variant, Histiocytoid variant, signet ring cell variant, and myoepithelial cell variant
Immunohistochemistry
- usually positive for ER and PR
- Negative for HER2/neu ( Except high grade pleomorphic variant)
Lobular carcinomas are graded as well differentiated and moderately differentiated
- Well differentiated tumors are diploid, ER +ve and associated with Lobular carcinoma insitu. HER2/neu expression is very rare.
- Moderately differentiated tumors are aneuploidy,lack hormone receptors and overexpress HER2/neu
Metastasis – More commonly spreads to the peritoneum, retroperitoneum, leptomeninges, ovary, gastrointestinal tract and uterus
Prognosis depends upon histologic grade of tumor and subtype