Invasive Lobular Carcinoma of the Breast II

Invasive lobular carcinoma (ILC):
- Represent roughly 7% to 15% of invasive breast cancers (IBC):
  - Second most common after NST
- More likely to be diagnosed in:
  - Elderly patients (> 60 years of age)
  - At advanced staged and with positive lymph node involvement
ILC has several variants but two main groups:
- Classic ILC and related variants:
  - Represent 60% to 70% of the cases
  - Variants:
    - Alveolar
    - Papillary
    - Mucinous
    - Tubulolobular
- Aggressive variants:
  - Represent 30% to 40% of the cases
  - Variants:
    - Pleomorphic
    - Solid
    - Signet ring
    - Histiocytoid
Aggressive variants:
- Can behave worse than matched NST
All variants share characteristic features of ILC:
- Classic ILC (60% to 70% of the cases):
  - More likely to present with larger tumors, multifocal tumors, and with positive margins after breast conserving surgery (BCS) than NST
  - 95% are ER+, HER2-, low mitosis and a low proliferation index
  - They are grade 1 to 2, luminal A or less likely luminal B
  - The majority of ILC are low and intermediate risk on Oncotype Dx compared to the clinical risk:
    - Onctoype Dx:
      - 1% to 8% of ILC were categorized as high risk but this include all variants of ILC
Classic ILC:
- Has a lower diagnostic concordance among pathologists:
  - This may be improved with E-Cadherin IHC
- Due to is growth pattern, it can be difficult to assess size accurately compared to NST (extent vs tumor burden)
- Can be more extensive that what is seen on H and E (small foci can be missed)