Lobular Carcinoma

LCIS (lobular carcinoma in-situ): Proliferation of small, round, discohesive cells filling and expanding the acini. The discohesion is due to loss of E-cadherin — a cell adhesion molecule on chromosome 16q. LCIS is generally considered a risk marker for bilateral breast cancer rather than a direct precursor, though pleomorphic LCIS behaves more aggressively.

Invasive lobular carcinoma: ~10–15%. Defined by loss of E-cadherin, same as LCIS. Cells infiltrate in single-file Indian file pattern or targetoid pattern around existing ducts, often with minimal desmoplastic reaction — this is why they're notoriously difficult to detect on mammography and clinical exam (no discrete mass, just a vague thickening). They also have a distinct metastatic pattern — more likely to go to peritoneum, GI tract, ovaries, and leptomeninges compared to ductal cancers.