## Distinguishing ILC from IDC ### Key Histological Features **Key Point:** Invasive lobular carcinoma (ILC) is characterized by single-file (Indian-file) infiltration of neoplastic cells through breast tissue, whereas invasive ductal carcinoma (IDC) shows cohesive nests or sheets of tumor cells. **High-Yield:** Loss of E-cadherin expression is the molecular hallmark of ILC. E-cadherin is a cell-cell adhesion molecule encoded by the CDH1 gene, which is frequently inactivated in ILC (by mutation, deletion, or hypermethylation). This loss of adhesion allows individual cells to permeate through tissue in a single-file pattern. ### Comparison Table | Feature | ILC | IDC | |---------|-----|-----| | **Growth pattern** | Single-file infiltration | Cohesive nests/sheets | | **E-cadherin** | Lost/reduced (CDH1 mutation) | Retained | | **Necrosis** | Rare | Common (especially comedo type) | | **Stromal reaction** | Minimal desmoplasia | Marked desmoplasia | | **Cell morphology** | Small, uniform cells | Pleomorphic cells | | **Clinical presentation** | Occult, diffuse, higher bilaterality | Palpable mass, more focal | ### Molecular Basis **Clinical Pearl:** ILC arises from loss of CDH1 (chromosome 16q22.1), leading to disruption of the E-cadherin–catenin complex. This explains the characteristic single-file pattern and the increased propensity for multifocal and bilateral disease. ### Why This Matters **Mnemonic:** **SILF** = Single-file Infiltration, Loss of E-cadherin, Lobular carcinoma. The single-file pattern combined with E-cadherin loss is the gold standard for distinguishing ILC from IDC and is essential for diagnosis and predicting biological behavior (higher risk of contralateral breast cancer and gastric involvement). [cite:Robbins 10e Ch 24]
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