ILC is the second most common invasive breast cancer (10% of all invasive breast cancers); 2/3 with associated LCIS

Compared to IDC, ILC is: more often multifocal/bilateral (10-15%), more common in older women (60s), generally larger and better-differentiated, and metastasizes later and to unusual locations (meninges, peritoneum)

Clinical presentation: Usually no symptoms; may present with thickening or hardened area of breast

Diagnosis: Needle or surgical biopsy; mutations in E-cadherin gene (CDH1) distinguish ILC from LCIS; Diffuse stromal invasion in single-file rows without disruption of anatomy and without inflammatory reaction; ER+, HER2-; MRI recommended after histopathologic diagnosis due to the propensity of ILC to be multicentric

Imaging (often underestimates size of disease):

–     Ultrasound: Ill defined heterogeneous infiltrating area; Hypoechoic; Disproportionate/isolated posterior acoustic shadowing

–     Mammogram: CC > MLO, Spiculated, ill-defined mass or poorly defined asymmetric density; Low to equal density compared to surrounding breast tissue; 30% cases are mammographically occult

Treatment: Surgery + radiation therapy + adjuvant therapy

(Ikeda 2010, Georgian-Smith 2014, Bleiweiss 2013, Esserman 2014, Kang ILC 2016, Johnson 2015, Breastcancer.org ILC 2016)