Infiltrating epitheliosis of the breast: a case report on an uncommon and challenging complex sclerosing lesion

Infiltrating epitheliosis (IE) of the breast is a complex sclerosing lesion uncommonly encountered in routine diagnostic practice. Histologically, all cases demonstrate florid usual ductal hyperplasia-like proliferation with irregular epithelial foci that appear to infiltrate into the adjacent scleroelastotic stroma. IE can easily be misdiagnosed as invasive carcinoma due to its infiltrative growth pattern and focal loss of myoepithelial cells. Most pathologists classify IE in the radial scar/complex sclerosing (RS/CSL) spectrum. Although association with oncogenic PIK3 mutations has been found, its propensity to behave as a neoplastic proliferation is still uncertain. It is currently considered a florid hyperplastic entity requiring a complete excision. Given its rarity and potential for misdiagnosis, recognising the subtle but distinctive features of IE is essential. Further studies are needed to clarify its behaviour, association with carcinoma and its potential as a precursor lesion.