Endometrial hyperplasia: Terminology and histologic classification

WHO: World Health Organization; EIN: endometrial intraepithelial neoplasia; BEH: benign endometrial hyperplasia.
* Also referred to as proliferative pattern, normal proliferative endometrium, or normal secretory endometrium.
¶ EIN is defined as: Epithelial crowding that displaces stroma to a point at which stromal volume is less than approximately half of total tissue volume in nonsecretory endometrium, and typically cells appear morphologically clonal and distinct from the surrounding endometrium. In the EIN system, the D-score is a measure of stromal volume as a proportion of total tissue volume (stroma + epithelium + gland lumen). EIN is distinguished from BEH by the presence of glandular crowding, even without atypia. Thus, both BEH and EIN have some overlap with WHO categories without atypia.