Endometrial intraepithelial neoplasia (EIN): will it bring order to chaos? The Endometrial Collaborative Group.

The diagnosis of precancerous lesions of the endometrium remains unstandardized because existing World Health Organization classification categories do not correspond to distinctive biologic groups and are inadequately supported by reproducible histopathologic criteria. A group of gynecologic pathologists was convened to consider revised diagnostic classification and criteria based on newly available information. We propose the terms endometrial hyperplasia (EH), endometrial intraepithelial neoplasia (EIN), and adenocarcinoma to define distinctive subgroups that are functionally relevant to clinical management of patients with endometrial disease. Endometrial precancers are collectively designated EIN in recognition of their monoclonal growth. At present there is no effective strategy for constructive subdivision of EIN lesions into grades or subgroups. EIN is to be distinguished from adenocarcinoma and the diffuse hormonal changes of EH seen in anovulation. An archive of genetically and morphologically classified endometrial precancers at www.endometrium.org provides a resource for centralized review of the histopathology of EIN lesions. A new architectural criterion for EIN diagnosis, diminution of stromal volume to less than approximately half of the total sample volume, will also assist in discriminating between EH and EIN. Implementation of this proposal will bring diagnostic terminology into agreement with current concepts of premalignant endometrial disease and facilitate more uniform patient management.