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What is Endometrial intraepithelial neoplasia and explain in detail?
Endometrial intraepithelial neoplasia (EIN) is a premalignant lesion of the uterine lining that predisposes to endometrioid endometrial adenocarcinoma.  It is composed of a collection of abnormal endometrial cells, arising from the glands that line the uterus, which have a tendency over time to progress to the most common form of uterine cancer—endometrial adenocarcinoma, endometrioid type.

History
EIN lesions have been discovered by a combination of molecular, histologic, and clinical outcome studies beginning in the 1990s which provide a multifaceted characterization of this disease.  They are a subset of a larger mixed group of lesions previously called "endometrial hyperplasia". The EIN diagnostic schema is intended to replace the previous "endometrial hyperplasia" classification as defined by the World Health Organization in 1994, which have been separated into benign (benign endometrial hyperplasia) and premalignant (EIN) classes in accordance with their behavior and clinical management.
EIN should not be confused with an unrelated entity, serous intraepithelial carcinoma ("serous EIC"), which is an early stage of a different tumor type known as papillary serous adenocarcinoma that also occurs in the same location within the uterus.

Clinical aspects
The average age at time of EIN diagnosis is approximately 52 years, compared to approximately 61 years for carcinoma.  The timeframe and likelihood of EIN progression to cancer, however, is not constant amongst all women.  Some cases of EIN are first detected as residual premalignant disease in women who already have carcinoma, whereas other EIN lesions disappear entirely and never lead to cancer.  For this reason, treatment benefits and risks must be individualized for each patient under the guidance of an experienced physician.
Risk factors for development of EIN and the endometrioid type of endometrial carcinoma include exposure to estrogens without opposing progestins, obesity, diabetes, and rare hereditary conditions such as hereditary nonpolyposis colorectal cancer.  Protective factors include use of combined oral contraceptive pills (low dose estrogen and progestin), and prior use of a contraceptive intrauterine device.

Biology
EIN lesions demonstrate all of the behaviors and characteristics of a premalignant, or precancerous, lesion.
Precancer Features of EIN (Table I). The cells of an EIN lesion are genetically different than normal and malignant tissues, and have a distinctive appearance under the light microscope