Endometrioid Intraepithelial Neoplasia (EIN): Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
September 2, 2026

Endometrioid intraepithelial neoplasia (EIN) is a precancerous condition of the lining of the uterus, which is called the endometrium. In this condition, the glands that make up the lining become crowded together, and the cells lining them look abnormal.

EIN is not cancer. It is called precancerous because, left untreated, it can develop over time into endometrioid carcinoma, the most common cancer of the uterus. Another name for the same condition is atypical endometrial hyperplasia, a term used in older reports and still used in some laboratories.

This diagnosis carries two separate questions, and they are worth keeping apart. The first is whether a cancer is already present somewhere in the uterus that the biopsy did not reach. The second is the risk of cancer developing in the future if the condition is not treated. Both are covered below, along with the treatment options.

What causes endometrioid intraepithelial neoplasia?

EIN develops when the lining of the uterus is exposed to estrogen over a long period without enough progesterone to balance it. Estrogen makes the lining grow. Progesterone, which is produced only after ovulation, matures the lining and prepares it to shed. When ovulation does not happen regularly, the lining stays under the influence of estrogen and keeps growing. Over time, the glands become crowded, and the cells begin to change.

Several situations produce this imbalance.

Irregular or absent ovulation — Common in polycystic ovary syndrome and in the years approaching menopause.
Excess body weight — Fat tissue converts other hormones into estrogen, which raises the amount reaching the lining. This is the most common contributing factor after menopause.
Estrogen-only hormone therapy — Taken by someone who still has a uterus, this produces exactly this imbalance.
Tamoxifen — A medication used in breast cancer treatment. It blocks estrogen in the breast but acts like estrogen in the uterus.
Diabetes and high blood pressure — Both are associated with a higher risk, partly through their link with body weight.

EIN is most often diagnosed between the ages of 50 and 55, around and after menopause, though it occurs at any age once periods have begun.

A small number of cases occur in people with an inherited cancer syndrome. The main one is Lynch syndrome. Cancer of the uterus is the most common Lynch-related cancer in women, and it often appears at a younger age than usual. Cowden syndrome is another, rarer example. If your diagnosis came at a young age, or there is a strong family history of uterine or bowel cancer, your doctor may suggest genetic counseling.

What are the symptoms?

The most common symptom is abnormal bleeding from the uterus, and it is usually what leads to the biopsy.

Heavy or prolonged periods — Bleeding heavier or lasting longer than usual for you.
Bleeding between periods — Spotting or bleeding when a period is not expected.
Irregular cycles — Unpredictable timing, or long gaps followed by heavy bleeding.
Bleeding after menopause — Should always be assessed promptly, whatever the eventual cause.

Some people have no symptoms, and EIN is found in tissue removed for another reason.

How is the diagnosis made?

A pathologist makes this diagnosis after examining a sample of the lining under the microscope. The sample usually comes from an endometrial biopsy, a brief clinic procedure using a thin flexible tube, or from a dilation and curettage.

EIN does not usually form a visible mass. An ultrasound may show a thickened lining, and hysteroscopy may show the affected area as a raised or polyp-shaped patch, but neither test can make the diagnosis. The affected area is often localized rather than involving the whole lining, which matters for the reason described in the next section.

The pathologist decides between several possibilities: a normal lining, endometrial hyperplasia without atypia, EIN, and cancer. That decision depends on how crowded the glands are, how the cells look, and whether the abnormal area differs from the surrounding lining.

What does endometrioid intraepithelial neoplasia look like under the microscope?

EIN is an area of the lining of the uterus in which crowded glands are lined by cells that look abnormal. Under the microscope, the pathologist looks for the following features.

Crowded glands — The glands sit close together, with less of the supporting tissue, called stroma, that normally separates them.
Abnormal-looking cells — The cells lining the glands have larger, darker, or more irregular nuclei than normal endometrial cells. Pathologists call this atypia, which separates EIN from hyperplasia without atypia.
A distinct area — The abnormal glands form a defined region that looks different from the lining around it. This contrast helps confirm the diagnosis.
A minimum size — The abnormal area has to reach a certain size, usually about 1 mm across, before the diagnosis is made.
No invasion — The glands stay within the lining and do not grow into the muscle wall. Invasion would make the diagnosis cancer rather than EIN.

Immunohistochemistry

Immunohistochemistry is a laboratory test that uses antibodies to detect specific proteins inside cells. It is not needed in every case, but it can help when the appearance is not clear-cut. If these tests were performed, the results appear in your report as protein names with a description of whether each is present or lost.

PTEN. Often lost. PTEN normally acts as a brake on cell growth, and losing it is one of the earliest changes in EIN development. Loss in a crowded area supports the diagnosis.
PAX2. Often lost. PAX2 is normally present in endometrial glands, and its loss in an abnormal area helps separate EIN from a normal lining. It is frequently used alongside PTEN.
Mismatch repair proteins. Usually retained. These four proteins, MLH1, PMS2, MSH2, and MSH6, repair errors in DNA. When one or more is lost, further testing may follow to determine whether the cause is Lynch syndrome or a change that arose in the tissue itself.

Neither PTEN nor PAX2 loss alone is enough to make the diagnosis, and neither is present in every case. They support what the pathologist sees rather than replacing it.

Could there already be a cancer in my uterus?

This is the more immediate of the two questions this diagnosis raises. EIN is often diagnosed on a small sample of the lining, and a cancer can be present in an area the sample did not reach. Among people diagnosed with EIN on biopsy who then have a hysterectomy, somewhere between 30 and 50 out of every 100 are found to have a cancer already present.

That number sounds alarming, and it needs context. These cancers are almost always found at an early stage and confined to the uterus, because the bleeding that led to the biopsy brought them to attention early. Being found this way is a good outcome, not a bad one.

The practical consequence concerns treatment choice. If you are considering keeping your uterus, your doctor will want to look more thoroughly first. Hysteroscopy, which uses a camera to inspect the lining directly and take targeted samples, is the most accurate way to do this. Imaging alone cannot rule out a small cancer.

What is the risk of cancer developing over time?

This is the second question, and it applies to the future rather than to now. Without treatment, roughly 8 out of every 100 people with this diagnosis develop cancer of the uterus within about four years. The figure rises to roughly 28 out of every 100 by 20 years.

The comparison with the milder condition is useful. Endometrial hyperplasia without atypia carries a risk of fewer than 5 in 100 over the same 20 years. Atypia raises that risk roughly sixfold, which is why EIN is treated rather than watched.

Treatment substantially reduces this risk. Most people diagnosed with EIN and treated appropriately never develop cancer of the uterus.

How is endometrioid intraepithelial neoplasia treated?

Treatment depends on your age, whether you have completed your family, and whether surgery is suitable for you.

Hysterectomy — Removal of the uterus is the definitive treatment, and it is usually recommended for those who have completed childbearing or who have gone through menopause. It both removes any hidden cancer and prevents new disease. The whole uterus, including the cervix, is removed, since leaving the cervix behind is not appropriate for this diagnosis.
Removing the tubes and ovaries — The fallopian tubes are usually removed at the same time. Whether the ovaries are removed depends on your age. After menopause, they are commonly removed; before menopause, keeping them is often preferred, since removing them brings on menopause early.
Progestin therapy — For those who wish to preserve fertility or cannot have surgery. Progestin can be given as tablets or through a hormonal IUD. The evidence favors the IUD, which achieves a higher rate of the abnormal cells returning to normal than tablets alone.
Follow-up biopsies — An essential part of non-surgical treatment rather than an optional extra. Repeat sampling confirms that the abnormal cells have gone and checks that they have not returned.
Addressing the cause — Reviewing estrogen-only hormone therapy, managing polycystic ovary syndrome, or weight reduction where relevant.

One treatment is specifically not recommended. Endometrial ablation, a procedure that destroys the lining, should not be used for EIN. It leaves abnormal tissue behind and makes future bleeding much harder to assess.

If you are hoping to become pregnant, this is worth raising early. Among people treated with progestin rather than surgery, reported pregnancy rates range from roughly 26 to 41 out of every 100. Most teams recommend proceeding with pregnancy once the lining has returned to normal, and discussing hysterectomy afterward.

What other findings may be described in the report?

Your report may describe other lining features alongside the EIN.

Hyperplasia without atypia — The lining outside the abnormal area may show crowded glands with normal-looking cells. This is reported separately.
Endometrial polyp — EIN sometimes arises within an endometrial polyp. When the polyp is removed whole, this may be favorable, since the abnormal tissue may have come out with it.
Sample adequacy — A note that the sample was scant or fragmented. This matters here, because a limited sample is less able to exclude a cancer elsewhere in the lining.
Metaplasia — Areas where the lining cells take on a different but still normal appearance. Common and not precancerous.
Findings in a hysterectomy specimen — If the uterus was removed, the report states whether cancer was found, and if so, its type, grade, and how deeply it extended into the wall.

Questions to ask your doctor

What features in my biopsy confirmed this diagnosis?
Was the sample adequate, or should it be repeated?
How likely is it that a cancer is already present?
Should I have a hysteroscopy before deciding on treatment?
Do you recommend a hysterectomy, or can I try progestin treatment first?
If I use progestin, would tablets or a hormonal IUD be better for me?
How often will I need follow-up biopsies, and for how long?
If I have a hysterectomy, will my ovaries be removed, and why?
What is my risk of developing cancer of the uterus in the future?
Were any special stains, such as PTEN, PAX2, or mismatch repair proteins, performed?
Should I be tested for Lynch syndrome, and does this affect my family?
Can I still become pregnant, and when would that be safe?
What can I do to reduce the chance of this coming back?