Undifferentiated Carcinoma of the Endometrium: Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
September 3, 2026

Undifferentiated carcinoma of the endometrium is an uncommon and fast-growing cancer that starts in the endometrium, the inner lining of the uterus. It is called undifferentiated because the tumor cells give no clear sign of which normal cell type they came from. Under the microscope, they look very abnormal and do not form the gland shapes seen in most uterine cancers.

A closely related diagnosis is dedifferentiated carcinoma. This describes a tumor with two parts: an undifferentiated component and a second, more ordinary component, most often a low-grade endometrioid carcinoma. Finding any undifferentiated component matters because it drives how the tumor behaves and how it is treated.

This article explains what this diagnosis means on your pathology report, what each section of the report describes, and why it matters for your care.

What causes undifferentiated carcinoma of the endometrium?

The cause is not fully understood. In many cases, the tumor develops through a process called dedifferentiation, in which an ordinary endometrial cancer, usually a low-grade endometrioid carcinoma, changes over time and loses its recognizable structure. This is why a dedifferentiated carcinoma contains both components side by side.

These tumors frequently carry changes in DNA repair genes and in genes that control how DNA is packaged inside the cell. Many are mismatch repair deficient, and many have changes affecting a group of proteins called the SWI/SNF complex. The biomarkers section below describes both.

A minority of cases are linked to Lynch syndrome, an inherited condition that raises the risk of cancers of the uterus and bowel. This possibility is considered particularly when mismatch repair proteins are lost, and it has implications for blood relatives as well as for you.

What are the symptoms?

Abnormal vaginal bleeding — The most common symptom, particularly bleeding after menopause. Any bleeding after menopause should be assessed promptly.
Unusual vaginal discharge — Reported by some people.
Abdominal or pelvic pain — Less common, and more likely when the tumor is large.

How is the diagnosis made?

The process usually begins with an endometrial biopsy, in which a small sample of the lining is removed, and a pathologist examines it under a microscope.

A biopsy carries a particular limitation for this diagnosis. In a dedifferentiated carcinoma, the two components may be unevenly distributed, and a biopsy that samples only the low-grade part will be reported as a low-grade endometrioid carcinoma. The undifferentiated component is then found for the first time after surgery. If your diagnosis changed between your biopsy and your final report, this is usually why.

Surgery generally involves removing the uterus, and often the ovaries, fallopian tubes, and lymph nodes. Pathologists examine the removed tissue to determine how deeply the tumor has grown, whether it has spread, and which biomarkers are present.

What does undifferentiated carcinoma look like under the microscope?

Undifferentiated carcinoma of the endometrium is made of sheets of abnormal cells that form no recognizable structures. The pathologist looks for the following features.

Sheets of cells with no glands — The tumor cells grow in solid sheets rather than forming the tube-shaped glands of most endometrial cancers.
Loosely connected cells — The cells are described as discohesive, meaning they do not stick to one another and appear as single cells or loose clusters.
Uniform but clearly abnormal cells — In the usual form, the cells are fairly similar to one another in size and shape, yet each is plainly a cancer cell.
Frequent cell division — Mitotic figures, meaning cells caught in the act of dividing, are numerous. This reflects how quickly the tumor grows.
Necrosis — Areas of dead tumor tissue, called necrosis, are common.
Immune cells within the tumor — Large numbers of lymphocytes are often present among the tumor cells. This is frequently a sign of mismatch repair deficiency.
Rhabdoid cells — Some tumors contain cells with abundant pink cytoplasm, described as rhabdoid. A smaller number have a gelatinous background between the cells.

In a dedifferentiated carcinoma, the second, more ordinary component is also present. The border between the two may be abrupt, or the two may blend into one another.

Immunohistochemistry

Immunohistochemistry is a laboratory test that uses antibodies to detect specific proteins inside cells. This matters because these tumors can resemble several very different cancers, and treatment depends on getting that right.

The defining feature is that the tumor shows only limited evidence of being a carcinoma at all.

Cytokeratins and EMA. Positive, but only in scattered cells or small clusters. The staining may be intense where it occurs. Widespread strong staining would point to a different diagnosis. Because it can be very weak or absent, pathologists use more than one marker. CK8/18 and EMA are usually the most informative.
Vimentin. Usually positive.
Estrogen receptor and progesterone receptor. Usually negative, unlike most endometrioid carcinomas.
E-cadherin. Usually absent, which fits with the cells not sticking together.
PAX8. Usually negative, though it may be present in scattered cells.
Synaptophysin and chromogranin. Usually staining fewer than 1 in 10 tumor cells. Limited staining like this separates undifferentiated carcinoma from a true neuroendocrine carcinoma, where the staining is widespread.
SWI/SNF proteins. Loss of SMARCA4, loss of SMARCB1, or combined loss of ARID1A and ARID1B may be seen. The biomarkers section describes these further.

How is this different from other cancers of the uterus?

Several cancers can look similar under the microscope, and the immunohistochemistry results above are what separate them.

High-grade endometrioid carcinoma — Still forms some gland structures and is usually positive for hormone receptors and widely positive for cytokeratins.
Serous carcinoma — Forms complex branching structures and shows widespread abnormal p53 staining together with strong PAX8.
Neuroendocrine carcinoma — Shows widespread staining for synaptophysin and chromogranin rather than the scattered staining seen here.
Lymphoma — Made of immune cells and negative for the epithelial markers.
Sarcoma — Arises from muscle or supporting tissue and carries different markers.

Grade

The FIGO grading system used for endometrioid carcinoma is based largely on how much of the tumor grows in solid sheets. Undifferentiated carcinoma is considered high grade by definition, so it is not assigned a grade of 1, 2, or 3.

If your report describes a dedifferentiated carcinoma, it may give a grade for the ordinary component, such as FIGO grade 1 or 2 endometrioid carcinoma. That grade describes only that part of the tumor. The undifferentiated component is what determines treatment.

Biomarkers

Biomarkers are tests performed on tumor tissue to understand how a cancer is likely to behave and which treatments may work. They include immunohistochemistry and molecular tests that look for changes in DNA. Not every case includes testing for every biomarker.

Mismatch repair proteins

Mismatch repair proteins fix small errors that occur when DNA is copied. The four tested are MLH1, PMS2, MSH2, and MSH6, which work in pairs. Results are reported as retained, meaning normal, or lost, meaning abnormal.

Loss is common in these tumors. A tumor missing one or more of these proteins is described as mismatch repair deficient. This matters for two reasons. It raises the possibility of Lynch syndrome, which may lead to further testing and to screening for blood relatives. And it identifies tumors likely to respond to immunotherapy if the cancer is advanced or returns.

POLE

POLE is a gene involved in copying DNA accurately. Tumors with a POLE mutation accumulate very large numbers of DNA changes, yet they behave far better than their appearance suggests.

POLE mutations are uncommon in undifferentiated and dedifferentiated carcinomas. When present, they carry a favorable outlook, and they lower the stage under the current staging system. Results are reported as mutated or wild-type.

p53

p53 is a protein that helps control cell growth and repair damaged DNA. An abnormal result, reported as aberrant, mutant-type, or abnormal expression, indicates an altered TP53 gene.

Some of these tumors show abnormal p53, and some do not. An abnormal result is associated with a less favorable outlook and, for tumors confined to the uterus, raises the stage under the current staging system.

SWI/SNF proteins

The SWI/SNF complex controls how DNA is packaged and which genes are switched on. Changes in these proteins are strongly linked to the loss of structure that defines this diagnosis.

Results are reported as retained or lost. Loss of SMARCA4 is seen in a proportion of these tumors, and loss of SMARCB1 or combined loss of ARID1A and ARID1B also occurs. These findings support the diagnosis and are associated with faster-growing disease.

PTEN, PIK3CA, and PIK3R1

These genes belong to a pathway that regulates cell growth and survival. Changes are common in these tumors and are often present in both components of a dedifferentiated carcinoma. Results are reported as mutated or wild-type, and they are most relevant when molecular profiling is done for advanced or recurrent disease.

Molecular subtypes

Large genomic studies group endometrial cancers into four molecular subtypes. The biomarkers above place a tumor into one of them.

Mismatch repair deficient — The group most undifferentiated and dedifferentiated carcinomas fall into—intermediate outlook, and often eligible for immunotherapy in advanced disease.
p53 abnormal — The least favorable group.
POLE ultramutated — Uncommon here, and the most favorable group.
No specific molecular profile — Tumors with none of the above changes. Intermediate outlook.

The subtype is more than a prognostic label. For tumors confined to the uterus, a POLE mutation lowers the stage and an abnormal p53 result raises it, as described in the staging section below.

What other findings will be described in the report?

Myometrial invasion

The myometrium is the thick muscular wall of the uterus. Myometrial invasion means the tumor has grown from the lining into that wall. The pathologist measures the depth and usually reports it as a percentage of the wall’s full thickness.

For this diagnosis, whether the tumor invades the muscle at all matters most because it changes the stage. This is different from the older staging system, in which the threshold was half the thickness of the wall.

Cervical stromal invasion

Cervical stromal invasion means the tumor has grown from the body of the uterus into the cervix’s supporting tissue. Involvement of only the surface lining of the cervix does not count. This finding may influence whether radiation therapy is recommended.

Involvement of other organs and tissues

The uterus sits against the ovaries, fallopian tubes, vagina, bladder, and rectum. The fallopian tubes, ovaries, and attached ligaments are together called the adnexa. The pathologist examines the removed tissue and reports whether tumor cells are present. Involvement of these tissues raises the stage.

Lymphovascular invasion

Lymphovascular invasion means tumor cells have been seen inside small lymphatic channels or blood vessels. These pathways allow cells to reach lymph nodes or distant organs.

Your report will describe it as absent, focal, or substantial. That distinction matters, because substantial lymphovascular invasion changes the stage for some endometrial cancers and often leads to a recommendation for additional treatment.

Margins

A margin is the edge of the tissue removed during surgery. The pathologist examines the margins for tumor cells. Depending on the operation, these may include the cervical margin, the vaginal cuff margin, the tissue on either side of the uterus, and the peritoneal surface.

A positive margin means tumor cells reach the cut edge and some cancer may remain. A negative margin means no tumor cells were found at the edges. Positive margins may lead to a recommendation for radiation therapy.

Lymph nodes

Lymph nodes are small immune organs that filter fluid draining from tissues. Cancer cells can travel to them and form a metastasis.

Nodes from the pelvis, and sometimes from higher in the abdomen, are commonly removed for this diagnosis and examined individually. Your report will state how many were examined and how many contained cancer. When cancer is found, the size of the largest deposit is measured, because size determines the nodal stage.

Isolated tumor cells — A group measuring 0.2 mm or less. These are recorded as N0(i+) and are not counted as node-positive disease.
Micrometastasis — A deposit measuring more than 0.2 mm and up to 2 mm. This gives a nodal stage of N1mi or N2mi, depending on which nodes are involved.
Macrometastasis — A deposit measuring more than 2 mm. This gives a nodal stage of N1a or N2a and is associated with a less favorable outlook.

Pathologic stage

The pathologic stage uses the TNM system from the American Joint Committee on Cancer, which describes the tumor (T), the lymph nodes (N), and distant spread (M).

Tumor stage (pT):

T1 — The tumor involves only the body of the uterus.
T2 — The tumor has grown into the supporting tissue of the cervix.
T3 — The tumor has reached the outer surface of the uterus, or has spread to the fallopian tubes, ovaries, or vagina.
T4 — The tumor has grown into the lining of the bladder or the bowel.

Nodal stage (pN): N0 means no tumor cells in the nodes examined. N0(i+) means only isolated tumor cells. N1mi and N1a describe involvement of pelvic nodes. N2mi and N2a describe involvement of nodes higher in the abdomen. The “mi” categories are used for deposits of 2 mm or less. NX means no nodes were examined.

Metastatic stage (pM): M1 means the cancer has spread to a distant site. This can only be assigned when tissue from that site has been examined, so most reports list it as MX and use imaging instead.

FIGO stage

The International Federation of Gynecology and Obstetrics substantially revised the staging system in 2023, and the change matters a great deal for this diagnosis.

The current system divides endometrial cancers into two groups according to how they tend to behave. Undifferentiated carcinoma sits in the higher-risk group, alongside serous carcinoma, clear cell carcinoma, carcinosarcoma, and high-grade endometrioid carcinoma. Tumors in that group are staged differently from the rest.

Stage IC — The tumor is confined to the lining, with no growth into the muscular wall.
Stage IIC — The tumor has grown into the muscular wall to any depth. Under the previous system, this would have been stage IA or IB depending on the depth. Many patients are therefore assigned a higher stage now than they would have been before 2023.
Stage IIIA — The tumor involves the outer surface of the uterus, or the ovaries or fallopian tubes.
Stage IIIB — The tumor involves the vagina or the tissue on either side of the uterus.
Stage IIIC — The tumor has spread to lymph nodes. IIIC1 is pelvic nodes, and IIIC2 is nodes higher in the abdomen.
Stage IVA — The tumor has grown into the lining of the bladder or bowel.
Stage IVB — The tumor has spread within the abdomen beyond the pelvis.
Stage IVC — The tumor has spread to distant organs such as the lungs or liver.

Molecular results can change the stage for tumors confined to the uterus. A POLE mutation lowers the stage to IAmPOLEmu,t whatever the histologic type. An abnormal p53 result raises it to IIc p53abn. Molecular results do not change stage III or stage IV disease.

Your report or your oncologist may use a stage that does not match what you have read elsewhere. The difference between the 2009 and 2023 systems likely explains it, and it is worth asking which one was used.

What is the prognosis?

Undifferentiated and dedifferentiated carcinomas grow quickly and carry a less favorable outlook than most endometrial cancers. Stage remains the strongest factor, and tumors confined to the uterus do considerably better than those that have spread.

The molecular subtype now adds meaningfully to this. Tumors with a POLE mutation do well even when they look alarming under the microscope. Tumors with abnormal p53 do the least well. Mismatch repair deficient tumors sit in between and have treatment options that others do not.

Published survival figures for this diagnosis come from small patient groups and vary between studies. They cannot account for your stage, your biomarker results, or your treatment. Your own team is far better placed to discuss what the outlook means for you.

How is undifferentiated carcinoma treated?

Surgery — Removal of the uterus, cervix, fallopian tubes, and ovaries, usually with lymph node assessment.
Chemotherapy — Commonly recommended after surgery, including for tumors that appear confined to the uterus, because of how these tumors behave.
Radiation therapy — Used in some situations, particularly to reduce the risk of the cancer returning in the pelvis.
Immunotherapy — An option for mismatch repair deficient tumors that are advanced or have returned, and one of the main reasons this biomarker is tested.
Genetic assessment — Recommended when mismatch repair proteins are lost, to determine whether Lynch syndrome is present.

Because this diagnosis is uncommon and the treatment depends on biomarker results, care at a center experienced with gynecologic cancers is generally recommended.

Questions to ask your doctor

Is my diagnosis undifferentiated carcinoma or dedifferentiated carcinoma?
What stage is my cancer, and was the 2023 FIGO system used?
Did the tumor grow into the muscle wall of the uterus?
Was lymphovascular invasion present, and was it focal or substantial?
Were the margins negative?
Were lymph nodes examined, and did any contain cancer?
What did the mismatch repair testing show?
Should I be tested for Lynch syndrome, and does this affect my family?
Was POLE or p53 testing done, and what molecular subtype is my tumor?
Does my molecular subtype change my stage or my treatment?
Do you recommend chemotherapy, radiation, or immunotherapy?
Should I be referred to a gynecologic oncology center?
What will follow-up involve, and how often?