Dedifferentiated carcinoma of the endometrium is an uncommon and fast-growing cancer that starts in the endometrium, the inner lining of the uterus. It is defined by a tumor made of two different parts.
One part is an ordinary, slow-growing cancer, most often a low-grade endometrioid carcinoma of FIGO grade 1 or 2. The other part is an undifferentiated carcinoma, in which the cells have lost all recognizable structure and grow quickly.
The undifferentiated part is what determines 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.
The two components are not two separate cancers. Genetic testing shows they share the same origin, meaning both grew from the same original tumor. The undifferentiated part developed out of the low-grade part through a process called dedifferentiation, in which cancer cells lose the features that identified where they came from.
Two consequences follow, and both matter.
First, the proportion does not matter. Even when the undifferentiated part makes up only a small fraction of the tumor, it drives the behavior of the whole thing. A tumor that is mostly low-grade endometrioid carcinoma with a small undifferentiated area is treated as a dedifferentiated carcinoma, not as a low-grade cancer.
Second, this diagnosis is easy to miss. If a biopsy samples only the low-grade part, the report will say low-grade endometrioid carcinoma, and the undifferentiated part is found for the first time after surgery. Even in a fully removed uterus, the undifferentiated area may be small and require several tissue sections to find. If your diagnosis appeared to change or worsen between your biopsy and your final report, this is the usual explanation. It does not mean the cancer grew in the interval.
The cause is not fully understood. Dedifferentiation appears to happen more often in tumors that are mismatch repair deficient, and between about half and two-thirds of dedifferentiated carcinomas show this. Loss of proteins in a group called the SWI/SNF complex is also strongly associated with it, and changes in the PI3K pathway are common in both components.
A minority of cases are linked to Lynch syndrome, an inherited condition that raises the risk of cancers of the uterus and bowel. This is especially true when mismatch repair proteins are lost, and it affects blood relatives as well as you.
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.
Because of the sampling problem described above, a biopsy showing low-grade endometrioid carcinoma does not rule out this diagnosis. The complete picture generally requires examination of the whole uterus after surgery, which usually also removes the ovaries, fallopian tubes, and lymph nodes.
The pathologist examines multiple sections looking for both components, then assesses how deeply the tumor has grown, whether it has spread, and which biomarkers are present.
Dedifferentiated carcinoma of the endometrium is diagnosed when both components are identified in the same tumor.
The differentiated component forms glands and looks like an ordinary low-grade endometrioid carcinoma. This is the part that would be reported on its own if the other were not found.
The undifferentiated component shows the following features.
The border between the two components may be abrupt, producing a striking two-part appearance, or the two may be mixed.
Immunohistochemistry is a laboratory test that uses antibodies to detect specific proteins inside cells. It matters here because the undifferentiated component can resemble several very different cancers, and because the two components stain differently from one another.
The defining feature of the undifferentiated component is that it shows only limited evidence of being a carcinoma.
The FIGO grading system used for endometrioid carcinoma is based largely on how much of the tumor grows in solid sheets. A dedifferentiated carcinoma contains an undifferentiated component and is therefore considered high grade by definition. It is not managed like a low-grade endometrioid carcinoma.
Your report may still give a FIGO grade for the differentiated component, such as grade 1 or grade 2. That grade describes only that part. The undifferentiated component determines treatment, regardless of what percentage of the tumor it represents.
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 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 this diagnosis. A tumor missing one or more of these proteins is described as mismatch repair deficient. This matters for two reasons. It identifies tumors likely to respond to immunotherapy if the cancer is advanced or returns. And it raises the possibility of Lynch syndrome, which may lead to further testing and to screening for blood relatives.
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 dedifferentiated carcinoma. 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 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.
The SWI/SNF complex controls how DNA is packaged and which genes are switched on. Changes in these proteins are strongly linked to dedifferentiation, and they are often found in the undifferentiated component but not the low-grade one.
Results are reported as retained or lost. Loss of SMARCA4, loss of SMARCB1, or combined loss of ARID1A and ARID1B supports the diagnosis and is associated with faster-growing disease.
These genes belong to a pathway that regulates cell growth and survival. Changes are common in this diagnosis and usually present in both components, supporting evidence that the two share a single origin. Results are reported as mutated or wild-type, and they are most relevant when molecular profiling is done for advanced or recurrent disease.
Large genomic studies group endometrial cancers into four molecular subtypes. The biomarkers above place a tumor into one of them.
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.
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 differs from the older staging system, in which the threshold was half the wall thickness.
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.
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 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.
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 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.
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):
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.
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. Dedifferentiated carcinoma sits in the higher-risk group, alongside undifferentiated carcinoma, serous carcinoma, clear cell carcinoma, carcinosarcoma, and high-grade endometrioid carcinoma. Tumors in that group are staged differently from the rest.
Molecular results can change the stage for tumors confined to the uterus. A POLE mutation lowers the stage to IAmPOLEmut, whatever the histologic type. An abnormal p53 result raises it to IIc p53abnn. 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 this, and it is worth asking which one was used.
Dedifferentiated carcinoma grows quickly and has a less favorable outlook than low-grade endometrioid carcinoma, even when the undifferentiated component is small. Stage remains the strongest factor, and tumors confined to the uterus do considerably better than those that have spread.
The molecular subtype now adds meaningful information. 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.
Because this diagnosis is uncommon and treatment depends on biomarker results, care at a center experienced with gynecologic cancers is generally recommended.